INA181: INA181 GAIN ISSUE

Jason Fan

Part Number: INA181

Hi Expert:

My customer is using the INA181A2 for the low side current measurement in the inverter, please see the schematic attached.

The issue is the inherent gain of INA181 is 50v/v , and customer put the external 1k resistor on the input of INA181, and the estimated gain is 45V/V,

However the actual gain is 22V/V (Vin = 103mV , Vout =2.2V) in the normal working , and the actual gain is 7V/V (Vin = 675mV, Vout = 4.7V) in the short current mode.

Could you please explain the reason ?

Thanks!

Isc回路.pdf

over 3 years ago

0 kai klaas69 over 3 years ago

Guru 140200 points

Hi Jason,

the datasheet recommends filter resistors <10R. But in your schematic I see 100k?

Kai

0 Peter Iliya over 3 years ago

TI__Mastermind 26710 points

Hello Jason,

I agree with Kai that the schematic shows 100k input resistors. The new gain when using input resistors can be calculated by multiplying the equation in Table 2 with the inherent device gain:

So with 1kΩ input resistors, the gain will change to 26.32 V/V nominally. However, there will be even more gain variation because the internal resistors can vary with absolute variation of +/20%. The equations for Table 2 account an input differential resistance of ~2.5kΩ from IN+ to IN- pins.

As for the variation of gain with short current mode, the inductors in series with the input resistors could change effective input impedance with changing input Vshunt frequency.

Hope this helps.

Best,

Peter

0 Jason Fan over 3 years ago in reply to Peter Iliya

TI__Intellectual 1705 points

Hi Peter:

Thanks for your comments.

However, even i remove the input inductors, the gain is still 8V/V in the case of short current mode, so i don't know why the gain drop a lot in this scenario.

It doesn't make sense considering the theoretical calculation.

0 kai klaas69 over 3 years ago in reply to Jason Fan

Guru 140200 points

Hi Jason,

your filtering resistances R72 and R73 are more than two decades bigger than the recommended values. What happens, if you replace R72 and R73 with 10R resistors?

Kai

0 Peter Iliya over 3 years ago in reply to Jason Fan

TI__Mastermind 26710 points

Hey Jason,

There is an input differential resistance due to the input bias stage, although it might not be explicitly shown in the datasheet. So there is resistance from IN+ to IN- pin. This changes the complete gain equation and is why the measured gain is not what you are expecting. You must use the equations in Table 2 of datasheet to calculate the effective gain from shunt voltage to INA181 output.

This gain change can be shown easily with simulation if you add an effective 2.5kΩ differential input resistor. You can also refer to this posts about it:

https://e2e.ti.com/support/amplifiers/f/14/p/735712/2718495?tisearch=e2e-sitesearch&keymatch=ina181%252525252520filter#2718495

https://e2e.ti.com/support/amplifiers/f/14/t/880174

https://e2e.ti.com/support/amplifiers/f/14/t/837825

Hope this helps.

Peter

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INA181: INA181 GAIN ISSUE