## 4 Measurement Results  
The prototype STUD-based LADAR receiver with a 16-to-1 TICA was fabricated in a 0.18 \(\upmu\)m CMOS process. The test board photographs with the prototype chip are shown in Figure 9. The prototype STUD-based LADAR receiver was implemented in a chip size of 1 mm \(\times\) 2.5 mm with peripheral circuitry and I/O pads as shown in Figure 9a. The prototype InGaAs APD was adopted in this design and had a size of 500 \(\upmu\)m \(\times\) 2240 \(\upmu\)m with \(C_{PDT}\) of 32 pF. The photosensitive area of the APD was partitioned into 16 cells, and the size of each cell was 500 \(\upmu\)m \(\times\) 140 \(\upmu\)m with \(C_{PDS}\) of 2 pF. The two types of measurements, electrical pulse response without the APD (Figure 9a) and the optical pulse response with the wire-bonded APD (Figure 9b), were applied in order to verify the prototype chip. Figure 8: Schematic of the balun and output buffer. In the electrical pulse response test, an electrical current pulse of width 3 ns and a rising and falling time less than 1.5 ns were concurrently induced in all inputs with the same amplitude at a repetition rate of 10 kHz. The prototype chip was mounted on a wire-bonded chip-on-board (COB) module as shown in Figure 10a. A resistor 10 k\(\Omega\) of input node acts as a voltage-to-current converter. To verify the transient response of the prototype receiver, an electrical pulse signal is generated by a pattern generator (Agilent 81110A), and it is applied to each input channel of the implemented test fixture as shown in Figure 10b. The OUT+ and OUT- signals were measured using a Rohde & Schwarz RTO2024 oscilloscope. Figure 11 shows the dependence of the 16-to-1 TICA output voltage amplitude as the input current sweep. The measured output voltage swing is linear at the input current sweeps from 3 uA to 73 uA for OUT+, and its maximum output peak voltage is approximately 240 mV. This implies that the transimpedance gain of TICA is approximately 70.4 dB-\(\Omega\) and it matches well with the targeted transimpedance gain. Figure 10: (**a**) Prototype receiver-COB schematic and (**b**) measurement setup for electrical pulse response test. Figure 9: Test board photographs with the prototype: (**a**) optical response with wire-bonded APD and (**b**) electrical response test board without APD. The measured output rms noise of the oscilloscope and the prototype chip are shown in Figure 12a,b, respectively. The output rms noise of the receiver was measured in the electrical pulse response test environment. The output rms noise of a 16-to-1 TICA was measured using the RMS calculation function of the oscilloscope with no input signal source [17], and the standard deviation of the output of the TICA was measured to be 1.793 mV. After subtracting the inherent oscilloscope noise of 353 \(\upmu\)mV\({}_{\text{rms}}\), the output noise of the TICA could be estimated as 1.759 mV\({}_{\text{rms}}\). The input-referred noise for each input was calculated as approximately 0.64 \(\upmu\)mA\({}_{\text{rms}}\)[18]. When considering the TIA bandwidth of approximately 230 MHz, the input-referred noise current is calculated as approximately 41.9 pA/\(\upmu\)Hz. In the optical response test, the measurement was carried out using an optical laser pulse of wavelength 1550 nm, FWHM 3.8 ns, rising and falling time 1.5 ns, and a repetition rate of 120 kHz through an attenuator and collimator. Figure 13 shows the linear dependency of the TICA output (\(V_{TICA}\)) on the optical laser power sweep. As it is difficult to control the optical laser power of the optical laser pulse generator, the optical laser power was increased by the attenuation level control of the attenuator in this work. As the attenuation level increased from 23 dB to 38 dB, the output peak voltage of TICA decreased from 240 mV to 5 mV. Figure 11: Dependence of the 16-to-1 TICA output voltage amplitude as the input current sweep. Figure 12: Measured output rms noise of (**a**) prototype chip and (**b**) oscilloscope.