## 2 Circuit Description
### Tester Circuits: A2V and T2V Converters  
In order to test the feasibility of the proposed OTA as the AFE circuit in short-range LiDAR sensors, the combination of analog-to-voltage (A2V) and time-to-voltage (T2V) converters is added to the OTA subsequently. These two converters can replace a complicated time-to-digital converter (TDC) circuit. The A2V converter can handle input currents up to 200 \(\mu\)A\({}_{\text{PP}}\), making it suitable for detecting signals reflected from targets located up to 10.2 m away, while the T2V converter takes on the TDC's role in dealing with larger currents by converting the time interval between the emitted pulse and the reflected pulse into an output voltage. Then, the T2V converter can effectively accommodate a wider range of signal strengths, hence extending the minimum detection range to close to 30 cm. Figure 5a shows a block diagram of the tester circuit, where the A2V converter consists of a post-amplifier (PA) and a peak detect and hold (PDH) circuit. The voltage signals converted by the m-TIA are further amplified by the PA, and then the amplified signals enter the PDH circuit that can detect and maintain the peak voltages. The operation of the PDH circuit is divided into two phases, i.e., a write mode and a read mode. In the 'write-mode', the output voltage from the PA exceeds the reference voltage (A2V_OUT) and transistor M\({}_{7}\) is turned on, thereby allowing the capacitor \(C\) to charge up to the voltage of PA\({}_{\text{OUT}}\). As the PA\({}_{\text{OUT}}\) voltage begins to decrease from its peak, M\({}_{7}\) is turned off and, therefore, the capacitor \(C\) retains the value of the peak voltage, thus transitioning the circuit into the'read-mode'. In this mode, the stored peak voltage can be read and utilized for further processing. The resistor R\({}_{2}\) is introduced to enhance the stability of the PDH circuit, which creates a zero in the loop gain. This dual-mode operation ensures accurate peak voltage detection and retention, which is crucial for reliable signal processing in optical communication systems, such as LiDAR sensors. The T2V converter enhances the input dynamic range of the whole LiDAR receiver by converting the time-of-flight (ToF) information into their corresponding output voltages. It is particularly effective for measuring the reflection of light pulses from objects at varying distances. Figure 6a shows a block diagram of the T2V converter, which consists of a latch,  
Figure 4: (**a**) Cross-sectional view of P\({}^{+}\)/NW/DNW APD and (**b**) its layout and measured results. a control block, a charging circuit, and a PDH circuit. In this T2V converter, the output signals from the PA are directly connected to the input latch comparator that converts the signals into digital pulses. Figure 6b presents a schematic diagram of the latch comparator. Specifically, the latch generates an output of '1' if a pulse above the reference voltage is detected, and an output of '0' otherwise. This latch output is fed into the control block along with the emitted signal (a.k.a. START) from the transmitter. This control block maintains an output of '0' between the START and the receiving (a.k.a. STOP) of rising edges, when this output (\(\text{Ctrl}_{\text{OUT}}\)) drives the charging circuit to generate a triangular waveform. Upon detecting a latch output pulse (i.e., the rising edge of the STOP pulse), the control block switches the output to '1'. The duration between the START and STOP of the rising edges determines the rising time of the charging circuit, hence directly correlating it with the time-of-flight (ToF) information. Once the output from the control block returns to '1', the charging circuit halts the voltage increment, thereby preserving the peak voltage. The charging circuit consists of an operational amplifier, a capacitor for charging through a load, and an NMOS transistor for reset functionality. Then, the output of this charging circuit is maintained by the PDH until the next reset signal arrives.