applied sciences Article

Switched 4-to-1 Transimpedance Combining Amplifier for Receiver Front-End Circuit of Static Unitary Detector-Based LADAR System Eun-Gyu Lee 1 , Jae-Eun Lee 1 , Bang Chul Jung 1 , Bongki Mheen 2 and Choul-Young Kim 1, * 1 2

*

Department of Electronics Engineering, Chungnam Nation University, Daejeon 34134, Korea; [email protected] (E.-G.L.); [email protected] (J.-E.L.); [email protected] (B.C.J.) Electronics and Telecommunication Research Institute, Daejeon 34134, Korea; [email protected] Correspondence: [email protected]; Tel.: +82-42-821-5663

Academic Editor: Ting-Chung Poon Received: 3 June 2017; Accepted: 30 June 2017; Published: 4 July 2017

Abstract: Laser detection and ranging (LADAR) systems are commonly used to acquire real-time three-dimensional (3D) images using the time-of-flight of a short laser pulse. A static unitary detector (STUD)-based LADAR system is a simple method for obtaining real-time high-resolution 3D images. In this study, a switched 4-to-1 transimpedance combining amplifier (TCA) is implemented as a receiver front-end readout integrated circuit for the STUD-based LADAR system. The 4-to-1 TCA is fabricated using a standard 0.18 µm complementary metal-oxide-semiconductor (CMOS) technology, and it consists of four independent current buffers, a two-stage signal combiner, a balun, and an output buffer in one single integrated chip. In addition, there is a switch on each input current path to expand the region of interest with multiple photodetectors. The core of the TCA occupies an area of 92 µm × 68 µm, and the die size including I/O pads is 1000 µm × 840 µm. The power consumption of the fabricated chip is 17.8 mW for a supplied voltage of 1.8 V and a transimpedance gain of 67.5 dBΩ. The simulated bandwidth is 353 MHz in the presence of a 1 pF photodiode parasitic capacitance for each photosensitive cell. Keywords: LADAR; STUD; switched-TCA; optical receiver

1. Introduction Laser detection and ranging (LADAR) systems are commonly used to acquire real-time three-dimensional (3D) images using the time-of-flight (TOF) of a short laser pulse. As LADAR technology has become more diverse, it has been utilized in various applications, such as autonomous vehicles, robots, remote sensing, reconnaissance, and motion detection, where high 3D resolution is important [1–10]. For the real-time acquisition of 3D images, a LADAR system must process all reflected TOF laser signals from every direction for a region-of-interest (ROI) in real time. There are different methods of implementing LADAR systems. The static unitary detector (STUD)-based technique [11] has some unique advantages compared with other techniques, such as the rotational motion-based technique [12] or the focal plane array (FPA)-based technique [13]. Because the STUD-based technique has only one signal processing chain and does not need micro-lenses to increase the signal-to-noise ratio (SNR), it is cost effective. In addition, the required power level of the transmitted laser pulse is not as high as for the FPA-based technique because the STUD-based technique illuminates one collimated laser pulse at a time in a specific direction. Figure 1 shows the block diagram of a STUD-based LADAR system. In the STUD-based LADAR system, the transmitter emits laser pulses over the entire ROI with two high-speed optical scanners and the receiver detects the returned optical pulses to a static-unitary large-area photodetector. Appl. Sci. 2017, 7, 689; doi:10.3390/app7070689

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Figure Figure 1. 1. Block Blockdiagram diagramof of the the static static unitary unitary detector detector (STUD)-based (STUD)-based laser laser detection detection and and ranging ranging Figure 1. Block diagram of the static unitary detector (STUD)-based laser detection and ranging (LADAR) (LADAR) system. system. (LADAR) system.

However, an increase in the area of the photodetector results in a decrease in the bandwidth of However, However,an anincrease increasein inthe thearea areaof ofthe thephotodetector photodetector results results in inaadecrease decreasein inthe thebandwidth bandwidth of of the receiver due to the large parasitic capacitance of the large-area photodetector. To overcome this the the receiver receiver due due to to the the large large parasitic parasitic capacitance capacitance of of the the large-area large-area photodetector. photodetector. To Toovercome overcome this this problem, the STUD-based LADAR receiver has multiple partitioned photosensitive cells, as shown problem, problem, the theSTUD-based STUD-based LADAR LADAR receiver receiver has hasmultiple multiple partitioned partitioned photosensitive photosensitive cells, cells, as as shown shown in Figure 2. Each of the partitioned cells has its own transimpedance amplifier (TIA) to receive and in in Figure Figure 2. 2. Each Each of of the the partitioned partitioned cells cells has has its itsown owntransimpedance transimpedance amplifier amplifier (TIA) (TIA) to to receive receive and and amplify the optical optical currentfrom fromeach eachpartitioned partitioned cell independently, and then a signal combiner amplify cell cell independently, and then signal combiner sums amplifythe the opticalcurrent current from each partitioned independently, and athen a signal combiner sums all the outputs of each TIA into a single output signal STOP, which indicates the arrival of the all the all outputs of eachofTIA into a single signal STOP, the arrival of the return sums the outputs each TIA into a output single output signal which STOP, indicates which indicates the arrival of the return signal. A time-to-digital converter (TDC) calculates the TOF between the START and STOP signal. time-to-digital converter (TDC) calculates the TOFthe between the START STOP signals. return A signal. A time-to-digital converter (TDC) calculates TOF between the and START and STOP signals. Since each partitioned cell with its own cascading TIA operates independently without Since eachSince partitioned cell with itscell ownwith cascading TIAcascading operates independently affectingwithout any of signals. each partitioned its own TIA operates without independently affecting any of the other cells, its bandwidth remains unchanged. In addition, since the STUD-based the other cells, bandwidth remains unchanged. In addition, sinceIn the STUD-based receiver affecting any ofitsthe other cells, its bandwidth remains unchanged. addition, sinceLADAR the STUD-based LADAR receiver does not need to determine which cell detects the arriving laser pulse, inter-channeldoes not need to determine which detects which the arriving laser pulse, inter-channel-interference is not LADAR receiver does not need to cell determine cell detects the arriving laser pulse, inter-channelinterference is not a problem, unlike in the FPA-based LADAR receiver. ainterference problem, unlike FPA-based receiver. LADAR receiver. is notina the problem, unlikeLADAR in the FPA-based

