Electronics > Projects, Designs, and Technical Stuff
OPA2388: peak-hold detector circuit affected by opamp input leakage current
Kleinstein:
During the reset phase the OP is seeing a near short. Maybe one could consider an additional signal from the reset gate drive to the OP, so that the OP does not have to fight the reset. This could be a diode + maybe series resistor from the gate to the inverting input of the 1st OP.
StillTrying:
Or an LM292 is a comparator plus opamp, the only one with all the pins in the right order, :) but not very good specs. 2mV and 1MHz.
brumbarchris:
This is a good idea and I will consider implementing it. It will be beneficial for current consumption regardless of the OPAMP that I will eventually select.
I will also adjust the driving circuit so that the capacitor charge reset duration is limited and only applied briefly just shortly before the expected peak.
brumbarchris:
I believe I have found the parameter of the OPA2388 which is making it impractical for this circuit. My current guess is that it is its slew rate. It is relatively low, at only 5V/us.
The repetition rate of my signal is 1ms, so I always thought about it as of a 1kHz signal. But I've just measured the rise time of my signal and it is some 6us. That's the equivalent of a whooping 60kHz signal (considering the bandwidth * risetime = 0.35 formula). So then, it is not a trivial peak detector that I have to make (which is what I have always regarded it as), but a rather more advanced one.
I was watching this EEVBLOG video:
Dave never takes the precision variant of the circuit (demonstrated in the second part of the practical demonstration) above 1kHz, pure sinewave. That is how I can explain that a very low slew rate opamp, like the TS912 that he uses works. But here I need to look for something faster, I suppose. I guess that is why the ADA8491 opamp used to work in my older version of the circuit: it is rated as 170V/us. I can probably get away with something in-between.
Cristian
Kleinstein:
For the OPA2388 the limited slew rate is a a weak point. Between the operation through the 2 diodes there is a dead zone of around 1 V. So the detection is behind by something like the time it needs to slew 1 V. However BW is also important: especially with FET input OPs the maximum slew rate is valid with a large signal at the input. So the OPA2388 with it's relatively high BW is not that bad compared to other OPs. So the AD711 with 16 V/µs but only 3 MHz GBW would be even worse.
A fast peak detector is indeed not trivial and some 6 µs rise time is fast.
With less than 1 ms required hold time one can likely reduce the capacitor - the faster the OPs the less they like capacitive loading.
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