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OPA2388: peak-hold detector circuit affected by opamp input leakage current

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brumbarchris:
I do not really need 1ms of hold time, as I plan to sample the peak detector output with an ADC in a 200us window after the peak. But I would obviously like the cap charge degradation in this window to be as small of possible, below 1mV, if possible.
So yeah, I will need to look at faster opamps.
I went through this article (https://www.analog.com/en/technical-articles/ltc6244-high-speed-peak-detector.html), which discusses some higher speed (200kHz max) peak detector using the LTC6244. Whereas using the "improved" topologies suggested in the article is not practical for us (due to the necessary -8.5V negative biasing voltage required, I take it from the article that the LTC6244 would be suitable at these speeds. It has a limited Vcm to 3.5V though, and I will need to detect peaks as high as 4.5V.

I have also watched this video suggested in the posts above (https://www.youtube.com/watch?v=5Pz7Mx0WRUk&feature=youtu.be&t=138), and this guy is proposing replacing the 1st opamp in the loop with an actual comparator (so this is something I will also look into, although first ever best preferred approach is to find out a suitable opamp that will work in my current schematic)

StillTrying:

--- Quote from: brumbarchris on March 06, 2020, 07:52:19 am ---This is a good idea and I will consider implementing it.
--- End quote ---

The first one I drew is near enough what Kleinstein said on the last line of his post.


--- Quote from: brumbarchris on March 06, 2020, 10:53:48 am ---But I've just measured the rise time of my signal and it is some 6us.
--- End quote ---

From that I'm not sure a large speed increase is needed. The op amp output having to change by about 2.5 V in 6 us is only about 0.4/us, and less than 1mA charging a 1nF, even if the output has to come out of saturation 2.5V/us seems like enough, but you do have to stay long way off the data sheet's max. slew rate because there's usually quite a few things it hasn't said about what happens there such as a large loss of GBW at least!

From your last post the ADC's sample time sounds like the most important, for most digiltal ???  ADCs it doesn't matter if the input droops while it's converting because it's then using it's own sample.

brumbarchris:
Hi
Our ADC will sample at about 2us intervals and will take in a 200us window 3 samples that it will average.

Coming back to the peak detector: I have also found somethting else in the datasheet of the OPA2388 that might make it unsuitable, apart from the relatively low slew rate: it is the phase shift (see attached graph).
It seems that it introduces a phase shift of -90 degrees across quite a large portion of the bandwidth, including the 60kHz I am dealing with. And considering that I have two opamps, somewhat in series (the feedback of the 1stone taking the feedback from the output of the 2nd one) this can result in a rather nasty 180 phase shift. Of course, the graph is only showing the open loop phase shift, I am not sure how to relate that to my current circuit, as I have the overall circuit operating with a unity gain, more or less.

Regards,
Cristian

Kleinstein:
The "improved" configuration in the AD article is using the diode outside the loop. So this is a 2 sided change: it makes things faster, but it also reduces the precision.
At high speed the classical version is also not perfect - because of delays and the limited BW. So it depends on the length of the pulses which version is actually better.

For the high speed range I also remember some discrete transistor based versions that may be an option.

I would not expect 1 mV accuracy to be possible. Some droop from the capacitor may be more acceptable than high capacitive loading to the OP. So I would definitely consider a smaller capacitor - more like 20-100 pF for high speed, as this can allow a faster OP.

The 90 deg. Phase shift is normal for open loop operation. The curve for the OPA2388 is actually quite good, as even at unity gain the phase shift is only 90 degrees and not more like 120 as with many other OPs.
However those phase shift plots have to be taken with a grain of salt: they can change with load current and also a capacitive load.

brumbarchris:

--- Quote ---The curve for the OPA2388 is actually quite good, as even at unity gain the phase shift is only 90 degrees
--- End quote ---

Which figure in the datasheet did you see that in? I am having trouble finding it myself.
And even if it were 90 degree, would this not give me trouble with two cascaded opamps? Is that not a sumation that I need to make (meaning 90degree from 1st opamp + 90degree from 2nd opamp = 180degree -> oscillations)?

Regards,
Cristian

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