In this application note, there is a circuit for temperature control which includes an NTC.

enter image description here

Why is there a capacitor (C5) in parallel with the inputs of the MC33201 Op Amp?

Is it a helper to the Op Amp as the Op Amp tries to make its inputs equal when there is a negative feedback?

Does it introduce an another negative feedback like this: When there is a sudden enough change in the resistance of the NTC that can pass C5, the voltage is then passed to the inverting input of the Op Amp and the output of the Op Amp does not need to change to compensate the small change on the non-inverting input due to the change of NTC resistance?

Explanation from the App Note:

The same 5 V supply is used also as reference voltage for the Wheatstone bridge, formed by components R2 (NTC), R3, R6, R7 and C5 used for low pass filtering.

If it is for low-pass filtering, why this capacitor is not connected parallel to R5 instead of where it is now? What is the advantage of it being on the place it is now compared to it being parallel to R5?

  • 2
    \$\begingroup\$ I think it's a noise reduction/sudden spike absorber, where it will adjust the node voltage of R3 and R7 to help minimize the effect of the sudden change on the final output Vout \$\endgroup\$ – KyranF May 8 '14 at 10:51
  • 3
    \$\begingroup\$ It adds a little bit of low pass filtering. \$\endgroup\$ – Olin Lathrop May 8 '14 at 13:01

The capacitor is to prevent a DC bias shift from being caused by EMI. Bipolar op-amps in particular can act as "detectors" to RF signals, and the capacitor shunts the RF to reduce the amplitude ( at the expense of increasing noise and reducing phase margin ) so it should not be too large a value capacitor.

Although all the parts are close together with minimum lead length, one could expect there is a major source of EMI nearby in the switching power supply.


On the time scale of temp changes, it might make sense to just ignore the cap, and imagine it removed for analysis. What you're left with is a simple difference amp. At high frequency, imagine a short instead, so the high freq difference goes to zero. It's a low pass filter.

  • \$\begingroup\$ Probably for EMC if the RTD is positioned some distance away from the op-amp. \$\endgroup\$ – Andy aka May 8 '14 at 12:10
  • \$\begingroup\$ @Andyaka The NTC is positioned a little away from the Op Amp since it will be connected to the LED(s) on a heat-sink. Probably 10-15 cm away. See the App Note. \$\endgroup\$ – abdullah kahraman May 9 '14 at 6:14

The capacitor is used as a coupling capacitor. The DC bias is fixed by R3 and R7. By doing this way, you assure that the bias on 1 and 3 is different.

  • 2
    \$\begingroup\$ "bias on 1 and 3" - what does this mean? DC bias is set by R2 and R6; gain is affected by R3 and R7. \$\endgroup\$ – Andy aka May 8 '14 at 12:09
  • \$\begingroup\$ did not check the connexion between 2 and 3, you are right. Thought that 2 is just capacitively coupled. My bad \$\endgroup\$ – DrWho May 8 '14 at 13:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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