ina333 common mode vs. gain

Jim Girardeau

Other Parts Discussed in Thread: INA333

I have a very simple ina333 application with a static deltav of 10mv at the inputs, gain of 100, single supply +5,0V. Vref set to 0V.

The output is 1V as expected with the common mode above 500mv. Once the common mode drops below 500mv, the gain starts dropping off. I see this in my board (unfortunately), and also in a Tina simulation.

Datasheet says cm > 100mv should work.

Why do I see gain changes below 500mv?

In the attached simulation, if you change V5 below 500mv you will see the gain gradually drop off.

thermoamp2.TSC

over 11 years ago

0 Jim Girardeau over 11 years ago

Prodigy 50 points

Kind of answering my own question; from looking at other posts, I suspect an internal node somewhere is saturating and causing this.

0 Deepak%20murali over 11 years ago in reply to Jim Girardeau

Expert 1095 points

Hi Jim Girardeau,

Please check Figure 21 in this link http://www.ti.com/lit/ds/symlink/ina333.pdf . In that figure output voltage VS common voltage is plotted for a given gain(there the gain is G=1). Note that output voltage decreases as common mode input voltage decreases beyond 2.5V

0 Pete%20Semig over 11 years ago in reply to Jim Girardeau

TI__Mastermind 25825 points

Hello Jim,

As Deepak mentioned, here is Figure 21 from the INA333 data sheet.

With a common-mode voltage of 500mV, the output can swing from ~50mV to ~1V. As Vcm decreases, so will the linear output voltage swing (as you said, an internal node is saturating). So you'll see the output change, but it's not because the gain is changing.

0 Jim Girardeau over 11 years ago in reply to Pete%20Semig

Prodigy 50 points

I wanted to thank you for answering this question. I didn't understand the relevance of the chart.

Almost all of my problems are related to generating a negative voltage, whereas I would like to use a single supply.

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ina333 common mode vs. gain