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Adjusting for input offset error using AD8230 Figure 35

Question asked by carl5055 on Mar 20, 2012
Latest reply on Mar 21, 2012 by carl5055

I need to adjust for a wide offset voltage error coming from a strain gauge bridge.  The common mode output of the bridge is 2.5V.  The normal differential output range from the bridge is 0-15mV, but initial offset error from the bridge can be as high as +/- 20mV.  I am using the AD8230 because of its remarkable input offset voltage specification and what I hope to be a very small offset voltage drift as the part warms up.  I plan to correct for bridge offset error by doing a calibration step at the beginning of operation.  The amplifier is configured for a gain of 250, so my normal amplifier output voltage swing would be 0-3.75 volts.  So far so good.  However, when bridge offset error is considered, the output swing would be -5V to +8.75V.  My supply rails are only +/- 5V.  Here are my questions.  If I were to use the method of output offset adjustment shown on Figure 35 of the datasheet to inject an output offset voltage adjustment signal in place of the return connection, as indicated by VR', would I be able to adjust for this much offset without saturating the first stage of the amplifier?  Also, are there any phase reversal issues that I might need to contend with?  In other words, would the amplifier behave properly and just saturate high or low depending on the polarity of the offset until I have a chance to inject the correct offset signal at VR'?  This would be a real win for my design if I could solve my problem in this manner.  Thanks.