Figure 2. STUD-based LADAR receiver with partitioned photodetector. Figure 2. STUD-based LADAR receiver with partitioned photodetector. Figure 2. STUD-based LADAR receiver with partitioned photodetector.

To implement LADAR receiver, the same number TIAs as partitioned To implementa STUD-based a STUD-based LADAR receiver, the same ofnumber of TIAs asphotosensitive partitioned To implement a STUD-based LADAR receiver, the same number of TIAs as partitioned cells is needed,cells andis they are and assembled onassembled a single board. The pad pitch thepitch partitioned photosensitive needed, they are on a single board. Theinpad in the photosensitive cells is needed, and they are assembled on a single board. The pad pitch in the photodetector is totally different from the pad interval of the TIAs. In case the lengths of the partitioned photodetector is totally different from the pad interval of the TIAs. In case the lengths of partitioned photodetector is totally different from the pad interval of the TIAs. In case the lengths of interconnection lines between each photosensitive cell and the corresponding TIA is different, accurate the interconnection lines between each photosensitive cell and the corresponding TIA is different, the interconnection lines between each photosensitive cell and the corresponding TIA is different, time information cannot be cannot obtained thebecause time delay ondepending which photosensitive accurate time information bebecause obtained thevaries time depending delay varies on which accurate time information cannot be obtained because the time delay varies depending on which cell receives the return signal. Therefore, the electrical length of the interconnection lines between photosensitive cell receives the return signal. Therefore, the electrical length of the interconnection photosensitive cell receives the return signal. Therefore, the electrical length of the interconnection each between photosensitive cell and the corresponding TIA should be designed equal the testequal fixture. lines each photosensitive cell and the corresponding TIA should be on designed onThis the lines between each photosensitive cell and the corresponding TIA should be designed equal on the limits the number of cells higher-resolution 3D images over a large ROI due to athe interconnection test fixture. This limits thefor number of cells for higher-resolution 3D images over large ROI due to test fixture. This limits the number of cells for higher-resolution 3D images over a large ROI due to problem between a partitioned photodetector and multiple TIAs [14,15]. resolve TIAs this problem, the interconnection problem between a partitioned photodetector and To multiple [14,15]. the interconnection problem between a partitioned photodetector and multiple TIAs [14,15]. a 4-to-1 transimpedance amplifier (TCA) was proposed in our previous work [14]in asour the To resolve this problem, combining a 4-to-1 transimpedance combining amplifier (TCA) was proposed To resolve this problem, a 4-to-1 transimpedance combining amplifier (TCA) was proposed in our

