INA828: INA828 saturation issue

Kartikeya Veeramraju

Part Number: INA828

Hello all,

I have run into a strange issue with the INA828 instrumentation amplifier. Below is a description of the problem.

I am using the INA828 instrumentation amplifier to measure a negative voltage, which will eventually make its way to an ADC input. To make the input positive and make my measurements proper and also to get rid of the common mode signals, I have used the instrumentation amplifier. I have connected the IC in the following configuration shown below:

The positive input was grounded as I had to flip the polarity of the signal to be able to feed it to an ADC. The sensing signal is also grounded as shown in the figure and I am using the amplifier at unity gain (Indicated by the gain resistor across pins 1,8 at infinity or NC). The output to input transfer is shown the figure below. Please note the negative voltage on the x-axis

One expects the output to saturate at 3.3 volts. But it was observed that the amplifier saturates prematurely at 2 volts. Is there an explanation for this? How can I possibly fix this issue?

over 6 years ago

0 kai klaas69 over 6 years ago

Guru 140200 points

Hi Kartikeya,

can you show a schematic?

Kai

0 Dennis Goeke over 6 years ago

TI__Genius 11135 points

Kartikeye

These issues are typically related to the common mode range of the part.
Use our Common-Mode Input Range Calculator for Instrumentation Amplifiers with your circuit values:
www.ti.com/.../INA-CMV-CALC

Its also useful to download our Pspice based simulator, TINA TI and model your circuit:
http://www.ti.com/tool/TINA-TI

Regards
Dennis

0 Kartikeya Veeramraju over 6 years ago in reply to kai klaas69

Prodigy 230 points

I'm sorry, I haven't posted the schematic due to an unexpected issue. Here's the first schematic:

After the failure of the above schematic, I implemented the schematic shown below, which didn't work as well. In fact both the results gave me the same output.

It was also noted that, upon disconnecting the positive input from the ground and feeding a signal (low voltage signal between 0 to -3.3V) to the circuit like a differential input, the problem wasn't observed (the opamp didn't prematurely saturate). I am assuming that the problem was caused due to the grounding of the positive terminal of the IN-Amp. But why should that even matter?