在光谱学中，探测器光柱的正确亮度测量是描述材料的基础。

　　光学传感器有重要应用，跨越的领域从消费电子，例如环境光测量和照相机曝光控制，到科学仪器，例如光吸收谱，IR（红外线）温度探测和双色测高温。例如，在光谱学中，探测器光柱的正确亮度测量是材料和设备描述的基础。必须消除任何直流或低频背景光引起的影响。为增加SNR（信噪比），可以使用窄带、相敏或锁存探测技术来断路或其他已调制探测器光源。

　　在本设计方案中，光断路的参考信号以方波频率f_CHOP，调制基于运放反向放大器的增益（图1）。放大器输入为电压信号，与光敏二极管产生的光电流信号成比例。调制光柱以断路器同样的频率照射光敏二极管。像这样，因为增益和输入是同样频率，低通放大器容易探测到的直流器件，呈现放大器输出。

点击看原图

　　运算放大器A_1A和A_1B将光产生的电流转换为电压，包括仅有的交流器件。根据所测的光量级改变R₁的值。忽略A^_1A的输入电容，C₁的值主要根据输入光敏二极管的终端电容，选择的值必须确保跨导倒数电路的稳定性（参考文献1）。

　　系统核心放大器A_1C，包括光敏电阻R_PR，代表决定部分增益的反馈元件。R_{P R}值依靠D₁发出的光。电压电流转换器A_2B驱动D₁。转换器通过A_2A和A₃具有固定电压V_B和ΔV信号。A_2A决定R_PR的直流值，然而A_2B和ΔR_PR以参考信号相同的频率改变。A₃ Schmitt触发器将参考信号的所有TTL/CMOS电平转换成平稳的±4.6V方波，削弱到±0.5V产生约1.8mA p-p的LED电流变化。对光敏电阻R_PR和LED元件，Silonex CdS（硫化镉）NSL-19M51单元与红色LED和黑盒子内的其它元件结合，确保光耦中缺少背景光。

　　为校准电路，首先断开或使输入光敏二极管变暗，以便A_1A转换非交流信号。然后，转换S₁到“测量”位置，并调整R_T2到涉及输出电压的所有无效的电压偏置。当A_1B缓冲器产生已知的约300mV测试电压和S1处于校准位置时，调整R_T1到0V固定输出电压。这种情况下，VB电压能设置为R_PR/R_C=R_A/R_B的条件。

　　英文原文：

　　Transimpedance synchronous amplification nulls out background illumination

　　In optical spectroscopy a correct intensity measurement of the probe beam is fundamental during material characterization.

　　Stefano Salvatori and Gennaro Conte, University Roma Tre, Rome, Italy; Edited by Charles H Small and Fran Granville -- EDN, 12/3/2007

　　Light sensors find use in a host of important applications, spanning from consumer electronics, such as ambient-light measurements and exposure control for cameras, to scientific instruments, such as optical-absorption spectroscopy, IR (infrared) detection for thermography, and two-color pyrometry. For example, in optical spectroscopy, a correct intensity measurement of the probe beam is fundamental during material and device characterization. You must eliminate any influence that dc or very-low-frequency background light induces. Also, to increase the SNR (signal-to-noise ratio), you can apply narrowband, phase-sensitive, or lock-in detection techniques to mechanically chopped or otherwise modulated probe-light sources.

In this Design Idea, thereference signal from the light chopper as a square wave of frequency, fCHOP, modulates the gain of an op-amp-based inverting amplifier (Figure 1). The amplifier input is a voltage proportional to the photocurrent signal produced by a photodiode, which is irradiated by a modulated light beam at the same chopper frequency. In this case, because the gain and input are at the same frequency content, a dc component, which a lowpass filter can easily detect, is present at the amplifier’s output.

　 　Op amps A1A and A1B convert the photogenerated current into a voltage including only the ac components. You can change the value of R1 depending on the light level you want to detect. Neglecting A1A’s input capacitance, the value of C1 strongly depends on the terminal capacitance of the input photodiode, and you must select the value to ensure the stability of the transimpedance circuit (Reference 1).

　　The heart of the system, op amp A1C, includes photoresistor RPR, which represents the feedback element that determines the gain of the stage. The value of RPR depends on the light that D1 emits. A2B, a voltage-to-current converter, drives D1. The converter has a fixed voltage, VB, and a ΔV signal through A2A and A3. A2A determines the dc value of RPR, whereas A2B and ΔRPR change at the same frequency as the reference signal. The A3 Schmitt trigger converts any TTL/CMOS level of the reference signal into a balanced ±4.6V square wave attenuated to ±0.5V to generate an LED current change of approximately 1.8 mA p-p. For the photoresistor, RPR, and LED elements, a Silonex CdS (cadmium-sulfide) NSL-19M51 cell couples to a red LED and resides in a black box to ensure the absence of background light on the optocoupler.

　　To calibrate the circuit, first disconnect or obscure the input photodiode so that A1A converts no ac signal. Then, switch S1 to the “measure” position and adjust RT2 to null any voltage offset referred to the output voltage. When the A1B buffer generates the known approximately 300-mV test voltage and S1 is in the calibrate position, adjust RT1 to fix the output voltage at 0V. In such a case, VB voltage can set the RPR/RC=RA/RB condition.

　　Reference

　　Wang, Tony, and Barry Erhman, “Compensate Transimpedance Amplifiers Intuitively,” Application Report SBOA055A, Texas Instruments, 1993.

　　英文原文地址：http://www.edn.com/article/CA6505568.html

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