Hi,
I am using AFE4400 EV kits to do a university project. I am writing to ask why the ambient signal given by EV kits
always drift over time. I did the experiments in the following situations.
Case 1: The finger clip, coming with the purchased EV kits, is connected to the EV kits. The
ambient light is drifting over time. The noise floor is about ADC count of 150.
Case 2: Disconnect the finger clip. Neither LED or PD is connected to EV kits. But a resistor of
500Kohm is connected between INN and INP of the AFE4400 chip. The ambient signal (ADC count of
-3000) with noise floor of 2000 count, always drift.
Case 3, 4, 5 and 6: The setup is the same as case 2 except that the 500Kohm resistor is replaced
by 100K, 20K, 10K and 5K respectively. In those cases, the average DC count of ambient signal
are 2.4e4, 7.1e4, 14.6e4 and 28.2e4. The noise floors are respectively ADC count of 600, 1000,
2000 and 4000.
I have the following questions.
1. Why does ambient signal (ADC count) change from positive to negative when comparing case 2
to case 3~6? Is it because that 500Kohm resistor makes the fully differential transimpedance
amplifier unstable?
2. By comparing case 4~6, we found the ambient signal level is inversely linearly proportional
to the resistance of the used resistor. It makes sense to me by considering the effect of the
offset voltage of the transimpedance amplifier. But by comparing case 3 and 4, the ambient
signal level is not inversely linearly proportional to the resistance of the used resistor. Do
you know why?
3. Why the ambient signal always drift no matter how long (10 minutes, 20 minutes or 1 hour) I
keep the EV kits on? Besides thermal effect, I think there should be another effect making the
ambient signal always drift. Could you explain what the signal might be? Could you also tell me
the typical offset voltage of the transimpedance amplifier used in AFE4400? And what's the drift
value per 1 degree Celsius?
The drift has been confused to me for a quite bit long time. It will be very helpful if you can
let me know the reasons. Thank you very much for your help in advance,
Bin