Research on the Design of Remote Optical Sensing Circuit

How can a circuit be used to reduce the bandwidth and noise effects of a photodiode remotely or through a large parasitic capacitor? A typical light sensing system circuit has a photodiode, op amp, and feedback resistor/capacitor pair at the front end. This article starts with the circuit described in the previous article. In this circuit, the photodiode, amplifier, and feedback capacitor components limit the bandwidth of the circuit.

When light sensing is implemented with a photodiode, either with a large parasitic capacitance or at a greater distance, the amplifier input has a larger input capacitance. This increase in capacitance increases the noise gain of the circuit unless you increase the feedback capacitance of the amplifier. If the feedback capacitance (CF) is increased, the bandwidth of the circuit is reduced.

To solve this problem, you can use a bootstrap circuit (see Figure 1). Photodiodes with lower diode capacitance do not benefit from this circuit. Unity-gain buffer A2 removes the cable capacitance and the photodiode parasitic capacitance that is present at the input of transimpedance amplifier A1.

Figure 1 Bootstrap diode capacitance and cable capacitance out of the transimpedance design problem

When designing this circuit, the choice of amplifier type for A2 can be somewhat relaxed. Only four performance specifications are important. These design guidelines include selecting an amplifier with low input capacitance, low noise, higher bandwidth than A1, and low output impedance.

In this design, the input capacitance of A2 is the only capacitance that contributes to the AC transfer function of the transimpedance system. The buffer input capacitance replaces the sum of the input capacitance of A1, the cable capacitance, and the photodiode parasitic capacitance. A good rule of thumb is CA2 << (CA1 + CCA +CPD), where CA1 and CA2 are the sum of their input differential and common-mode capacitances.

Using this design, you can swap one noise problem (A1) with another (A2). The unity gain buffer cancels the noise contribution of A1. A good rule of thumb is to have A2 noise <= A1.

The difference between the input and output signals in this system is reduced by the cable/diode capacitance. You can maintain this low signal difference by choosing A2 with a wider bandwidth than A1 and keeping the output impedance of A2 low. The A2 gain roll-off sets an upper limit on the bandwidth improvement, making the bandwidth relationship between these amplifiers equal to A2-BW >> A1-BW. This circuit requires stability optimization as you balance CF and A2 input capacitance.