INA181: measure current on -12V rail. looking for proposal

Clamping or disconnecting negative voltages would prevent you from measuring them.

The LMP8602 supports such negative voltages with a 5 V supply. But it might be a better idea to use an amplifier that can be supplied from ±12 V, e.g., INA199.

Hello Fabian, 

The easiest way to do this is with the INA199 as Clemens mentioned. 

If you were going to use the INA181, you would need a floating supply off of the -12V supply and you would need to level translate the INA181 output for downstream measurement. 

Best Regards, 

Joe

thanks,

can you help me understand the difference to INA181, and how to connect INA199 to measure on a negative supply rail?

cause looking at the datasheet INA199 and INA181 seem to have the same common mode voltage and differencial voltage.

the internal circuit to the REF pin also looks the same.   so i assume if i take the schematic from the screenshot above and just put the INA199 in it would also not work.

how do i need to connect the ref pin in the application?  it feels like i am missing something simple in the INA199 datasheet to understand how to measure on the negative rail.

Fabian

The REF pin voltage stays the same.

The difference is the supply voltage range (between the VS/V+ and GND pins); the INA181 supports up to 5 V, the INA199, 26 V. The common-mode voltage range is specified relative to the GND pin, so connect the GND pin to −12 V.

Clemens thank you for your correct inputs.

Hi Fabian, 

As Clemens mentioned the supply voltage is the key difference and you will need to translate you logic level for the output similar this reference design. 

https://www.ti.com/lit/ug/tidu361a/tidu361a.pdf 

I hope this helps, 

Joe

ah ok I understand.

that makes the solution much more complicated than i was hoping for. 

I did find this in parallel: Current Sensing on a Negative Voltage Supply Rail, using a Precision Instrumentation Amplifier | Analog Devices

can you have a look at it, if this would work with INA181 or 199 and still provide a analog output which is correctly GND referenced?

or even simpler this simulation: Can I use INA210 to current sense negative rail? - Amplifiers forum - Amplifiers - TI E2E support forums

in the INA210 design, it looks like it it still generates a positive voltage in relation to GND, even GND of the INA210 is tied to the -12V.

so should INA199 also work as shown in the INA210 solution? with have Vs 26V 

Hello Fabian, 

Clemens is correct in that you can set the output to an appropriate voltage level for your design via the REF pin. I would size your shunt and select a gain such that the output stays within the levels you are hoping to measure downstream (for example REF = 2.5V and your output swings from 500mV to 4.5V).

Best Regards, 

Joe