AD8231: Output signals / not enough gain

Document created by analog-archivist Employee on Feb 23, 2016
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I'm having problems getting the advertised gain on the AD8231.

 

You need to observe the input voltage range you need, to run AD8231 at a
specific gain to get wanted output voltage.
The easiest way to explain this, is with an example. The real question is what
swing you want at the output, what’s your supply voltage, and where your
reference voltage is.

For example:

+Vs = 5V, -Vs=0V, Vref = 2.5V and you wish to see full swing at the output.

In order to achieve this, you need to look at figure 14 in the datasheet (revB)
"Instrumentation Amplifier Input Common-Mode Range vs. Output Voltage, VREF =
2.5 V". From here, you can see that for full output swing (0 to 5V), your input
common mode needs to be between 1.7 and 3.7V (I don’t like to use 3 significant
digits, this is not a precision limit), but the further away from those limits
the better.

Obviously, the maximum voltage you can apply across the input terminals is (5V
- 2.5V) / 128 = 19mV, but you may want to stay away from that for linearity
reasons. So, if the non-inverting input is at 2.5V and the other is at  2.51V
(that’s 10mV differential, 2.505V common mode) the output should be
approximately 3.78V to GND (or 10mV*128 = 1.28V to REF). And note that we say
“approximately” because of the uncertainty of the gain and offset errors.

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