# Research on Fast Detection Technology of Dark Currents in a Ge-Si Detector Array
## 2 Principle and System
### Scanning Detection Method and System  
Figure 3 is the principal block diagram of the Ge-Si-detector-array dark-current fast detection system, which consists of a controllable voltage source, a Ge-Si detector array and its scanning circuit, an electrometer-level amplifier, a programmable amplifier, an MCU (micro-controller), and an LCD (liquid crystal display). During the working process, the MCU controls the bipolar DAC (digital-to-analog converter) output voltage in  
Figure 2: PIN photodetector circuit model and dark current curve. (**a**) PIN-equivalent circuit model under no illumination. (**b**) Bias voltage–dark current curve. the controllable voltage source, which is amplified by a voltage follower to generate a controllable bias voltage. The controllable bias voltage is loaded onto the optical relay. The MCU controls the 3-8 decoder for row selection, selecting one row of pixels to make one row of pixels conductive. The MCU also controls the 3-8 decoder for column selection, selecting one column of pixels to make one column of pixels conductive. Finally, the bias voltage is applied to a single detector. The dark current signal generated by the pixel generates a voltage signal through the electrometer amplifier. The generated voltage signal is amplified by the programmable amplifier. The amplified voltage signal is input to the ADC (analog-to-digital converter) module in the MCU and converted into a digital signal displayed on the LCD screen. This system can achieve rapid scanning and dark current detection of large-scale Ge-Si detector arrays. The controllable voltage source consists of a bipolar DAC and a voltage follower, with a bipolar DAC power supply voltage of \(\pm\)5 V. Therefore, the bias voltage range of the detector can reach \(-\)5 V to +5 V. We continuously read the dark current under reverse-bias voltage and positive-bias voltage. The actual voltage scanning range was \(-\)3 V\(\rightarrow\)1 V. As shown in Figure 4, the bipolar DAC chip used ADI (Analog Devices Inc.)'s LTC2664-12. The DAC chip had four output channels, one of which was used for actual measurement. If the DAC is 12 bits, the voltage resolution is:  
\[\Delta V=\frac{(+5\text{V})-(-5\text{V})}{2^{12}}\approx 2.44\text{ mV} \tag{5}\]  
The voltage follower was implemented using Texas Instrument's high-output current amplifier LM7332, which can output a maximum current of up to 70 mA. The dark current amplification circuit is shown in Figure 5, consisting of a cross-impedance amplifier (TIA), a programmable amplifier, and a general-purpose amplifier. The TIA was realized by an electrometer amplifier ADA4530-1 of ADI. This chip is an operational amplifier with an input bias current on a flying ampere level, which is suitable for measuring weak currents. The impedance connected in Figure 5 was 1 M\(\Omega\), so the output voltage of the TIA is:  
\[V_{1}=-R_{1}I_{dark} \tag{6}\]  
Figure 3: Schematic diagram of the composition of the detection system. Due to the extensive dark current variations in PD detectors, especially in the case of reverse bias and forward bias, there is a greater difference in the current. Therefore, a programmable amplifier was used to amplify \(V_{1}\) to ensure a variable amplification factor. The programmable amplifier consisted of ADI's instrument amplifier AD8220 and a single-pole 8-position analog switch. The amplification factor of AD8220 is:  
\[G=1+\frac{49.4\ k\Omega}{R_{G}} \tag{7}\]  
The eight resistors connected to the single-pole 8-position analog switch were 1 M\(\Omega\), 12 K\(\Omega\), 5.1 K\(\Omega\), 1 K\(\Omega\), 510 \(\Omega\), 240 \(\Omega\), 100 \(\Omega\), and 51 \(\Omega\). Based on the resistors \(R_{x1}\)\(\sim\)\(R_{x8}\), the corresponding amplification factors can be calculated as 1.05, 5.22, 9.36, 50.40, 97.86, 206.83, 495.00, and 969.63. During testing, adjusting the amplification factor according to the size of the measured value can improve the measurement accuracy, with A, B, and C being the address inputs of the analog switch controlled by the MCU.