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previous work [14] as the front-end readout integrated circuit (ROIC) for the STUD-based LADAR previous work [14] as the front-end readout integrated circuit (ROIC) for the STUD-based LADAR receiver with a photodetector, which has four photosensitive partitioned cells. receiver with a photodetector, which has four photosensitive partitioned cells. In this study, integrated we propose a switched TCA. The switched 4-to-1 TCA has on each front-end readout circuit (ROIC)4-to-1 for the STUD-based LADAR receiver withaaswitch photodetector, In this study, we propose a switched 4-to-1 TCA. The switched 4-to-1 TCA has a switch on each input path, as shown inpartitioned Figure 3. The photodetector in this work has four photosensitive cells. whichcurrent has four photosensitive cells. input current path, as shown in Figure 3. The photodetector in this work has four photosensitive cells. The target of awe single photosensitive is 350 μmThe × 100 μm and4-to-1 the parasitic the In thissize study, propose a switchedcell 4-to-1 TCA. switched TCA hascapacitance a switch onofeach The target size of a single photosensitive cell is 350 μm × 100 μm and the parasitic capacitance of the single cell is assumed to be 1 in pF.Figure In the3.STUD-based LADAR is necessary to increasecells. the input current path, as shown The photodetector in receiver, this workithas four photosensitive single cell is assumed to be 1 pF. In the STUD-based LADAR receiver, it is necessary to increase the photosensitive area of thephotosensitive photodetectorcell in is order to enlarge theand ROIthe inparasitic the STUD-based LADAR The target size of a single 350 µm × 100 µm capacitance of the photosensitive area of the photodetector in order to enlarge the ROI in the STUD-based LADAR receiver. theSTUD-based noise of the LADAR receiver receiver, due to the single cellMeanwhile, is assumed this to beincreases 1 pF. In the it islarge-area necessary photodetector. to increase the receiver. Meanwhile, this increases the noise of the receiver due to the large-area photodetector. The proposed switched TCA caninbeorder used, shown Figure 4,STUD-based in the STUD-based photosensitive area of the4-to-1 photodetector to as enlarge theinROI in the LADARLADAR receiver. The proposed switched 4-to-1 TCA can be used, as shown in Figure 4, in the STUD-based LADAR receiver front-end. According to the position where the returned laser pulse arrives, one of the TCAs Meanwhile, this increases the noise of the receiver due to the large-area photodetector. The proposed receiver front-end. According to the position where the returned laser pulse arrives, one of the TCAs is switched on to receive theused, optical currentinand the4,others not connected to the photodetector. switched 4-to-1 TCA can be as shown Figure in theare STUD-based LADAR receiver front-end. is switched on to receive the optical current and the others are not connected to the photodetector. Therefore, thethe noise generated the unconnected affect the receiver. According to position where from the returned laser pulse photodetector arrives, one of cannot the TCAs is switched on to Therefore, the noise generated from the unconnected photodetector cannot affect the receiver. The switch control signal EN causes the switch to be turned on. Depending on the ROI, it is predicted receive the optical current and the others are not connected to the photodetector. Therefore, the noise The switch control signal EN causes the switch to be turned on. Depending on the ROI, it is predicted which photodetector will detectphotodetector the return signal, thatthethe EN signal is ablecontrol to turn on the generated from the unconnected cannot so affect receiver. The switch signal EN which photodetector will detect the return signal, so that the EN signal is able to turn on the corresponding switch. causes the switch to be turned on. Depending on the ROI, it is predicted which photodetector will corresponding switch. detect the return signal, so that the EN signal is able to turn on the corresponding switch.

Figure 3. Proposed switched 4-to-1 transimpedance combining amplifier (TCA) with four Figure switched 4-to-14-to-1 transimpedance combining amplifieramplifier (TCA) with four partitioned Figure 3.3.Proposed Proposed switched transimpedance combining (TCA) with four partitioned photodetectors. photodetectors. partitioned photodetectors.

Figure 4. 4. Operation Operation example example with with the the four four proposed proposed switched switched 4-to-1 4-to-1 TCAs TCAs and and four four multiple multiple Figure Figure 4. Operation example with the four proposed switched 4-to-1 TCAs and four multiple partitioned photodetectors. photodetectors. partitioned partitioned photodetectors.

The TCA amplifies and combines current signals generated using the photosensitive photosensitive cells cells from The TCA amplifies and combines current signals generated using the photosensitive cells from incoming incoming optical optical signals signals into into one one voltage voltage signal for further processing. The The switched switched 4-to-1 4-to-1 TCA TCA is incoming optical signals into one voltage signal for further processing. The switched 4-to-1 TCA is fabricated using aa standard standard complementary complementary metal-oxide-semiconductor metal-oxide-semiconductor (CMOS) (CMOS) 0.18 0.18 μm µm technology. technology. √ fabricated using a standard complementary metal-oxide-semiconductor (CMOS) 0.18 μm technology. It provides noise current spectral density with a bandwidth ofof 353 MHz and a provides 3.8 3.8 pA/√Hz pA/ Hzaverage average noise current spectral density with a bandwidth 353 MHz and It provides 3.8 pA/√Hz average noise current spectral density with a bandwidth of 353 MHz and a transimpedance gain of 67.5 dBΩ. The core of the TCA consumes 17.8 mW of power from a 1.8 V a transimpedance gain of 67.5 dBΩ. The core of the TCA consumes 17.8 mW of power from a 1.8 V transimpedance gain of 67.5 dBΩ. The core of the TCA consumes 17.8 mW of power from a 1.8 V supply. occupies an an active active area area of of about about 92 92 µm μm × × 68 supply. The core of the TCA occupies 68μm µm and and the the die die size size including including supply. The core of the TCA occupies an active area of about 92 μm × 68 μm and the die size including I/O I/Opads padsisis1000 1000μm µm××840 840μm. µm. I/O pads is 1000 μm × 840 μm.

