Author Topic: Op Amp integrator - limiting +Vsat to zero (ground)  (Read 234 times)

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Offline sspj9

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Op Amp integrator - limiting +Vsat to zero (ground)
« on: December 22, 2018, 05:54:23 am »


Part of the Dual Slope ADC consists of an integrator which integrates the input voltage according to Vin/-Vref .
 I am using TL071 connected as an inverting integrator.

Quick summary of operation:
The output of the integrator charges negatively up to a certain count value when Vin is connected.
When the count value is reached, the controller switches to -Vref.
This causes the integrator to discharge and hence increasing output voltage (note due to inverting output).

NOW... Instead of the integrator to continue to integrate towards a positive output voltage while -Vref is connected, I want it to stop at zero voltage so that when Vin is connected as input, the integrator will start integrating negatively instantly.

Is this possible by connecting the op-amp supply terminals +Vcc to ground while -Vcc to -ve supply?
I tried this but observed clipping in the negative rail too, is it a bad approach?
Any other possible solutions?
« Last Edit: December 22, 2018, 09:59:16 am by sspj9 »
 

Offline David Hess

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Re: Op Amp integrator - limiting +Vsat to zero (ground)
« Reply #1 on: December 22, 2018, 09:04:58 am »
An operational integrator has overshoot due to its limited bandwidth so even if the input is disconnected when zero is reached, time will still be needed to reset.

You might want to study run-up and run-down integrators which continuously integrate the input.

 

Offline sspj9

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Re: Op Amp integrator - limiting +Vsat to zero (ground)
« Reply #2 on: December 22, 2018, 09:27:42 am »
even if the input is disconnected when zero is reached, time will still be needed to reset.


I was thinking, would it be possible to run an enable switch (like a mosfet) across the integrator capacitor so that when the integrator reaches zero, the output of the MCU sends a signal to the switch which hence closes it and shorts out the capacitor thus keeping the output voltage fixed to zero till -Vin is connected again? (ie till when MCU is ready for the next reading)
« Last Edit: December 22, 2018, 09:34:23 am by sspj9 »
 

Offline David Hess

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Re: Op Amp integrator - limiting +Vsat to zero (ground)
« Reply #3 on: December 22, 2018, 02:13:19 pm »
Shorting out the integration capacitor will certainly work but is only done in lower performance applications.

Another way I have seen it done is to have the integrator temporarily enclosed inside the control loop of another integrator which adjusts the input current to force the output to zero.
 
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