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Slew rate/amp question

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szzuk

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My setup is this - OPA548 opamp - 1:20 transformer - piezo at 100KHz

I'm getting slew rate distortion when i output over 30Vpp from the opamp, driving the piezo at over 600Vpp. This is to be expected from the datasheet. The circuit itself is tried and tested over several months and is fully functional.

My question is this - if i add an inductor in series with the piezo thus reducing/cancelling the impedance of the piezo will I be able to drive the volts higher without slew rate distortion?

My thought is that theoretically I should be able to drive the piezo with more volts - but I tried this and it didn't seem to make any difference.

Regards,

Szzuk.
 
The data sheet: "10V/us". and that's the best under certain conditions.
I believe your load is capacitive so adding inductance should help. You could try to get the LC to resonate at 100khz.
 
I did add an inductor and got the LC resonance at 100KHz. I tried a few inductors and gradually homed in the right inductance to cancel out the impedance of the piezo. It just didn't seem to make any noticeable difference when I compared the signal using the inductor and then not using it.

It is quite possible I simply wasn't looking hard enough to see if the distortion had reduced - it was a bit subjective. So what is the theory exactly?

Regards,

Szzuk.
 
Do you get the same slew rate distortion when driving 30V into a purely resistive load? If so, then that seems to be due to a limitation of the amp which external tinkering is unlikely to fix.
 
My setup is this - OPA548 opamp - 1:20 transformer - piezo at 100KHz

I'm getting slew rate distortion when i output over 30Vpp from the opamp, driving the piezo at over 600Vpp. This is to be expected from the datasheet. The circuit itself is tried and tested over several months and is fully functional.

My question is this - if i add an inductor in series with the piezo thus reducing/cancelling the impedance of the piezo will I be able to drive the volts higher without slew rate distortion?

My thought is that theoretically I should be able to drive the piezo with more volts - but I tried this and it didn't seem to make any difference.

Regards,

Szzuk.


Hi,

The slew rate comes into play when the output voltage goes over a certain level at a certain frequency. In other words, at a certain frequency the maximum output voltage without distortion is limited by the slew rate of the op amp.
The maximum gain is limited by the gain bandwidth product, which means you might need a certain minimum level input voltage so you can use a low enough gain to satisfy the GBP.

For this op amp, 30v is probably too high of a voltage at 100kHz. It's a shame because at 50kHz it might work, but that's half what you want.

So to use this at 100kHz you'll have to reduce the output to 15v peak and use a higher ratio transformer.

Alternately you could use two of these op amps in a bridge configuration driving the second one 180 degrees out of phase with the first one, and connecting the transformer across the two outputs rather than one output and ground. The output of each one could then be only 15v but you'll get the full 30v across the two outputs. This should work at 100kHz.
Of course the gain bandwidth product has to be satisfied too, so max gain would be 10 with a 1.5v input, but better would be a gain of 5 with a 3v input for each op amp.

Please note in the above discussion the voltages mentioned are all in units of peak volts, not rms volts. This means not 15v rms but 15v peak. You may be able to stretch it to 15v rms but that's something you'll have to try.
 
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Do you get the same slew rate distortion when driving 30V into a purely resistive load? If so, then that seems to be due to a limitation of the amp which external tinkering is unlikely to fix.

The amp doesn't show any distortion driving 30V into a purely resistive load. I think I should repeat and check more closely. Can someone confirm theoretically what should happen?

Regards,

Szzuk.
 
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Hi,

So to use this at 100kHz you'll have to reduce the output to 15v peak and use a higher ratio transformer.

Alternately you could use two of these op amps in a bridge configuration driving the second one 180 degrees out of phase with the first one, and connecting the transformer across the two outputs rather than one output and ground. The output of each one could then be only 15v but you'll get the full 30v across the two outputs. This should work at 100kHz.

I'm using a bought transformer, I'd like to say I could make one at 1:30 but I know I'd end up with a worse signal if I tried.

I do have a spare OPA548 amp which I could use in a bridge configuration as you have mentioned. I've not thought about that, it is worth looking at because I'd like to be able to put 1200Vpp across the piezo for testing if nothing else.

Regards,

Szzuk.
 
Slew rate distortion causes a sine-wave to be a triangle-wave.
If you series-tune the capacitance of the piezo with an inductor then the resulting lower impedance might be too low for the max output current of the opamp.
If you use a transformer with a higher voltage stepup then again the resulting lower impedance might overload the opamp.
 
The amp doesn't show any distortion driving 30V into a purely resistive load. I think I should repeat and check more closely. Can someone confirm theoretically what should happen?

Regards,

Szzuk.

Hi,

Yes, theoretically there is some error in your measurement or something really strange is happening, because according to the data sheet you cant get 30v rms out of this op amp. According to the data sheet you cant even get 30v peak out of this op amp without distortion or possibly not even be able to attain that level at all.

Take a look at the graphs on the data sheet. You'll find one that shows max about 17v peak output at 100kHz and it doesnt even get to 30v peak much less 30v rms. 30v rms is over 42v peak.

How are you measuring this output voltage and what are your power supply voltages?
 
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I need to retest. I can also check the current. (I was measuring pp)

You have answered the question so I'll leave the thread, thanks for all the replies.

Szzuk.
 
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