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Sci. 2017, 7, 689 2. Appl. Architecture Description 2. Architecture Description

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2. Architecture Description The block block diagram diagram of the the proposed proposed switched switched 4-to-1 4-to-1 TCA TCA is is shown shown in in Figure Figure 5. 5. It It amplifies amplifies and and The of

combines the photocurrent from the four four partitioned cells into voltage combines photocurrent from the partitioned photosensitive cells into5.one one voltage signal. signal. Thethe block diagram of the proposed switched 4-to-1photosensitive TCA is shown in Figure It amplifies and The switched 4-to-1 TCA consists of four primary stages: (1) four over-current protection (OCP) The switched TCA consists of four partitioned primary stages: (1) four over-current (OCP) combines the4-to-1 photocurrent from the photosensitive cells into one protection voltage signal. circuits; (2) four switches, four current buffers; (3) a signal combiner; and (4) a post-amplifier. The OCP circuits; (2) four4-to-1 switches, current buffers; (3) astages: signal (1) combiner; and (4) a post-amplifier. The The switched TCAfour consists of four primary four over-current protection (OCP) circuits prevent fabricated chipchip from being by abyvery high signal. The switch is circuits; (2) prevent fourthe switches, four current buffers; (3)damaged a signal combiner; (4) ainput post-amplifier. The OCP OCP circuits the fabricated from being damaged a and very high input signal. The switch turned on when the reflected laser pulse arrives at the corresponding photodetector among the circuits prevent the fabricated chip from being damaged by a very high input signal. The switch is is turned on when the reflected laser pulse arrives at the corresponding photodetector among the multiple detectors. The current buffer is aa low low impedance input stage stage intended intended to receive receive the optical optical turned detectors. on when the laser pulse arrives at the corresponding photodetector among the multiple Thereflected current buffer is impedance input to the current from photosensitive cell. The combiner outputs of the buffers. multiple detectors. The currentcell. buffer a low impedance inputthe stage intended to the optical current from aa photosensitive Theissignal signal combiner sums sums the outputs of all allreceive the current current buffers. current from a photosensitive cell. The signal sums all transimpedance the current buffers. The post-amplifier is designed designed to to preserve thecombiner bandwidth and to enhanceofthe gain. The post-amplifier is preserve the bandwidth andthe to outputs enhance the transimpedance gain. The post-amplifier is designed to preserve the bandwidth and to enhance the transimpedance gain. A balun balun is is aa differential with differential input signals signals biased biased at A differential amplifier amplifier with differential input at the the same same direct direct current current (DC) (DC) A balun is a differential amplifier with differential input signals biased at the same direct current (DC) level to convert the single-ended output of the signal combiner into a differential signal. The output level to convert the single-ended output of the signal combiner into a differential signal. The output levelistoaconvert the single-ended output of the signal combiner into a differential signal. The output buffer differential amplifier with with resistor loads of 50 50 Ω on buffer is a differential amplifier resistor loads of Ω on both both the the positive positive and and negative negative outputs. outputs. buffer is a differential amplifier with resistor loads of 50 Ω on both the positive and negative outputs. The schematic schematic diagram diagram of of the the designed designed circuit circuit is is illustrated illustrated in in Figure Figure 6. 6. The The schematic diagram of the designed circuit is illustrated in Figure 6.

Figure 5.5.Block buffer. Block 4-to-1TCA TCAwith witha abalun balunand and output buffer. Figure Blockdiagram diagramof ofthe theproposed proposed switched switched 4-to-1 anan output buffer.

Figure6.6.Schematic Schematic diagram diagram of Figure of the the proposed proposedswitched switched4-to-1 4-to-1TCA. TCA. Figure 6. Schematic diagram of the proposed switched 4-to-1 TCA.

2.1. OCP and Input Switch 2.1. OCP and 2.1. OCP and Input Input Switch Switch The OCP circuit, as shown in Figure 6, is designed to protect the 4-to-1 TCA from being damaged The OCP circuit, as shown in Figure 6, is designed designed to to protect the the 4-to-1 4-to-1 TCA TCA from from being being damaged damaged OCP circuit, shown[16]. in Figure 6, is by The a very high inputascurrent The transistor M5 turnsprotect on when its source voltage is larger than by a very high input current [16]. The transistor M turns on when its source voltage is larger 5 by1.04 a very high input The M5 turns on when its400 source voltage larger than than V. As shown in current Figure 7,[16]. when thetransistor input current is approximately μA, the sourceisvoltage of 1.04 V. As shown in Figure 7, when the input current is approximately 400 µA, the source voltage of 1.04 As shown 7, when input is approximately 400 μA, theinput source voltage M5V. reaches 1.043in V.Figure When the inputthe current is current larger than 400 μA, the increase in the voltage is of M reaches 1.043 1.043 V. When theinput input current largerthan than400 400μA, µA,the theincrease increase the input voltage andV. the sink the current tocurrent the OCP increases. Therefore, the effective range of the is M55suppressed reaches When isiscircuit larger ininthe input voltage is suppressed and the sink current to the OCP circuit increases. Therefore, the effective range ofV,the input voltage fromsink the DC bias voltage of thecircuit input current buffer, approximately 610 mV range to 1.043of suppressed andis the current to the OCP increases. Therefore, the effective the input voltage is from the DC bias voltage of the input current buffer, approximately 610 mV to 1.043 V, before the OCP circuit turns on. input voltage is from the DC bias voltage of the input current buffer, approximately 610 mV to 1.043 V, before before the the OCP OCP circuit circuit turns turns on. on.

