Problem with measuring current with opamp - Page 1
Forum section: electronics / projects

author: pisoiu
author_rank: Regular Contributor
timestamp: June 05, 2020, 09:31:16 am
content: Hi all,I am stuck with a problem and I could use some hints.My problem is: I have a device which must run in low power mode. The device has a SMPS controlled by an RTC which is separately powered by a small battery. The RTC wakes up the device once every day, for few minutes. The device has a wireless transmitter which must send some data. When powered, the device has a variable consumption ranging from 30ma to 200ma, depending on wireless device performance. When the device is in shutdown mode, SMPS is stopped, the only thing working is RTC. In this state, device draws under 1 microamp from 7.2v lithium battery. It is a 2xSAFT 3.6V 19000mah battery, not a rechargable li-ion accumulator.Because consumption in active mode varies, I need to measure it, to estimate how much energy is left in the battery. It is not possible to insert a measuring shunt after smps because the device is already built, it cannot be modified. The only place where the measuring circuit can be installed is at device input. I do not really need to measure voltage, that is pretty much constant. I need to measure current. The measuring circuit cannot be powered permanently from 7.2v battery because it draws way too much current. My device consumes under 1uA in shutdown mode and the most efficient coulomb counter I was able to find, consumes around 15uA, not acceptable for my power budget. The only way to deal with this is to power the measuring circuit from SMPS output, only when the device is in active state. But this means that in shutdown mode, the ESD protection from input pads of measuring circuit will draw power from 7.2v battery to Vcc pad of measuring circuit, which is 0v since SMPS is stopped.What I thought is to use jfet opamps. One may not be enough because if it needs to do amplification, the feedback resistors will draw power from inputs. Maybe if I install 2 of them at the shunt ends, in voltage repeater configuration, and only after that to perform differential amplification to feed an ADC, it may work. But the problem remains, I was not able to find a JFET opamp which can withstand 7.2v at input (without drawing current) when the Vcc of it is 0V.Does anyone know such opamp, or have any other idea for this?Thanks.

author: Vovk_Z
author_rank: Super Contributor
timestamp: June 05, 2020, 10:12:08 am
content: There must be a lot of low-power or ultra-low-power opamps. Some of them may suit you. You need one with rail-to-rail input. Current shunt may be in ground wire or in "+" wire. It may be a little easier with shunt in ground wire (you'll need only input-from-ground capability or in other words "Common Mode Range Extends to Negative Supply" opamp capability ).There are similar circuits in "The Art of Electronic" bible. You have only to choose a right low-power opamp.Or just google "current monitor circuit".

author: MasterT
author_rank: Frequent Contributor
timestamp: June 05, 2020, 01:30:14 pm
content: I discovered, that Texas Instruments Inc. has a product line, "Picoamp bias current". Check it out.

author: David Hess
author_rank: Super Contributor
timestamp: June 05, 2020, 06:18:48 pm
content: Instead of using operational amplifiers which have a high input impedance when powered down, the inputs can be disconnected when power is removed using FETs. This is commonly done in micropower circuits to disconnect the voltage divider which is part of a feedback loop.

author: pisoiu
author_rank: Regular Contributor
timestamp: June 05, 2020, 07:47:30 pm
content: Thanks for suggestions, so far INA186 looks what I need, but I still have to check its datasheet thoroughly.