Need help understanding this circuit.

[integrant2]

http://courses.cit.cornell.edu/bionb442/labs/f2007/lab9circuit1.png

This is finger plethysmograph i found on Cornell site. I can kind of understand how the circuit works, but i am confused on the circuitry around the op amps, especially the 10K potentiometer, i have no idea what that does.
My guess is 10uf cap and 20K ohm is a high pass filter and a low pass filter (the 100K ohm and .47uf cap) is fed back to the inverting input for amplification. Please correct me if i am wrong.
Any help is greatly appreciated.:smile:

[dlgoff]

Welcome to PF integrant2.

Did you notice that the 10 kΩ pot is labeled gain?

[integrant2]

Welcome to PF integrant2.

Did you notice that the 10 kΩ pot is labeled gain?
Yes i did. Does that mean that pot controls the gain on the op amp in the middle or both?

[uart]

Yes i did. Does that mean that pot controls the gain on the op amp in the middle or both?
Specifically it controls what fraction of the signal from the output of the first op-amp makes it to the input of the second op-amp circuit. It provides a variable attenuation which ultimately of course controls the gain of the overall circuit.

[integrant2]

Thanks alot for the reply, it helps me a lot.
But how about my guess on the filters? I am quite sure the 10uf and 20KOhm is a high pass filter, but how about the 100KOhm and 0.47uf in parallel? Is that a low pass filter?

[dlgoff]

The 10uf and 20KOhm looks like it would be a very low frequency (0.1 Hz) filter and the 100KOhm and 0.47uf would pass around 21 Hz.

[uart]

Thanks alot for the reply, it helps me a lot.
But how about my guess on the filters? I am quite sure the 10uf and 20KOhm is a high pass filter, but how about the 100KOhm and 0.47uf in parallel? Is that a low pass filter?

It's a crude way to do it, but the combination of a low pass filter and a high pass filter provides a simple band pass filter. In this case the high pass filter cuts in about 0.8Hz and the low pass cuts off at about 3.4 Hz. The fact that two of these stages are cascaded however reduces the pass band somewhat.

BTW Integrant2. Is this for measuring pulse rate? The frequencies look about right for that purpose.

[integrant2]

It's a crude way to do it, but the combination of a low pass filter and a high pass filter provides a simple band pass filter. In this case the high pass filter cuts in about 0.8Hz and the low pass cuts off at about 3.4 Hz. The fact that two of these stages are cascaded however reduces the pass band somewhat.

BTW Integrant2. Is this for measuring pulse rate? The frequencies look about right for that purpose.

Yea, it measures the pulse rate by shining IR into the finger tip, then the phototransistor on the other side detects the signal and transmit it to the op amps for filtering and amplification.


But anyway, thank you so much for the help, you guys are really helpful :)