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To To expand expand the the ROI ROI with with multiple multiple partitioned partitioned photodetectors, a switch is added on each input current path. Several Several types of switches, such as n-channel n-channel metal-oxide-semiconductor metal-oxide-semiconductor field effect transistor transistor (NMOSFET (NMOSFET or NMOS), p-channel MOSFET (PMOSFET (PMOSFET or PMOS), and CMOS transmission gates, are available. In this study, NMOS NMOS switches switches M M66 are used on all input current paths, as shown in Figure 6. A high output will be degraded by the NMOS switch, since the NMOS switch turns off when where EN EN is is aa control control signal of the switch and when the the input input becomes becomesEN EN− − Vthth,,where and V Vth th is the threshold voltage of the switch transistor M . The maximum input value of the NMOS switch without signal the switch transistor M66 degradation about625 625mV, mV, and and the the available available maximum degradation is is approximately approximately1.175 1.175VVwhen whenthe theVVthth of M66 isisabout input photocurrent is 1.043 V. Therefore, an NMOS switch on theon input input voltage voltagedependent dependenton onthe theinput input photocurrent is 1.043 V. Therefore, an NMOS switch the path is capable of passing an input signal having a value from 610 mV to 1.043 V. A PMOS switch is input path is capable of passing an input signal having a value from 610 mV to 1.043 V. A PMOS not a viable the threshold is larger than mVthan and a610 lowmV input switch is notsolution, a viablesince solution, since thevoltage threshold voltage is 610 larger andsignal a lowcannot input be passed through a PMOS switch. signal cannot be passed through a PMOS switch.

Figure 7. Simulated input voltage and sink current to OCP (over-current protection) circuit according Figure 7. Simulated input voltage and sink current to OCP (over-current protection) circuit according to the input current. to the input current.

2.2. 4-to-1 TCA, Post-Amplifier, Balun, and Output Buffer 2.2. 4-to-1 TCA, Post-Amplifier, Balun, and Output Buffer Four copies of the regulated cascode (RGC) topology are selected as current buffers because of Four copies of the regulated cascode (RGC) topology are selected as current buffers because their low input impedance and wide bandwidth characteristics, as compared to other topologies such of their low input impedance and wide bandwidth characteristics, as compared to other topologies as the inverter, common-source, and common-gate topologies [17]. The RGC structure reduces the such as the inverter, common-source, and common-gate topologies [17]. The RGC structure reduces input impedance significantly by using the M2 and R2 stage as a local feedback to boost the the input impedance significantly by using the M2 and R2 stage as a local feedback to boost the transconductance of M1. The small-signal impedance of the RGC structure ( ) is given by (1): transconductance of M1 . The small-signal impedance of the RGC structure (Zin ) is given by (1): 1 (1) ≅ , 1 1 Zin ∼ , (1) = gm1 (1 + gm2 R2 ) andgm2 areare transconductances of1M 1 and 2, respectively. where gm1 and where thethe transconductances of M and M2M , respectively. The signals from the current buffers combine through two stages, as shown in Figure 6. In the first stage, two inputs are are summed summed through through the the output output load load resistor resistor RR11 of the RGC TIA. In the second stage, common-source amplifiers are used at the outputs of the two first-combining stages, and their currents are are summed summed through through aa single single resistive resistive load load R R33. There is a common-source currents common-source amplifier between the first and second combining combining stages stages that that functions functions as as aa buffer buffer and and aa bias bias shifter. shifter. To analyze analyzethe theeffect effectofofthe thenoise noiseintroduced introduced partitioned photosensitive cells TCA, To byby partitioned photosensitive cells andand the the TCA, the the simplified circuit is illustrated in Figure withfactors noise factors [15]. The equivalent total input simplified circuit is illustrated in Figure 8 with8noise [15]. The equivalent total input referred referred noise of the TCA is approximately noise of the TCA is approximately given by given (2): by (2): in,in 2

2 , ∙ ( ) , , , ≅ 4 2 ∙ 2in,C2 2 + in,R3 2 · R3 2 2 , ,  v i n,OUT , 2 ∼ , = 4 in,PD 2 + in,CB 2 + n,R1 · R1 + = ZT,TCA 2 ZT,TCA ,

(2) (2)

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where , is the noise from a single photosensitive cell, , is the generated noise in the current 2 where noise from a singlenoise photosensitive cell,combining in,CB 2 is thestage, generated in the current n,PD is the buffer istage, is the generated in the second and noise and are , , , 2 2 buffer stage,noise in,C2 from is theRgenerated noise in the In second combining and in,R1 and in,R3 2 are the 1 and R3, respectively. this analysis, westage, assume that: the thermal thermal noise from R1 and R3 , respectively. In this analysis, we assume that: ,

,

,

,

,

in,PD 2 = in,PD1 2 = in,PD2 2 = in,PD3 2 = in,PD4 2 ,

,

,

,

,

in,CB 2 = in,CB1 2 = in,CB2 2 = in,CB3 2 = in,CB4 2 , 2 , , 2 , , 2 vn,C1 = vn,C1,1 = vn,C1,2

2 2 2 in,C2 = in,C2,1 , , , = i n,C2,2 , , .

