Noninverting op amp doesn’t work on highly resistive load

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#1 21684882 03 Aug 2020 00:54

Michael Lamontagne Michael Lamontagne

Anonymous

Post #1
21684882 03 Aug 2020 00:54

I am using an Apex PA443DF operational amplifier to drive a sinusoidal signal at 100 V amplitude across a very large resistor with resistance of about 5000 Mega ohms. The amplifiers are set up as noninverting with a gain of 22 and function very well for small resistive loads. The sinusoids being amplified are also very low frequency (approximately 1 Hz).
They work very well for a load of 3 Mega ohms, but decrease in performance I see measured on my oscilloscope to an amplitude of only about 0.1 V when the full 5000 Mega ohm load is put in place. I understand there would likely be some frequency dependent instability for capacitive loads, but I am only directly changing the resistance. I am a physicist rather than an electrical engineer, but I can't seem to think what would be the problem with increasing the load on the op amp, because less current would surely be required by ohm's law.
Any guidance you can offer would be greatly appreciated.
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#2 21684883 04 Aug 2020 01:40

PeterTraneus Anderson PeterTraneus Anderson

Anonymous

Post #2
21684883 04 Aug 2020 01:40

Try using 3 Mega ohm load in parallel with 5000 Mega ohm load.
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#3 21684884 04 Aug 2020 02:54

John David Heinzmann John David Heinzmann

Anonymous

Post #3
21684884 04 Aug 2020 02:54

Indeed, I would expect an op-amp to work just fine with no load on its output (in other words with infinite resistance) so I don't see why it wouldn't work with 5 MΩ. And a quick scan of the datasheet doesn't reveal any mention of this. And at such a low frequency, I would not expect any dynamic attenuation. But this is a power op-amp so it may have a gotcha there. Is it possible that the current limit circuit is kicking in for some unwanted reason? If Peter's suggestion of 3 MΩ in parallel with your load isn't feasible, I would suggest contacting an Apex field applications engineer. Best -JD
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#4 21684885 05 Aug 2020 13:47

Richard Gabric Richard Gabric

Anonymous

Post #4
21684885 05 Aug 2020 13:47

5GOhms, no, not unless your amplifier output pins are on an isolated teflon standoff and the rest, you don't say if this is the case, but even then, see my comment below about making measurements.These devices are typically used for driving Piezo devices, where you can expect leakage currents up to some uAmps. That determines a more realistic value for your output resistor. Put a scope probe or a DVM on the output and you probably have 1 to 10MOhms of effective output resistance, unless you are using more sophisticated measuring devices, so you are not driving it into your chosen output resistance under that circumstance.It would help if you posted the actual circuit you are using, particularly if it is not taken directly from the Apex data sheet or application notes.Cheers,Richard
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#5 21684886 05 Aug 2020 23:57

Michael Lamontagne Michael Lamontagne

Anonymous

Post #5
21684886 05 Aug 2020 23:57

Thank you for your suggestions. It turns out that the problem was my scope impedance being low enough that my voltage divider was flattening the signal completely differently from what I expected.
It's interesting to realize that my method of observation was changing the signal.
I was effectively putting 1 Mohm in parallel with my load anyway, so your suggestions helped a lot.
Best,Mike
#6 21684887 06 Aug 2020 03:37

PeterTraneus Anderson PeterTraneus Anderson

Anonymous

Post #6
21684887 06 Aug 2020 03:37

Does the behavior with the 5Gohm load differ from the behavior with no load? What is the capacitance of the 5Gohm load?
While driving the 5Gohm load and seeing 0.1 V output on your oscilloscope, try waving your fingers close to the PA443DF without touching the circuitry. Does the 0.1 V output change in response to the presence of your hand? If so, the PA443DF is oscillating at a frequency in the MHz range. What is the bandwidth of your oscilloscope?
The PA443DF contains two amplifier chips. How is the other chip wired?It would help if you posted your actual schematic as Richard Gabric requested. Also post a photo of the PA443DF and surrounding circuitry.
#7 21684888 09 Aug 2020 04:19

PeterTraneus Anderson PeterTraneus Anderson

Anonymous

Post #7
21684888 09 Aug 2020 04:19

Since you are amplifying just very low frequencies, you could try increasing the compensating capacitor Cc to prevent oscillations by lowering the amplifier's bandwidth.
#8 21684889 11 Aug 2020 20:11

andreew kelvin andreew kelvin

Anonymous

Post #8
21684889 11 Aug 2020 20:11

These gadgets are normally utilized for driving Piezo gadgets, where you can expect spillage flows up to some uAmps. That decides a more sensible incentive for your yield resistor. Put an extension test or a DVM on the yield and you most likely have 1 to 10MOhms of successful yield opposition, except if you are utilizing more advanced estimating gadgets, so you are not driving it into your picked yield obstruction under that conditions.
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Topic summary

An Apex PA443DF operational amplifier configured as a noninverting amplifier with a gain of 22 was used to drive a low-frequency (approximately 1 Hz) sinusoidal signal at 100 V amplitude across extremely high resistive loads (up to 5000 MΩ). While the amplifier performed well with loads around 3 MΩ, the output amplitude dropped significantly to about 0.1 V with the 5000 MΩ load. The issue was identified as measurement error caused by the oscilloscope probe's finite input impedance (~1 MΩ), which formed a voltage divider with the high-value load, drastically reducing the observed output voltage. It was noted that the effective output resistance and measurement setup critically affect the observed signal. Suggestions included using a parallel 3 MΩ resistor to stabilize the load, verifying the actual circuit and measurement setup, and considering the amplifier’s internal current limiting or oscillation behavior. For low-frequency signals, increasing the compensation capacitor to reduce bandwidth and prevent oscillations was also recommended. The PA443DF is typically designed for driving piezoelectric loads with leakage currents in the microampere range, implying practical output load resistances are much lower than gigaohms. Proper measurement techniques and understanding of the amplifier’s output impedance and load conditions are essential for accurate signal observation.
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Noninverting op amp doesn’t work on highly resistive load

Noninverting op amp doesn’t work on highly resistive load

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