Differential amplifier issue. Getting an "asymmetrical" up-down output

TeeKay6

Joined Apr 20, 2019
572
I want to measure the current through a device, which has to be in phase with the voltage generated in another part of the circuit (to get the current-voltage relation, like a curve tracer).
You may be able to achieve lower phase delay by using an opamp that is not fully compensated for unity gain (e.g. it may only be stable for gains > 10); whether a suitable opamp (low noise, low offset, etc) exists, I do not know. You might start by looking at the LT1226. Alternatively, you could construct your own opamp (not a trivial project when high frequency performance is involved). Finally, you might find advice by searching the web for articles about wideband amps, oscilloscope amps, and similar. If the current sense resistor is not grounded, then common-mode response will also become relevant. If the current sense signal is not sinusoidal and/or has a varying frequency, then acceptable performance at one frequency will not guarantee acceptable performance at other frequencies.
 

Thread Starter

Elerion

Joined Sep 11, 2017
117
Use a dual opamp, one for current and the second opamp for voltage. Since they are in the same IC then their phase shifts are identical
Thank you. It is worth a try.

You may be able to achieve lower phase delay by using an opamp that is not fully compensated for unity gain (e.g. it may only be stable for gains > 10)
...
If the current sense signal is not sinusoidal and/or has a varying frequency, then acceptable performance at one frequency will not guarantee acceptable performance at other frequencies.
Thank you. I know what to look for now.
The test voltage will be almost sinusoidal and the frequency always fixed, but the current response could be highly non linear. Would that be an issue?
Would the CMRR be an issue if powering the opamp from asymetrical voltages (like +15/-5V or +5V/-15V).
 

TeeKay6

Joined Apr 20, 2019
572
Thank you. It is worth a try.



Thank you. I know what to look for now.
The test voltage will be almost sinusoidal and the frequency always fixed, but the current response could be highly non linear. Would that be an issue?
Would the CMRR be an issue if powering the opamp from asymetrical voltages (like +15/-5V or +5V/-15V).
I don't know what this means: "the current response could be highly non linear." If you mean that the current waveform might not be a sine wave, as is the stimulus, then the issue of how the current varies with frequency (or time) becomes relevant. "highly nonlinear" covers an extreme range of variation.

The CMRR varies as defined in the datasheet for the opamp. Unfortunately, most opamp datasheets give very little info re CMRR. The issue is not the symmetry of the power supplies, but the variation in the opamp + and - input voltages relative to the power supplies. CMRR issues will be less if the input voltages have small variation compared to large variation; e.g. if you are using +/-10V power supplies and the voltages at both ends of the current sense resistor (measured relative to the power supplies) vary less than 0.1V, CMRR problems are likely to be small, at least compared to a case in which those voltages vary by several volts.
 

Thread Starter

Elerion

Joined Sep 11, 2017
117
If you mean that the current waveform might not be a sine wave, as is the stimulus, then the issue of how the current varies with frequency (or time) becomes relevant. "highly nonlinear" covers an extreme range of variation.
As @Audioguru again said, imagine a forward biased diode. That's what I meant by non linear current response.
 
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