Figure 8. 8. Simplified Simplified circuit circuit for for noise noise analysis. analysis. Figure

In (2), the receiver noise with a large-area photodetector, even though it is partitioned, is increased In (2), the receiver noise with a large-area photodetector, even though it is partitioned, is increased in the developed TCA. The noise generated in the first combining stage with the current buffer is also in the developed TCA. The noise generated in the first combining stage with the current buffer is also the dominant factor of the equivalent total noise. the dominant factor of the equivalent total noise. The post-amplifier is realized using a two-stage common-source amplifier. The first stage has an The post-amplifier is realized using a two-stage common-source amplifier. The first stage active inductor load, consisting of a transistor M4 and a resistor R4, to increase the overall bandwidth [18]. has an active inductor load, consisting of a transistor M4 and a resistor R4 , to increase the overall The second stage controls the pulse polarity and the DC bias. bandwidth [18]. The second stage controls the pulse polarity and the DC bias. The balun converts the single-ended TCA output signal to differential signals. The balun and The balun converts the single-ended TCA output signal to differential signals. The balun and output buffer are illustrated in Figure 6. The balun is a differential amplifier with differential inputs output buffer are illustrated in Figure 6. The balun is a differential amplifier with differential inputs biased at the same DC level. In this study, the same DC voltage as compared with the output voltage biased at the same DC level. In this study, the same DC voltage as compared with the output voltage of of the TCA is applied through additional DC bias port VB. The output buffer is also a differential the TCA is applied through additional DC bias port VB. The output buffer is also a differential amplifier, amplifier, and it is designed to match the output impedances to 50 Ω on both the positive and and it is designed to match the output impedances to 50 Ω on both the positive and negative outputs. negative outputs. The full width at half maximum (FWHM) of the input pulse used in this work is about 2.2 ns The full width at half maximum (FWHM) of the input pulse used in this work is about 2.2 ns and its rise time is ~1 ns. The bandwidth required for the designed TCA to preserve its rise time is and its rise time is ~1 ns. The bandwidth required for the designed TCA to preserve its rise time is approximated by [19,20] as: approximated by [19,20] as: 0.35 BW ∼ , (3) = t0.35 r (3) ≅ , where tr is the rise time of the input pulse. For a rise time of 1 ns, (3) gives a bandwidth of approximately 350 MHz. The simulated transimpedance gain of the developed TCA obtained using this balun and where buffer is the rise time of the input pulse. For a rise of 1 ns, (3) 68 gives bandwidth of output is shown in Figure 9. The transimpedance gaintime is approximately dBΩa and the −3 dB approximately 350 MHz. The simulated transimpedance gain of the developed TCA obtained using this balun and output buffer is shown in Figure 9. The transimpedance gain is approximately 68 dBΩ

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and the −3 dB frequency is approximately 353 MHz with a photodetector parasitic capacitance of frequency is approximately MHz with a photodetector parasitic capacitance of 1 pF. The gain and dB bandwidth frequency353 is from approximately 353 stage MHz is with a photodetector parasitic 1 pF. and The the gain−3and each circuit summarized in Table 1. capacitance of bandwidth from each circuit stage is summarized in Table 1. 1 pF. The gain and bandwidth from each circuit stage is summarized in Table 1.

Figure 9. Simulated transimpedance gain of the proposed TCA obtained using this balun and

Figure 9. Simulated transimpedance gain of the proposed TCA obtained using this balun and Figure 9. Simulated transimpedance gain of the proposed TCA obtained using this balun and output buffer. output buffer. output buffer. Table 1. Summary of the circuit designed performances.

Table 1. Summary of the circuit designed performances. Table designed performances. Stage1. Summary of the circuitGain Bandwidth Direct Current (DC) Current buffers and first combining stages (R1) 70 dBΩ (3 kΩ) Stage Gain Stage Second combining stage 2 dB Current buffers and first combining stages (R 1) 70 dBΩ (3 Current buffers and first combining stages (R1 ) 70 dBΩ (3 kΩ) kΩ) Post-amplifier 6 dB Second combining stage 2 dB Second combining stage 2 dB Balun and output buffer −10 dB

Post-amplifier Post-amplifier Balun and outputbuffer buffer Balun and output 3. Measurement Results

6 dB −10 −10 dB dB

330 MHz

1.48 mA

Bandwidth Bandwidth Direct Direct Current Current (DC) (DC) 310 MHz 0.65 mA 330 MHz 1.48 mA 330 MHz 415 MHz 2.061.48 mA mA 310 MHz 310 MHz 0.65 mA 353 MHz 3.780.65 mA mA 415 MHz 2.06 415 MHz 2.06 mA mA 353 3.78 353MHz MHz 3.78 mA mA

The switched 4-to-1 TCA was implemented in a 0.18 μm CMOS technology. The core occupies

