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# INA326: Minimum measurable current range

Part Number: INA326
Other Parts Discussed in Thread: INA212, INA211, , INA333

Hi Team,

For one of our design we require accurate measure of supply current (in range of uA to mA). Initially we planned of using INA211/INA212 but we found huge error at the output when the current drops below 1mA. After few search we found a reference design from TI (https://www.ti.com/lit/an/sboa210a/sboa210a.pdf) where they used an Instrumentation amplifier (INA326) to measure current in the range 10uA - 10mA. With the same reference we designed the circuit which can measure current in the range 1uA - 200uA. By doing spice simulation we were able to measure the current but we are not sure how to relate Input measured current with output voltage we couldn't find any formula to calculate output voltage in the datasheet.

Below attached the INA326 circuit from our design.

Kindly help us to verify the design,

• Hi Anand.

Q: With the same reference we designed the circuit which can measure current in the range 1uA - 200uA.

The transfer function for the part is: Vout = Gain*(V+ - V-) + Vos, where Gain = 2(110k/20k) = 11 V/V

Say the constant I=1uA, the voltage delta across the INA326 is only 1.5Ω, Vout = 11*1.5uV + Vos = 16.5uV + Vos. Since Vos is up to 124uV, the voltage offset will dominate the Vout, you do not see the correct Vout.

I increased the resistor from 1.5Ω to 1kΩ, the Vout = 11*1uA*1kΩ=11mV, and the simulation is much closer (I used dual supply rails here).

Here is what you need to do.

1. increase the resistor value or gains in the circuit, so Vout >> Vos.

2. Make sure the Vcm is operating within the allowed input voltage range for a single supply rail (ideally in the mid of Vcc/2).

3. You may need to remove the Vos, see Figure 6 or use MCU to null out the Vos. As you can see, Vp or Vout is linear to 1uA -200uA (including Vos).

4. You may also use other Instrumentation Amplifier, such as INA333 or similar, but Vos will be an issue with low delta input voltage at IA.

INA326 1u-200uA 03242021B.TSC

There may be some issues with pspice model with single supply rail, and I will find out why.

Best,

Raymond

• Hi Raymond,

Actually in my design i have two different gains. ie Max gain = (6600K/20K) 660 v/v & Min gain = (110K/20K) 11 v/v.
So for 1uA current i should get output as 660*1.5uV = 0.99mV. But i am getting different value.

And i couldn't find any formula to calculate output voltage if the circuit has two different gains like the one in this design note https://www.ti.com/lit/an/sboa210a/sboa210a.pdf .

As you said, if I increase shunt resistance or gain my current measuring range will be less (in few uA).

• Hi Anand,

If you have additional issues, please send me the schematic image, and I can hep you to check it out.

I am going to close this inquiry. If you have any questions, you are still able to open the thread.

Best,

Raymond

• Hi Raymond,

We tried INA333 as you suggested and we could able to get proper results by varying Gain and Rshunt value.