I have a circuit that is used to amplify the incoming signal from a thermopile sensor.

The AD8630 op-amp is connected to resistors to have a gain of 1000 but I am confused about the capacitor. Is it there to provide low-pass filtering at 53 Hz? If it is, then what could be the reason to use such low-pass filtering?

The data sheet of AD8630 provides the following statement:

If interstage ac coupling is used, as in Figure, low offset and drift prevent the output of the input amplifier from drifting close to saturation. The low input bias currents generate minimal errors from the output impedance of the sensor. As with pressure sensors, the very low amplifier drift with time and temperature eliminate additional errors once the temperature measurement is calibrated. The low 1/f noise improves SNR for dc measurements taken over periods often exceeding one-fifth of a second.

Can someone please break it down in easier words?


  • 1
    \$\begingroup\$ "why AD8630 is used", Well, it has to be something. I'm sure there are many other OP amps that would work. About the capacitor, see here: electronics.stackexchange.com/questions/390986/… \$\endgroup\$
    – Klas-Kenny
    Commented Dec 3, 2021 at 9:21
  • \$\begingroup\$ That's not a lowpass, it's a shelf-lowpass (pole-zero). Because it's a non-inverting amplifier, so the transfer function is 1+feedback, which makes it of the form (s+a)/(s+b). \$\endgroup\$ Commented Dec 3, 2021 at 9:40

1 Answer 1


Read the link posted by Klas-Kenny in comments.

In short, when you use a large feedback resistor such as 200K, the capacitance of the the inverting input causes the feedback signal to have some phase delay at high frequencies. Basically the output is faster than the input which causes a tendency for the output to oscillate uncontrolled at a certain high frequency.

To compensate, the capacitor provides a strong feedback path at high frequencies, so the output can be well regulated again. It has the side effect of forming a lowpass as you describe.

Another way to prevent oscillation would be a lower feedback resistor, but this choice can be limited when it results in unpractical resistor values.

The cited datasheet paragraph is unrelated


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.