A transimpedance amplifier (TIA) converts the current from an optical sensor, such as a photodiode, into a voltage that an embedded system can capture.
The key design parameters are:
- Sensor sensitivity
- Signal to noise ratio
- System bandwidth
- Op amp input impedance
- Op amp noise
- Capacitor and resistor values for noise reduction
- Keeping parasitic capacitance low for signal integrity
- Adjustable gain in unipolar, bipolar and floating configurations
Op amps with high input impedance and low noise keep distortion down and the signal to noise ratio up.
To check the circuit’s noise density, frequency response and stability, simulate it first, with a tool such as LTspice. Balancing the capacitor and resistor values is what keeps noise low in simulation. Sometimes the op amp you want has no LTspice model and you need to convert one from another simulator; we wrote up one way to do that.
Once designed, the TIA goes onto a PCB or into an embedded system. Layout needs care to keep parasitic capacitance low and the signal clean. An ADC then digitises the TIA output so the system’s microcontroller or DSP can process it.
We have designed low noise, high speed TIA circuits with fixed and adjustable gain in unipolar, bipolar and floating configurations, for health monitoring, environmental monitoring and communication products. See our multi stage transimpedance amplifier for one example.