Design of weak light detection circuit based on logarithmic amplifier LOGl00

Weak light detection usually converts the optical signal into an electrical signal through a photoelectric device, and then amplifies it through a preamplifier circuit. The A/D conversion circuit converts the analog signal into a digital signal for analysis and processing. Weak light detection technology is widely used in modern communications, medical treatment, scientific research and other fields. An important performance indicator of the weak light detection circuit is the ability to filter out noise, but in weak light detection, the optical signal and the noise are almost at the same order of magnitude, and the signal is easily submerged in the noise, which is not conducive to subsequent circuit processing. The traditional method is to use circuit cascade to filter out interference and amplify the signal; but this circuit requires precision resistors, and the design is complex, the circuit volume is large, and the reliability is poor. With the development of integrated logarithmic amplifier circuits, with its remarkable characteristics of wide dynamic range and high-precision output, the optical detection circuit has also been continuously developed and improved. The logarithmic circuit has excellent data compression performance and can compress a wide input dynamic range signal into a very narrow voltage range. Therefore, a weak light detection circuit design scheme using LOGl00 as a preamplifier is proposed here.

1 Circuit Design and Analysis

1.1 Photoelectric conversion circuit

Figure 1 is a photoelectric detection circuit. The detection circuit is composed of an amplifier A, feedback resistors RF and CF, and its output voltage is u1=SPRF, where S is the sensitivity of the photodiode and P is the incident light power. When detecting weak light signals, RF is used to increase the gain, and the value of RF should be selected as large as possible. The input bias current IB and input offset voltage VB of the amplifier have an impact on the output voltage of IBRF and respectively , and Rs is the internal resistance of the photodiode. It can be seen that reducing RF can reduce the above effects, but at the same time it will reduce the gain of the circuit. To solve this problem, an operational amplifier with very low bias current and offset voltage should be selected. Here, a 0PAlll type high-precision operational amplifier is selected, with a bias current of about 0.8 pA and an input offset voltage of about 100 μV. After calculation, when the value of RF is within the range of several hundred MΩ, the above effects can be approximately ignored, which can meet the requirements of the circuit.

1.2 Preamplifier circuit

Since the output signal of the photoelectric conversion circuit is usually in the mV order of magnitude, and the signal is often submerged in noise, the preamplifier part needs to have strong noise filtering and amplification capabilities. The precision logarithmic amplifier circuit LOG100 and peripheral components are selected to form the preamplifier circuit. The circuit shown in the dotted box in Figure 1 is the simplified internal circuit of LOG100, with a dynamic input range of 1 nA to 1 mA, a full span output error (FSO) of less than 0.37%, and a maximum deviation from the precise logarithmic relationship of less than 0.1%. At the same time, laser calibration resistors are also integrated inside, so that the logarithmic amplifier can still maintain accurate output when the ambient temperature changes. LOG100 has 4 selection terminals, and different gains can be easily obtained through different connection methods. See the literature for details.

From the literature, we know that the input-output relationship of LOG100 is:

2 Results Analysis

In the weak light test experiment, the photodiode uses the S1227-66B PIN silicon photodiode, which has high sensitivity and low dark current. In order to reduce interference, the circuit is encapsulated in a metal box during the experiment, powered by a ±lO V DC regulated power supply, and shielded cables are used for both the power line and the signal line. The light source is a periodic light pulse signal input by an ordinary red light-emitting diode controlled by an oscillator circuit composed of an oscillator 555, with a period of T=105 ms. A digital oscilloscope is used to observe and record the output of the photoelectric conversion circuit and the output signal of the preamplifier circuit.

According to the input-output relationship of LOG100, I2 is used as the reference current in the experiment. According to the reverse input structure of the op amp, it can be achieved in the circuit by giving a reference voltage μ2. The output voltage of the photoelectric detection circuit is generally only in the mV order. At the same time, according to the requirements of the LOG100 device, its input current should be in the range of l nA to l mA, and the ratio of I1 to I2 should be within l05. Therefore, the input resistors R11 and R21 in Figure 1 are selected to be tens of kΩ to ensure the output accuracy of the logarithmic circuit. According to these requirements, the μ2 value is set to a few mV.

Because the gain of the photoelectric conversion circuit is very high, although a precision amplifier circuit is used and RF and CF are used to limit the signal frequency band, the output noise is very high for almost all input optical signals. Figure 2 (a) is the output waveform of the photoelectric conversion circuit when P = 0.7nW. It can be seen that the noise and signal are at the same order of magnitude, and the noise-to-signal peak ratio is close to 1. If such an output is directly used for A/D conversion, the accuracy of the data will be greatly reduced. Although the noise filtering subroutine in the microcontroller program can be used to reduce the probability of data error, the software simulation function has certain limitations and may not be able to obtain accurate data output. Figure 2 (b) is the output signal processed by the preamplifier circuit. Its waveform is relatively smooth, and the noise-to-signal peak ratio is reduced to below 0.02. The A/D converted data can be directly processed later without software noise filtering. It can be seen that LOGl00, as a preamplifier circuit, can effectively suppress noise while amplifying useful signals. The gap between its specific measurement data and theoretical calculation results is small, which can fully meet the design requirements. Figure 3 shows that the difference between the theoretical value (dashed line) and the measured value (solid line) is less than 0.1 V when the input optical power is 1.4 nW.

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Figure 4 shows the output VOUT waveform of the preamplifier circuit under different incident light powers P. It can be seen from Figure 4 that when the intensity of the input light signal changes slightly by a few hundred pW, the output signal amplitude of LOGl00 changes significantly by a few hundred mV, which enables the maximum collection of data of different light source intensities during A/D conversion. However, LOGl00 also has its shortcomings. As can be seen from Figure 4, as the input light intensity decreases, the noise in the output signal gradually increases. When the noise increases to a certain extent compared with the signal, the A/D conversion circuit may output errors. At this time, software must be used for noise filtering. Therefore, LOGl00 is generally used in the circuit for detecting weak light signals. In the detection of weak light signals, the output signal-to-noise ratio of LOGl00 is relatively small, and a precision filtering circuit is required to assist. This not only increases the complexity of the circuit, but also greatly reduces the accuracy of its output data, making it impossible to carry out practical applications.

To solve this problem, consider using LOGl01 and LOGl04 with higher precision as preamplifier circuits. Compared with LOGl00, they have a wider dynamic input range of 100 pA to 3.5 mA and an accuracy of up to 0.01% FSO. LOGl01 and LOGl04 use constant gain and are not as flexible as LOGl00 in circuits. There is no integrated laser calibration resistor inside, but the DC offset voltage is low and can accurately output in a wide temperature range (-5 to 75°C), so they are more suitable for use in weak light signal detection circuits.

3 Conclusion

The noise filtering performance of LOGl00 in weak light detection applications is discussed. The measured results show that LOGl00 has strong noise suppression ability and can be used as a preamplifier circuit in weak light detection. However, when the input signal gradually weakens, the noise suppression ability is also weak, and it is not suitable for use in weak light signal detection.