AC-coupling cap for AC inductor drive

Thread Starter

Rikh

Joined Jan 19, 2020
3
Hi,
with reference to the attached schematic, I need to choose the right capacitor value. These are the requirements:
- op-amp input/output is a 200 KHz sinewave
- the amplitude of the AC signal across the inductor (i.e. the OUT signal) is fed into another op-amp (for later ADC conversion)
- the op-amp O/P sinewave is DC shifted to 2.5V and the AC amplitude vary from 0.5V to 4.5V peak-peak to adjust the current across the inductor depending on the application
- resistor R3 limits the current going through the inductor and is usually fixed to 100R
- capacitor C2 provides AC coupling so the OUT signal is centred to ground

Because of the op-amp AC peak-to-peak amplitude van vary from 0.5 to 4.5 the opamp output current (i.e. also the current through the inductor) can reach up to 45mA. That is assuming R2 is not mounted on the PCB.

Questions:
1) how do I choose C2 value? does the signal frequency and/or the op-amp output current (when driving R3 & coil) matter in the choice of the capacitor C2? How? How I don't think that if the current is several amps (just as an example) a tiny pF capacitor would work, or maybe it does?
2) does the value affect/change the phase across the inductor compared to the original signal (OP-AMP output sinewave)? If so would a small one keep it to a minimum?
3) If any value would work, any benefits in a large value (or small one for that matter)?
4) I don't see the point of using R2. It was from a reference schematic. Does it serve any practical purpose in AC coupling or otherwise?

Many thanks for all feedback.
 

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crutschow

Joined Mar 14, 2008
24,125
1) how do I choose C2 value? does the signal frequency and/or the op-amp output current
It's the frequency and the value of R3 (R2 not needed).
2) does the value affect/change the phase across the inductor compared to the original signal (OP-AMP output sinewave)? If so would a small one keep it to a minimum?
Yes, it does change the signal.
You need a large capacitor, not a small one to minimize the affect of the capacitor on the signal.
A value of 1uF would have an impedance of less than 1 ohm at 200kHz so would have only a minor effect on; the signal for R3=100Ω, but note that the op amp will not drive such a low impedance load at more than perhaps 2V peak.
3) If any value would work, any benefits in a large value (or small one for that matter)?
See 2).
4) I don't see the point of using R2. It was from a reference schematic. Does it serve any practical purpose in AC coupling or otherwise?
Not that I can see.
 

ronsimpson

Joined Oct 7, 2019
419
I did not have our op-amp so I used another one. 10nF, 100 ohms, 80uH, 7.7 ohms.
Graph shows all low frequencies are attenuated. I set C1 so gain is 1 (0db) at 200khz. High frequency passes well.
Is this what you wanted to do?
1579473646031.png
 

Thread Starter

Rikh

Joined Jan 19, 2020
3
Thank you Ron and Crutschow. :)

Ron, yes that would work. Thank you for the sim.

Crutschow, why do you say that the op-amp would not drive 100R load? Does the output current decrease as frequency increases?

Yes, it does change the signal. You need a large capacitor, not a small one to minimize the affect of the capacitor on the signal.
A value of 1uF would have an impedance of less than 1 ohm at 200kHz so would have only a minor effect on; the signal for R3=100Ω, but note that the op amp will not drive such a low impedance load at more than perhaps 2V peak.
Thank you
 
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