High-speed photodiode signal conditioning electronic circuit diagram

The circuit shown in Figure 1 is a high-speed photodiode signal conditioning circuit with dark current compensation function. The system converts current from a high-speed silicon PIN photodiode and drives the input of a 20 MSPS analog-to-digital converter (ADC). The device combination provides spectral sensitivity from 400 nm to 1050 nm and photocurrent sensitivity of 49 nA, a dynamic range of 91 dB, and a bandwidth of 2 MHz. The signal conditioning circuit is powered by a ±5 V power supply and consumes only 40 mA, making it suitable for portable high-speed, high-resolution light intensity applications such as pulse oximeters.

Figure 1. Photodiode preamplifier system with dark current compensation (schematic diagram: all connections and decoupling not shown)

This circuit is also suitable for other applications such as analog opto-isolators. It can also accommodate applications requiring higher bandwidth and lower resolution, such as adaptive speed control systems.

This circuit note discusses the steps to optimize the design of the circuit shown in Figure 1 to meet the requirements of a specific bandwidth application. These steps include: stability calculations, noise analysis, and device selection considerations.

Photodiodes are high-impedance sensors used to detect the intensity of light. It has no internal gain but can operate at higher light levels than other photodetectors.

Photodiodes operate in either zero-bias (photovoltaic) mode or reverse-bias (photoconductive) mode. Photovoltaic mode provides the most accurate linear operation, while operating the diode in photoconductive mode allows for higher switching speeds at the cost of reduced linearity. Under reverse bias conditions, there are small amounts of current (called dark current) that flow even in the absence of illumination. Dark current errors can be eliminated by using a second photodiode of the same type at the non-inverting input of the op amp, as shown in Figure 1.

There are three factors that affect the response time of a photodiode:

Charging acquisition time of the carrier in the photodiode depletion region

Charging acquisition time of the carrier in the area where the photodiode is not exhausted

RC Time Constant for Diode Circuit Combination

Since the junction capacitance depends on the diffusion area of ​​the photodiode and the applied reverse bias, faster rise times can be obtained by using a photodiode with a smaller diffusion area and applying a larger reverse bias. In the CN-0272 circuit note, using an SFH 2701 PIN photodiode, the typical junction capacitance is 3 pF, with a maximum of 5 pF at 0 V bias. The typical capacitance at 1 V reverse bias is 2 pF, 1.7 pF at 5 V reverse bias. Measurements for this circuit were made with 5 V reverse bias.