INA180: Large gain error

Marvn Arcangel

Part Number: INA180

Hi Team,

Our customer is having a problem with the INA180A1. According to the datasheet, the input resistances (Rint in Figure 46 in the datasheet) are equal 20 kohms, and connecting a RC filter (with R<10 ohms) on the input would not be a problem. In the customer application, they have used resistors 1 ohm in the input filter and got quite a large gain error. They measured the resistance between the noninverting input and ground, and the total resistance was arround 20.5 kohm, therefore Rint on the noninverting input cannot be equal to 20 kohm (more likely it's 0.976 kohm). Probably this is the cause of large gain error. Was this OpAmp changed in any way to get rid of the problem?

Can you please help?

Regards,

Marvin

over 3 years ago

0 Javier Contreras4 over 3 years ago

TI__Mastermind 32340 points

Marvin,

Could you share the schematic to help debug? One cannot simply measure the resistance as there are many thing internally connected to those nodes and depending on the power conditions you will get different results.

What is the gain error? Is it too high? Did they measure the input voltage of the pins? How is the shunt connected?

0 kai klaas69 over 3 years ago

Guru 140200 points

Hi Marvin,

I aggree with Javier. Gain errors are most often caused by shunt resistor issues. The shunt resistance looks bigger because of the soldering tin on the solder pad. A 4-wire shunt resistor can do the trick then.

Kai

0 Marvn Arcangel over 3 years ago in reply to Javier Contreras4

TI__Genius 12605 points

Hi Javier,

Here is the schematic from the customer.

I hope this helps.

Regards,

Marvin

0 kai klaas69 over 3 years ago in reply to Marvn Arcangel

Guru 140200 points

Hi Marvin,

can you still answer Javier's questions, please?

Javier Contreras4 said:
What is the gain error? Is it too high? Did they measure the input voltage of the pins? How is the shunt connected?

And a layout would be helpful to see how the shunt is connected to the load and to the INA180.

Kai

0 Marvn Arcangel over 3 years ago in reply to Marvn Arcangel

TI__Genius 12605 points

Hi Javier,

Here is the follow up details from the customer:

-Gain error is around 4.5% with a 1-ohm resistor in the input filter. Everything is alright when there are no filter resistors on the input.

-They haven't tried to measure the voltage.

-The shunt resistor is connected via goldpins - I have attached a design of the board with the shunt resistor.

I hope this helps.

Regards,

Marvin

0 Marvn Arcangel over 3 years ago in reply to kai klaas69

TI__Genius 12605 points

Hi Kai,

Thanks for the reminders.

Regards,

Marvin

0 kai klaas69 over 3 years ago in reply to Marvn Arcangel

Guru 140200 points

Hi Marvin,

this is strange and should not happen. I guess the INA180 got damaged somehow. Take a fresh INA180 and check again.

Kai

0 Carolus Andrews over 3 years ago in reply to kai klaas69

TI__Mastermind 22650 points

Marvin,

I agree with Kai that this is abnormal. What is the test criteria they are using here to establish a gain error of 4.5%? Are they testing two points and comparing to the ideal? If so, what points are they testing?

0 Javier Contreras4 over 3 years ago in reply to Carolus Andrews

TI__Mastermind 32340 points

Hello Marvin,

I am looking the layout and I do not see the filter or how this is the INA180 From the pinout and the Rshunt it does not look like either pinout option.

+1 Marvn Arcangel over 3 years ago in reply to Javier Contreras4

TI__Genius 12605 points

Hi Javier,

The customer has found the issue regarding the large gain error. They are measuring the frequency characteristics without a voltage divider, which generally is used on the board. When the divider resistors are soldered, they got large errors in current measurements. They found out that the voltage divider was connected to one of the resistors in the input filter between the resistor and a capacitor and the problem disappeared when the connection was cut and soldered the divider's resistor directly to the gold pins.

Thank you for your support.

Regards,

Marvin

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INA180: Large gain error