Ultra low current Instrumentation Amp - Page 1
Forum section: electronics / general

author: mfusaro
author_rank: Newbie
timestamp: May 22, 2021, 04:24:22 am
content: I'm looking for a Instrumentation Amp that has a supply current around 1 uA, no other special requirements. The lowest current I could find is 30 uA. There are plenty of sub uA opamps, why do Instrumentation Amps need so much current? Thanks!

author: Kleinstein
author_rank: Super Contributor
timestamp: May 22, 2021, 09:59:01 am
content: Internally an instrumentation amp is made from something like 2-3 OPs. In addition to the amplifier there are also resistors to set the gain, that need current when the input votlage is not zero. So it would need rather high resistors and thus causing high noise. High resistors are also not so easy to make on a chip. It is not impossible, but relatively expensive.2 OPs + resistors may be an option - especially with low requirements on accurcy.

author: Marco
author_rank: Super Contributor
timestamp: May 22, 2021, 11:34:08 am
content: What's the common mode voltage and source impedance? The single opamp+mosfet high side current sense circuit is a difference amplifier, CMRR is not dependent on resistor matching either, only gain is.

author: voltsandjolts
author_rank: Unknown
timestamp: May 22, 2021, 01:31:37 pm
content: The lowest power amplifier is one that is not thereIf you are using a sigma-delta ADC, maybe you don't need the IAmp.

author: mfusaro
author_rank: Newbie
timestamp: May 22, 2021, 08:17:02 pm
content: Thanks for all the responses. This is a low side 4 wire current sense application. The common mode voltage is near zero and the sense resistor is 0.1 ohm. The output of the amp will feed the ADC input of a ultra low power microprocessor that is sleeping.

author: julian1
author_rank: Frequent Contributor
timestamp: May 22, 2021, 08:28:23 pm
content: Consider, using a 2x power ops (dual) for the hi and lo sense of the 0.1ohm sense resistor. Then feed those to separate adc channels and compute the difference in software. That avoid the feedback resistors present in the diff/ina amp that draw current.

author: Marco
author_rank: Super Contributor
timestamp: May 22, 2021, 09:56:45 pm
content: For a 0.1 Ohm shunt you don't really need buffers, but the ADC will require amplification.

author: Zero999
author_rank: Super Contributor
timestamp: May 23, 2021, 08:25:42 am
content: Why not only power the amplifier, when a reading is being taken? There are instrumentation amplifiers available with an enable/shutdown, I found the LTC6915 by doing a quick Google, but they often consume more than that, when disabled. You could power the 30µA instrumentation amplifier you mentioned off an output pin, which is only high for long enough for it to stabilise, before taking the reading.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 23, 2021, 10:22:06 am
content: Some of the SD ADCs have internal gain and differential input and can directly work with quite low signals. So one may not need an amplifier.If amplification is really needed, this can often be relative to the sense terminal at the shunt, with only 1 OP - it is still not sure if a low power OP is lower noise than the ADC directly.

author: mfusaro
author_rank: Newbie
timestamp: May 24, 2021, 05:10:01 am
content: Thanks for your help, all good ideas. this was posed to me as a design challenge but everytime I proposed something new requirements were added. I tapped out.

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 24, 2021, 01:02:14 pm
content: Since we are talking about ultra low consumption, possibly we don't need Instrumentation amp, but can live with usual opamp amplifier (connected as differential amp)?