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High voltage integrator op amp choice?

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bobopedic

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I need an integrator that will handle +100v output.
This is a serious problem due to the lack of high voltage opamps.
It's my understanding that integrators need to be unity gain stable.

Would this opamp work?

https://www.apexanalog.com/resources/products/pa340u.pdf

It seems to show unity gain capability under the "gain and compensation" graph but does that mean unity gain stability?
 

An integrator uses maximum DC gain not unity , so it does not need to be unity gain stable. It does need to have very low DC offset.

You may be confusing the fact that Op AMps are integrators with internal small compensation and are thus often unity gain stable. When they compromise compensation for bandwidth, unity gain stability is compromise, but still work as integrators or high gain amplifiers.

Without knowing your source impedance and input offset voltage requirements, this certainly handles the output range.

How will you null the offset or reset the integrator?

Compare your choice with the LTC7090 Voltage - Input Offset 330µV
https://www.digikey.com/product-search/en?keywords=ltc6090

PA340CC Voltage - Input Offset 12mV
 
Last edited:

Hi,

An integrator uses maximum DC gain not unity , so it does not need to be unity gain stable. It does need to have very low DC offset.
Click to expand...

I did not focus on DC for stability.. more on the upper frequency end, where an integrator approximates to unity gain..or even zero gain
Therefore in the past I always selected a unity gain stable amplifier for integrator circuits.

******
What circuits needs a 100V output..what current?
Could you give more information?

******
I just used the LT selection guide just for 100V Opamps. It shows LTC6090/6091/6101/6102...

Klaus
 

An integrator uses maximum DC gain not unity , so it does not need to be unity gain stable.
Click to expand...
An OP used in the classical inverting integrator circuit must be in fact unity gain stable. If the OP is e.g. stable for G >= 5, you need to modify the integrator circuit to reduce the loop gain at high frequencies.

It seems to show unity gain capability under the "gain and compensation" graph but does that mean unity gain stability?
Click to expand...
Yes. PA340 should work in an inverting integrator circuit when compensated for unity gain (Cc >= 20 pF).

I don't know the application details, but a standard OP followed by a discrete class AB output stage, similar e.g. to an audio amplifier, or even a single ended class A output stage may serve the purpose.
 

SunnySkyguy said:
An integrator uses maximum DC gain not unity , so it does not need to be unity gain stable. ..........................
Click to expand...
That is not true. An integrator has unity gain feedback (no feedback attenuation) at high frequencies (well above the -3dB point) and thus requires a unity-stable op amp to avoid oscillations.

- - - Updated - - -

bobopedic said:
I need an integrator that will handle +100v output.
This is a serious problem due to the lack of high voltage opamps.
It's my understanding that integrators need to be unity gain stable.

Would this opamp work?

https://www.apexanalog.com/resources/products/pa340u.pdf

It seems to show unity gain capability under the "gain and compensation" graph but does that mean unity gain stability?
Click to expand...
That op amp should work for your requirements.
Unless you have a high integration frequency you can use a large value for the compensation capacitor Cc (say 68pF or higher) to insure stability.
 

Yeah I guess I should have given more info about the application.
I am using it as a bias for an amplifier to eliminate DC at the output. So at the end of the day I just need it to match its inverting input to the non-inverting one without too much hassle.

Usually I have to use a trimmer on the "offset null" terminals of an opamp to fine tune the integrator until I see 0VDC on the output of the amplifier.
Is the capacitor compensation pretty much the same thing?
 

bobopedic said:
...............
Usually I have to use a trimmer on the "offset null" terminals of an opamp to fine tune the integrator until I see 0VDC on the output of the amplifier.
Is the capacitor compensation pretty much the same thing?
Click to expand...
Capacitor compensation is to prevent the op amp from oscillating.
It has no effect on the output dc offset. That's determined by the op amp input voltage and current offsets.
 

What is the difference between this opamp https://www.mouser.com/ds/2/405/opa445-444023.pdf
vs this opamp https://www.apexanalog.com/resources/products/pa340u.pdf ?

The opa445 that I am used to using for integrators requires an offset trimmer and if I recall it can make a several volts difference between both ends of the trimmer.

The PA340 doesn't have offset pins but they both show only a few mv of offset in the datasheet.
What am I missing? Do I need to use a divider pot to adjust the non-inverting input of the PA340 for offset?
 

Hi,

Offset_voltage (some uV to some mV) of an OPAMP is the difference voltage of both input pins (In+, In-) that generate a zero volt at the output. (Dual supply)

The trimmer voltage can be much more, some volts is not unusual. With the trimmer you can adjust the offset_voltage to become zero.

Klaus
 

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