OVP strategy for electrometer type op amp? - Page 1
Forum section: electronics / general

author: julian1
author_rank: Frequent Contributor
timestamp: October 25, 2022, 07:57:43 pm
content: Would using optocouplers in series with the supply pins of an op-amp be valuable for sensing OVP conditions? Especially where fitting external diodes (bav199, 4117 etc) would add too much leakage and capacitance?The internal ESD protection of some fet ops (lmc662, lmp6001) are apparently already internally bootstrapped or guard compensated, to achieve their low leakage/capacitance. And the activation of the internal ESD network, should be able to be sensed as excess currents on the rails.The switch control could be direct/indirect, a relay type, or depletion mode fets, or fets with isolated source for NC, as needed.

author: Gyro
author_rank: Super Contributor
timestamp: October 25, 2022, 08:52:16 pm
content: OVP protection of such low input current opamps is most easily achieved using the internal ESD protection diodes and reasonable value series resistance at the input pin. For instance, the Absolute Maximum input current of an LMC662 is 5mA. If you play safe and drop that to 500uA or 1mA max, adding a suitable high voltage (and low capacitance) 1M series resistor would protect the input up to 500V or 1kV. With the limiting 4pA max bias current and 2pA offset current of LMC662 (industrial temperature range part) a 1M resistor will only contribute 4uV of bias voltage and 2uV of offset - insignificant against the typical 1mV offset spec of the opamp.You might want to look at this picoammeter thread featuring the LMC662...https://www.eevblog.com/forum/projects/picoammeter-design/P.S. If you're interested in actually detecting the input overvoltage condition, then sensing the output hitting the rails is as good a indication as any.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 25, 2022, 08:56:21 pm
content: With most electrometer applications there is no real need to turn off the input in case of overload. One can usually have enough (e.g. some 500 kOhm) series resistance to just limit the current to a save level. The super low bias op-amps are not that low in noise, especially for the relevenat lower frequencies. The low current part is naturally not fast - the few cases where one wants relatively fast pA current measurments tend to need a custom amplifier close to the signal source, to avoid parasitic capacitance. So the simple series resistance is usually acceptable.The supply current measurement may be tricky and slow due to the decoupling capacitor at the OPs supply. The simple Optocouplers and diodes as shown wold get slowed down quite a bit.Chances are additional bootstapped diodes would be the lesser evil. The leakage gets quite small and the capacitance is hardly effective anymore.

author: julian1
author_rank: Frequent Contributor
timestamp: October 25, 2022, 10:29:55 pm
content: QuoteThanks, I am working my way through that thread, and I think it was your tests, that revealed the presence of ESD protection for lmc662 and friends. Other comments I found on the web suggested it was just a naked Fet long-tail-pair.QuoteGood point. And I think that's a bit of a killer. Also discriminating if the op is just drawing power to drive its own AB output quickly, even if lightly loaded.QuoteI am mostly interested in a buffer/without gain.If the op is bootstrapped, would placing a bav199 diode in front of the input form an effective input protection strategy? Since the PN diode leakage/offset at the input node is constant with input voltage?Or should the diodes be floated with offsets < Vforward to the input.I remember there is an op-amp datasheet that explains the motivation for bootstrapping protection diodes. But I can't find it.Edit;Ok, ADA4530 shows the approach. Using internal anti-parallel diodes to a driven guard, and where the guard has protection diodes to the rails. That provides a ESD path from input to rails.ie. ADA4530, p29. Fig 99. Using protection diodes, to the guard.https://www.analog.com/media/en/technical-documentation/data-sheets/ADA4530-1.pdf

author: David Hess
author_rank: Super Contributor
timestamp: October 26, 2022, 12:08:01 am
content: Quote from: julian1 on October 25, 2022, 07:57:43 pmI would say no because the resistance in series with the non-inverting input should be present to limit the current.

author: julian1
author_rank: Frequent Contributor
timestamp: October 26, 2022, 12:41:41 am
content: Something like this might be better,

author: Kleinstein
author_rank: Super Contributor
timestamp: October 26, 2022, 06:51:57 am
content: The optocouplers could well have low enough leakage current (they have a relative high forward voltage) and one may get away without the extra diodes in parallel. This would still be a point to check as it's usually not part of the data sheets.The bootstrapping is still missing, so a resistor from the buffers output to the right side of the opto-coupler. Because of the drop at the protection one may want an extra resistor in series to the OP-amps input.

author: julian1
author_rank: Frequent Contributor
timestamp: October 26, 2022, 07:58:54 am
content: Yes, I think guard goes to the rhs of optocoupler.

author: Gyro
author_rank: Super Contributor
timestamp: October 26, 2022, 08:30:24 am
content: Quote from: julian1 on October 25, 2022, 10:29:55 pmI'm a bit confused.The 5mA absolute maximum through the input protection network is a datasheet figure. Datasheet trumps web comments every time [Edit: I'd be interested in a link all the same].I'm not seeing decently high value resistors in any of your diagrams, trying the brute force effective shorting of the input approach is unlikely to be successful - for a fast risetime 'hit' it is probably going to join the party too late.What maximum voltage are you trying to protect against?

author: ch_scr
author_rank: Frequent Contributor
timestamp: October 26, 2022, 10:07:02 am
content: Here is a 1982 GDR circuit suggestion for the problem - input protection diodes V1 & V2 are strapped to the impedance buffer output (idea being that no potential difference equals no leakage current). Buffer output is then limited through R14 by V3 & V4 to Gnd, which then in turn limits the input through V1 & V2. R10 limits the current through the arrangement. The text mentions V3 & V4 could be series connected with zeners to allow for higher buffer output voltages. It obviously only works when the buffer has a gain of 1. The rest of the suggested DC / R meter is attached for completeness.

author: r6502
author_rank: Regular Contributor
timestamp: November 09, 2022, 12:28:39 pm
content: Hi,You may also have a look in the electrometer schematics like K617 from Keithley. low leakage Q311 diodes are used with series resistors at the input - easy done.Guido

author: KT88
author_rank: Frequent Contributor
timestamp: November 09, 2022, 08:37:43 pm
content: The question is what the input needs to be protected against. The ADA4530-1 offers already 10mA continuous and 4kV HBM. Depending on the max. voltage involved and the input current range you already get quite some wiggle room…