3. Measurement Results 3. Measurement Results an area of 92 μm × 68 μm, and the die size including I/O pads is 1000 μm × 840 μm. A microphotograph of the fabricated chipTCA with was bond wires on the test isμm shown intechnology. Figure 10. All the biases were The switched 4-to-1 TCA wasimplemented implemented a0.18 0.18µm CMOS technology. The core occupies The switched 4-to-1 ininafixture CMOS The core occupies an applied through bond wires, and short pulse response measurement was performed with a coaxial an area of 92 μm × 68 μm, and the die size including I/O pads is 1000 μm × 840 μm. A microphotograph area of 92 µm × 68 µm, and the die size including I/O pads is 1000 µm × 840 µm. A microphotograph of the themicro-receptacle fabricated chip chip(CMJ) withconnector. bond wires wires on on the the test test fixture fixture is is shown shown in in Figure Figure 10. 10. All All the the biases biases were were of fabricated with bond applied through bond wires, and short pulse response measurement was performed with a coaxial applied through bond wires, and short pulse response measurement was performed with a coaxial micro-receptacle (CMJ) (CMJ) connector. connector. micro-receptacle

Figure 10. Microphotograph of the fabricated chip with bond wires on the test fixture.

The fabricated chip was mounted on a wire-bonded chip-on-board (COB) module to measure the electrical pulse response, as shown in Figure 11a. A 10-kΩ resistor acts as a voltage-to-current converter. To measure the pulse transient the fabricated circuit, an electrical pulse signal, Figure 10. Microphotograph of the response fabricatedofchip fixture. with bond wires on the test fixture. generated from an Agilent 81110A pattern generator (Keysight, Santa Clara, CA, USA), was applied

The fabricated fabricated chip chip was was mounted mounted on on aa wire-bonded wire-bonded chip-on-board chip-on-board (COB) (COB) module module to to measure measure The the electrical electrical pulse pulse response, response, as as shown shown in in Figure Figure 11a. 11a. A A 10-kΩ 10-kΩ resistor resistor acts acts as as aa voltage-to-current voltage-to-current the converter. To measure the pulse transient response of the fabricated circuit, an electrical converter. To measure the pulse transient response of the fabricated circuit, an electrical pulse pulse signal, signal, generated from an Agilent 81110A pattern generator (Keysight, Santa Clara, CA, USA), was applied

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generated from an Agilent 81110A pattern generator (Keysight, Santa Clara, CA, USA), was applied to Appl. Sci. 2017, 7, 689 of the implemented test fixture, as shown in Figure 11b. The OUT+ and 8 of 11 − each channel OUT toinput each input channel of the implemented test fixture, as shown in Figure 11b. The OUT+ and OUT− signals were measured using signals were measured usingananAgilent AgilentDSO7104B DSO7104B oscilloscope. oscilloscope. to each input channel of the implemented test fixture, as shown in Figure 11b. The OUT+ and OUT− signals were measured using an Agilent DSO7104B oscilloscope.

(a)

(b)

Figure 11. (a) TCA-chip (b) measurement (a) on board (COB) schematic for electrical pulse response; (b)

Figure 11. (a) TCA-chip on board (COB) schematic for electrical pulse response; (b) measurement setup setup for electrical pulse response. 11.pulse (a) TCA-chip on board (COB) schematic for electrical pulse response; (b) measurement for Figure electrical response. setup for electrical pulse response.

Figure 12 shows the measured transient pulse response for the fabricated chip. The pulse Figure 12 shows measured response for the fabricated pulse magnitude magnitude of the the input voltage transient from the pulse pattern generator is adjusted sochip. that The the input current Figure 12 shows the measured transient pulse response for the fabricated chip. The pulse of the input voltage fromthe theFWHM patternofgenerator adjusted sons that theainput becomes becomes 20 μA and the inputissignal is 2.2 with 1.8 nscurrent rise time, which20isµA theand magnitude of the input voltage from the pattern generator is adjusted so that the input current minimum rise time ofsignal the Agilent 81110A pattern generator. The input current is applied to each the becomes FWHM of the input is 2.2 ns with a 1.8 ns rise time, which is the minimum rise time of the 20 μA and the FWHM of the input signal is 2.2 ns with a 1.8 ns rise time, which is the successive input channel from IN1 through IN4, and 200 segment pulse responses of OUT+ and OUT− Agilent 81110A The input pattern currentgenerator. is appliedThe to each channel from minimum risepattern time ofgenerator. the Agilent 81110A inputsuccessive current isinput applied to each are measured. The blue lines represent the pulse response of 200 segments, andmeasured. the red lineThe represents IN1successive through IN4, and 200 segment pulse responses of OUT+ and OUT − are blue input channel from IN1 through IN4, and 200 segment pulse responses of OUT+ and OUT−lines the average pulse response value. The average peak-voltage of the measured output pulses is represent the pulse 200 segments, and the redofline thethe average pulse response are measured. Theresponse blue linesofrepresent the pulse response 200 represents segments, and red line represents approximately 47.4 mV, and hence, the transimpedance gain is calculated to be 67.5 dBΩ using by (4). value. average peak-voltage of theThe measured pulses is 47.4 mV, and hence, the The average pulse response value. averageoutput peak-voltage of approximately the measured output pulses is This value is very near the simulated results of 68 dBΩ. 47.4 mV,isand hence, the transimpedance gain calculated to beis67.5 using by (4). the approximately transimpedance gain calculated to be 67.5 dBΩ using byis(4). This value verydBΩ near the simulated 47.4 mV Thisof value is very near the simulated results of 68 dBΩ. results 68 dBΩ. (4)  20 log  47.4 20 mV μA ZT = 20 × log10 47.4 mV (4)(4) 20 log 20 µA 20 μA

Figure 12. Measured transient pulse response for the fabricated chip. Figure 12.Measured Measuredtransient transient pulse pulse response chip. Figure 12. responsefor forthe thefabricated fabricated chip.

