0
\$\begingroup\$

I am studying RC filtering in order to try and understand this circuit which was previously discussed in(What kind of filtering does this op-amp perform?).

The answers gave advice on testing the response to a sine wave sweep which should show it's filtering of undesired noise/interference such as mains electricity at 50/60Hz. I want to follow the path/modifications to the signal itself which to my knowledge is a direct current produced by the photodiode. Can this current pass through the capacitors and if so how?

I am trying to identify sub-components of the circuit and learn what effects they are having, please correct any misunderstanding:

1.The two 100K resistors are there to introduce an offset voltage so as to center the Op-Amp output at 2.5V? They do not have any additional effects?

2.The feedback path from Vout through two series resistors R6 (feedback resistor)& R5 forms another voltage divider. The fact that R6 is far far larger means only a very small voltage difference will be seen at VIn-.

3.Lastly, the feedback branch R5 and C4 appears to me to be a differentiator but I am unsure how to test this in simulation as I am interested in how the circuit processes the signal of interest - a varying direct current, rather than the AC noise of a sweep signal.

Any advice or input appreciated.

Circuit for amplifying & filtering input from a photodiode

\$\endgroup\$

1 Answer 1

Reset to default
0
\$\begingroup\$

Can DC go thru a Capacitor? NO

Is it DC that the Light Sensor picks up from the pulsing LED going thru a finger? NO

What might it look like?

Time domain and Freq. Domain with 0 to 100 Hz linear bins.

enter image description here

This is my hand-drawn image of a heartbeat at 120 BPM with log dB vs linear Fourier spectrum to 100Hz.

What is the circuit?

First Caps reduce the voltage in half but also act as passive LPF followed by a Sallen-Key BandPass Filter (BPF) . The upward slope amplifies the downward slope of the spectrum of the heartbeat.

You can say it is basically a band-limited active differentiator. The heart signal is differentiated and higher f noise is attenuated. enter image description here

The output would amplify the middle pulse of the QRST wave.

\$\endgroup\$
9
  • \$\begingroup\$ Stewart I have tried to brush up on filtering concepts and get accustomed to circuit simulators but have not been able to generate the real circuit in KiCad etc and LTSpice is unavailable in Linux. I have been trying to build my intuition using the Lush/Falstad simulator but when I build the circuit as shown in your post I cannot make any sense of the output. For a composite sinusoid which resembles the expected input of a PPG quite well; the output is a square wave. Really quite stumped by this, I would have expected to observe the bandpass effects when I change the frequency. \$\endgroup\$
    – liambanus
    Commented Mar 16, 2021 at 23:06
  • \$\begingroup\$ The effects of reducing the capacitance and increasing the input frequency by the same factor fully cancel out correct? So this circuit should have maximal gain at 2*10^3 Hz? I cannot seem to get an output that morphologically resembles the input regardless of the frequency. One last question, why would the voltage be halved (by the action of C3 & C4) only for an offset to be introduced later? \$\endgroup\$
    – liambanus
    Commented Mar 16, 2021 at 23:11
  • \$\begingroup\$ No 3 decades down or 2Hz , I used nF rather than uF to get 2kHz for convenience. I have no idea why they did except to meet some specs on shape. Remember ideal SNR matches the spectrum of all the signals. It is obvious to me how it works so here is a simplified version for you. tinyurl.com/yj7bqckl. The cap divider serves no purpose. \$\endgroup\$
    – D.A.S.
    Commented Mar 17, 2021 at 1:55
  • \$\begingroup\$ The simplified version is NOT the same but has a similar shape for your intuitive understanding. It is basically a dominant HPF with LPF tinyurl.com/ydnytofb to make an asymmetric wide BPF \$\endgroup\$
    – D.A.S.
    Commented Mar 17, 2021 at 2:14
  • 1
    \$\begingroup\$ Falstad's sims do everything you need. after learning curve \$\endgroup\$
    – D.A.S.
    Commented Mar 17, 2021 at 11:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.