Photodiode Monitoring - Principles & Preliminary Design

Transimpedance Amplifier

A single photodiode will be used to detect reflected light from both the red and infrared emitters.The photodiode produces a forward current proportional to the intensity of light to which it is exposed. Typically, this current is on the order of 100uA. The OPA381 Precision, Low Power, 18MHz Transimpedance Amplifier can be used for precision current-to-voltage conversion for photodiode monitoring.

OPA 381 - Basic Operation

Basic operation is detailed in the data sheet:

The OPA381 is a high-precision transimpedance amplifier with very low 1/f noise. Due to its unique architecture, the OPA381 has excellent long-term input voltage offset stability. 

The OPA381 performance results from an internal auto-zero amplifier combined with a high-speed amplifier. The OPA381 has been designed with circuitry to improve overload recovery and settling time over that achieved by a traditional composite approach. It has been specifically designed and characterized to accommodate circuit options to allow 0V output operation. 

The OPA381 is used in inverting configurations, with the non-inverting input used as a fixed biasing point. Figure 1 shows the OPA381 in a typical configuration. Power-supply pins should be bypassed with 1µF ceramic or tantalum capacitors. Electrolytic capacitors are not recommended.

Biasing Photodiodes in Single-Supply Circuits

For our application, a single 3.3V supply is available. Thus, our application requires that our transimpedance amplifier is configured with a reverse bias voltage. From the data sheet:

The +IN input can be biased with a positive DC voltage to offset the output voltage and allow the amplifier output to indicate a true zero photodiode measurement when the photodiode is not exposed to any light. It will also prevent the added delay that results from coming out of the negative rail. This bias voltage appears across the photodiode, providing a reverse bias for faster operation. An RC filter placed at this bias point will reduce noise. (Refer to Figure 4.) This bias voltage can also serve as an offset bias point for an ADC with range that does not include ground.

Preliminary Design - Photodiode Monitoring

A preliminary design  has been laid out using CadSoft Eagle PCB Design Software. 

Current-to-Voltage conversion using the OPA381 Transimpedance Amplifier

The bias voltage will be implemented using a voltage divider and unity-gain voltage buffer. Implementing the voltage buffer allows for large resistor values (on the order of MOhms) for the  voltage divider, thus reducing static power dissipation.

Bias Voltage Generation