Digital Lock-In Amplifier filter random fluctuations

Greetings,

I have made a LIA to measure IR signal amplitude in a optically noisy environment. However even in a relatively quiet environment I get +/-5 mV fluctuations in to my readings. Signal is approx. 450mV. I don't know if this is a good or bad LIA performance?

Here is my setup:

- IR light source is switching on and off at 1kHz freq. (Squarewave)
- Vishay BPV10NF PIN Photodiode -> transimpedance amplifier (AD8616) at 100k Gain -> Passive HPF -> 16bit ADC
- 1kHz reference square signal
- 20kHz sampling frequency

1. Multiplying signal data (10 000 samples) with Sin and Cos (IQ-demod)
2. Instead using LPF I'm just averaging I and Q chunks
3. Then get the result by Sqrt(I^2 + Q^2) and normalizing it

I've never used a commercial LIA before (or any LIA), but I would assume that LIA should be stable and not show random fluctuations.

Is there any ideas how to improve? Or am I doing anything wrong? Should I be implementing a FIR LPF instead just averaging?


Thanks in advance

I think you probably need a passive LPF before your ADC. Using a digital filter (averaging or FIR) won't achieve the same function. The passive LPF is to prevent aliasing, any signal or noise will fold into the first nyquist zone (10 kHz) when the analog signal is sampled.

For instance, the AD8616 claims BW> 20 MHz. So all the noise energy from 10 kHz to 20 MHz will fold into the first nyquist zone (0-10 kHz) which will cause very poor noise performance. A simple 2 pole LPF before the ADC can prevent the majority of this noise.

Hi thank you for answer.

Good idea to add aliasing filter. According to transimpedance amplifier's transfer function with 100k Gain I get f3db at before 50kHz, and beginning of noise gain at approx 3kHz.

The FIR LPF I was refering is the lock-in amplifiers LPF after signal has been multiplied with Sin and Cos reference signal. I don't know if a LPF say cut-off frequency at 0.1Hz would perform better than just averaging all data together.

Any good references for determining weight ratios? I have never done a FIR LPF.

Any other digital LIA design experience is greatly appreciated.