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The integrated single-ended output noise of the switched 4-to-1 TCA was measured via the oscilloscope root-mean-square (RMS) calculation function with no input signal source, as shown in Figure [21]. The standard deviation of the output was measured to be 0.524 mV. After subtracting Appl. Sci.13 2017, 7, 689 9 of 11 the inherent oscilloscope noise of 0.4 mVrms , the corrected single-ended integrated output noise of the TCA The was estimated be 0.338 mVrms . The noise integrated input-referred noiseTCA of the differential output of integratedtosingle-ended output of the switched 4-to-1 was measured via the the switched root-mean-square 4-to-1 TCA for each inputcalculation channel can be calculated as input in Reference [22]. as shown in oscilloscope (RMS) function with no signal source, Figure 13 [21]. The standard deviation of the output was measured to be 0.524 mV. After subtracting q 2 2 the inherent oscilloscope noise of rms, mV the )corrected single-ended integrated output noise of the − (0.4 mV 2 mV (0.524 ) 1 0.4 In,in ≈ mV · rms. The integrated input-referred = 0.07noise µArmsof the differential output (5) TCA was estimated to be 0.338 4 67.5 dBΩ of the switched 4-to-1 TCA for each input channel can be calculated as in Reference [22].

Figure 13. Integrated single-ended output noise of the switched 4-to-1 TCA. Figure 13. Integrated single-ended output noise of the switched 4-to-1 TCA.

1 2 current 0.524 density mV 0.4 mV The average input-referred noise is: ∙ 0.07 μA , 4 67.5 dBΩ √ In,in In,in,avg ≈ √ = 3.8 pA/ Hz BW The average input-referred noise current density is:

(5) (6)

The switched 4-to-1 TCA used a supply voltage of 1.8 V and dissipated 17.8 mW of power. , (6) 3.8 pA/√Hz , , The performances of the TCA are summarized in √ Table 2. Table 2. Summary of the switched 4-to-1 TCA performances.

The switched 4-to-1 TCA used a supply voltage of 1.8 V and dissipated 17.8 mW of power. The performances of the TCA are summarized in Table 2. Parameter Performance Combining Channel 4 Table Summary of the switched 4-to-1 TCA performances. CPD /cell 2. (pF) 1 Effective total CPD (pF) 4 Parameter Performance Transimpedance gain (dBΩ) 67.5 Combining Channel 4 Bandwidth (MHz) 353 (simulated) √ Input-referred noise current density/cell (pA/ Hz) 3.8 CPD/cell (pF) 1 Power consumption (mW) 17.8 PD (pF) 4 Effective total C 1.00 × 0.84 Chip size (mm2 ) Transimpedance gain (dBΩ) 67.5 Complementary metal-oxide-semiconductor Technology(MHz) Bandwidth 353 (simulated) (CMOS) 0.18 µm

Input-referred noise current density/cell (pA/√Hz) Power consumption (mW) Chip size (mm2) Technology

3.8 17.8 1.00 × 0.84 Complementary metal-oxide-semiconductor (CMOS) 0.18 μm

4. Conclusions A compact switched 4-to-1 TCA was implemented using 0.18 μm CMOS technology and was used as a receiver front-end ROIC for a STUD-based LADAR system. A switch was inserted on the

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4. Conclusions A compact switched 4-to-1 TCA was implemented using 0.18 µm CMOS technology and was used as a receiver front-end ROIC for a STUD-based LADAR system. A switch was inserted on the input path of the TCA to expand the effective photosensitive area without increasing the noise from the large-area photodetector of the STUD-based LADAR system. The space between the partitioned photosensitive cells and its cascading current buffers was made smaller by about several hundred micrometers by integrating several TIAs and a signal combiner onto a single chip. The fabricated chip had a power consumption of 17.8 mW for a 1.8 V supplied voltage, an average input-referred √ noise current spectral density of 3.8 pA/ Hz, and a transimpedance gain of 67.5 dBΩ. The chip was operated based on the same working principle as the STUD-based LADAR receiver. Therefore, the compact switched 4-to-1 TCA is suitable for the front-end ROIC of the STUD-based LADAR system as one integrated chip. Acknowledgments: This research was supported in part by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2015R1C1A1A01052508), and in part by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (MSIP) (NRF-2016R1A2B4014834). Author Contributions: Eun-Gyu Lee made the study design and performed the experiment, as well as the manuscript writing; Jae-Eun Lee and Bang Chul Jung contributed to the data analysis and results discussion; Bongki Mheen suggested the idea and organized the paperwork. Choul-Young Kim participated in research plan development and revised the manuscript. All authors have contributed to the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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