Instrumentation Amp for measuring current in the 1nA to 1mA range.

Art,

at this range, one of the largest contributing factors you are going to need to account for are the input bias currents of the device. This typically rules out current sense amplifiers, because these devices require slightly higher input bias currents to power their front ends that allow them to escape input common mode voltage restrictions. As the input bias currents are going to flow into the input pins of the device, the current flowing into the IN- pin would also pass over the shunt, so for an accurate measurement, this current needs to << than the current under test. Effectively this means you need to find a device that exhibits input bias currents in the picoamp to femtoamp region for accurate measurement at this lower bound. The current sense amplifier in our portfolio with the smallest input bias current available is the INA190, but even this amplifier exhibits Ib of 3nA worst case, which would result in ~300% measurement error when attempting to measure 1nA, before considering any additional forms of error. 

I am going to loop in our precision amps team, as this level of current is typically performed with either an instrumentation amplifier or a transimpedance amplifier. I'm sure they will be able to assist you further. 

Thanks for looking into this Carolus!

Yes please forward to the precision amps team for any other inputs or device suggestions.

Best regards,

Art

Hi Art,

Please take a look at the following thread from Marek. I think that the best current sensing solution from 1nA to 1mA is using TIA for the application. 

It may be possible to use 100Ω  shunt resistor to sense the current range with instrumentation amplifier, but there will be high %error when dealing with 1nA range low current. In addition, Vos for many of low Ib instrumentation amp is high. hence you may have to deal with current calibration between the range.

With TIA, technique, you do not have to deal with the issue.  

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1034863/ina111-picoamp-current-measurement-using-instrumentation-operational-amplifier/3828666?tisearch=e2e-sitesearch&keymatch=marek%2520lis#3828666

Please let us know if you have additional questions. We can propose a TIA circuit, if you are able to provide the available supply voltage in the system. 

Best,

Raymond 

Hello Raymond,

Thank you very much for the inputs. I discuss this with the customer. He had changed his  requirements from 1nA to 1mA to 1uA to 1mA current range.

He had done a research of his own and below are his feedback.

I choose INA821. With Rg = 12350ohm, Rshunt = 1000ohm. Vs = +-15V. Therefore the gain is 5.

Case 1:
In this case, I am using LTspice and allowing 1uA to pass through the Rshunt and expecting 1mV differential voltage at the input of the INA821 along with gain 5 output will be 5mV.

Case 2:
In this case, max current that is 1000uA. I expect a voltage drop of 1V at the Rshunt which is basically input voltage to the INA821 along with gain 5 output is 5V.

Rest of the part I will do it using 24bit ADC.

So what do you think? Will this work?

Attached is his circuit diagram.

Thanks and regards,

Art

Hi Art,

Will this work?

Your solution will work, but it will have minor voltage offset issues at low end, unless the minor offset is acceptable or may be calibrated out via two points calibration or corrections (Vout's %error will be larger due to Vos voltage because of low differential input voltage in 1mA). 

Per the case 1:

INA821 E2E Case1 10022021.TSC

Per the case 2: The Vout's %error is not an issue due to large output voltage. 

INA821 E2E Case2 10022021.TSC

Here is the Voffset specification in INA821, which is specified as RTI. To calculate RTO (refer to Output) you need to multiple the total Voffset*Gain or 5V/V in this case. 

If we sweep the measured current source from 1uA to 1mA in your application, the output linear relationship is:

Enclosed is the simulation for the sweep, where input current is IS that is swept from 1uA to 1mA and INA821's Vout is plotted. 

INA821 1uA to 1mA sweep 10022021.TSC

If you have additional questions, please let us know. 

Best,

Raymond