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OPA2388: peak-hold detector circuit affected by opamp input leakage current
brumbarchris:
Hello,
I am struggling with a peak detector circuit based on the OPA2388, and it just does not want to work!
I have eventually reduced it to the schematic in the first attachment. The second attachment indicates what I get from it, when appropriately stimulated with a repeatable input signal.
I do not understand why the C331 capacitor is not holding the charge, and the only thing I can blame it on is on the input leakage current of the opamp input. The datasheet indicates am input bias current of less than 1nA, but it indicates this is with an Ri=100kOhm. I am not sure what this Ri is referring to, is it an additional series resistance that needs to be added to the input of the opapm? If I consider the leakage of both opamp inputs connected to the C331 (about 1nA each) and the maximum reverse current of the diode (30nA) I estimate the decay on the capacitor must not be greater than 35mV during the 1ms period (which is the repetition rate of the input signal).
I must mention that I am explicitly not probing the capacitor voltage directly with the oscilloscope, to prevent the probe impedance from discharging the cap. I am only able to look at the output of the second opamp (which is in buffer configuration.
So any ideas? Is it not the OPA2388 the right one for the job? I have replaced both the opamp IC and the diode, just in case I was dealing with a faulty one, but that was not it.
Best regads,
Cristian
Kleinstein:
The diode may not be the very best choice. Like other very fast diodes it has a significant leakage current. However the choice is not that easy. Low leakage diodes tend to be slow and fast diodes have quite some leakage. So its about finding a good compromise. The choice depends on the application / speed and hold time needed. For the relatively fast cycle like show the diode may be OK.
The input bias current for the OPs is valid with the OP in linear mode - in this case the first OP would no longer be in linear operation and can have a much higher input current. The OPA388 does not look like it has the common protection diodes, but there can still be additional current.
The usual way would be to have the feedback for the 1 st OP from the output of the first OP. To make this work, one may have to use 2 different OPs, with the 2nd OP considerably faster than the 1st. For the 1 st OP one may have to look for good capacitive drive capability.
To speed up the reaction one may have to add an alternative feedback parth with a diode and resistor, so that the OP does not start from saturation, but something like 1 V below the peak.
It usually also needs some kind of reset circuit, if there is not that much drift downwards. :-DD
awallin:
what capacitor are you using? Try a 'plastic' PP or PTFE low leakage capacitor?
electrometer op-amps have maybe 10s of fA input bias currents..
electrometer datasheet or app-notes probably have guard-ring ideas explained, to minimize leakage of the charge through the PCB material.
Kleinstein:
For the capacitor 1 nF is already quite large for a fast peak detector, but it can be Ok. Anyway this effects the response to short pulses and not the hold time. On the scope pictures the rise looks quite fast - so maybe the capacitor is much smaller than planed ?
The observed discharge (some 400 mV in 400 µs and thus some 1 µA) is way more than the usual board and diode leakage.
There is usually no need for PTFE caps. NP0 ceramic should be good enough: about as good as PP. With TDK brand ones I got a loss factor even about 5 times lower, not much different from what PTFE caps should be.
If fast reaction and a long hold time would be needed one can use 2 stages in series: a second slower one with a larger cap for a longer hold time.
StillTrying:
Probably something to do with this, page 20.
www.ti.com/lit/ds/symlink/opa2388.pdf
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