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Low noise Piezo stack driver amplifier design

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I'm in a need of a "low noise" amplifier that can drive piezo stacks (for precise motion control purposes).
- The piezo stack itself can operate between -10 to 120V, so the amplifier can almost be unipolar. As known piezo stacks acts very much like capacitors, such piezo may have capacitance of several uFs.
- Low noise: the amplifier should have S/N ratio to faithfully serve 20-22 bit DACs.
- Low speed: bandwidth DC to 3kHz.
- Low gain: 5-10 times amplification

Initially I was immersed by the "low noise" requirement of this project and found the following:
a.) Use a single amplifier that have inherently low noise.
b.) Whatever amplifier I have, I can lower the noise by adding more amplifiers parallel. With 2 amplifiers the theoretical yield would be +3dB in S/N, with 4 amplifiers +6dB.  What I think is that this should work also when amplifiers are bridged, ie. two amplifiers working to a floating load in anti phase would yield +3dB, when two pair paralleled amplifiers are bridged would yield +6dB.
c.) Since the bandwidth is not very high and the output is already a capacitor I can add an R-C filter (where R is small and C is in the order of the load) to the output, to filter out noise further.

My source signal is differential to start with, so having a bridged, floating output is not a hassle at all.
There are several high voltage opamps but they are either specialty items and hard to come by or expensive (or both).

I am toying with the idea of building the amplifier around the OPA445 opamp. The specs for this IC is not outstanding by any means, but fair, but the maximal power supply is 90V which is just too small.
But having two of these amplifiers driving in bridge mode would be enough, eg. the positive side driver IC would run from +70V and -15V supply, the negative driver would run from +15V and -70V supply, so the total output voltage could swing between ca. -25V to 135V .

Even though the bandwidth is fairly low at 3kHz, but driving 10uF would still be tough for these opamps (from dissipation point of view), so I would add a simple "two transistor current booster stage" to the output.

I have ran noise simulations and even a single opamp and current booster would reach S/N >150dB (which is more than 24bit resolution), at full drive voltage!

Can it be this simple or am I missing something?

PS.: I have been playing in simulations with low noise low voltage opamps with bootstrapped (tracking) supply as well, but the difference in the results were negligible compared to the effort for the bootstrapping circuit.

With that many bits, I'd worry more about offset drift than noise. Presumably you want repeatability?

Here's something that might be of some help regarding Piezo Drivers and Controllers. This was a project we did some time ago to help with positioning in the nanometer regions.


Lately we've been developing a 128 independent channel AWG with effective 15 bit resolution capable of driving +-90V. The design is based around the TI OPA462 Op-Amp as the driver amp. We've done a lot of evaluation and selected the OPA462 over other candidate amps. The "Over Current-Voltage-Die Temperature" are nice features. This Op-Amp might be worth investigating.



--- Quote from: Marco on August 19, 2021, 11:18:57 pm ---With that many bits, I'd worry more about offset drift than noise. Presumably you want repeatability?

--- End quote ---

In other words, 1/f noise -- easily solved using a chopper/autozero amp, which are available these days in quite a bit more bandwidth (~3MHz) than needed here, so it should be a fine choice.

With an op-amp closing the loop, the buffer can be really anything.  A discrete amplifier with level shift and current boost, would do a fine job.  Not that a whole lot of current will be needed at 3kHz.

A very important question: do you know if the transducer is subject to mechanical hysteresis?  If so, you may need a similarly precise position sensor, and a servo loop.  Or active cancellation (e.g. when performing a move, overshoot a preprogrammed amount, optionally wiggling somewhat, to settle to the correct position).  Backlash algorithms are commonly applied in CNC, probably easy to look up examples.

Also, does its capacitance vary with applied voltage (over the given control range), or say with applied force?  This can make compensation challenging; a possible consequence being, the loop bandwidth has to be turned down to the worst-case condition, slowing settling time.

Note that loop gain determines settling time: the higher the loop and transducer bandwidths are, the faster settling can be, to within some margin of error.  And the error is dependent on the loop gain, at the frequency equivalent to that time.  (Loop gain is generally decreasing with rising frequency, i.e. the error amp looks like an integrator.)  I'm guessing it needs to be quite stable (within say >20 bits after X time?) so this can suggest a more challenging control scheme if faster settling is required.



--- Quote from: Marco on August 19, 2021, 11:18:57 pm ---With that many bits, I'd worry more about offset drift than noise. Presumably you want repeatability?

--- End quote ---

Well, the piezos tend to drift anyway, so I am not sure about the drift. I have also considered OP177 and LT2057HV (chopper stabilized), the former would only work with bootstrapping, the later maybe would be good enough alone with limited output.


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