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Advice on dealing with unbuffered reference inputs on AD719X converters

Question asked by Scaleman on Apr 16, 2013
Latest reply on May 3, 2013 by MaryMc

I have a great deal of experience working with the AD7730  converter and we are migrating to the AD7190 series of converters.

 

Our application is a wheat-stone bridge ratio-metric setup (weigh scale).

Currently have a AD7190/92 eval board which was very well done by the way.

 

Some of the testing done so far has revealed that careful attention is needed around the reference inputs (REFIN+-).  Unlike the AD7730 these inputs are not buffered, and the sampling charge currents are greatly influenced by resistance and capacitance before the reference inputs. This affects linearity greatly as documented in your datasheets and was quickly discovered on the eval board.  This was never a problem with the AD7730, and to be honest caught me a bit off guard.

 

This is an industrial application, and the weighscale can be several hundred feet away from the instrument itself.  So the REFIN+- sense lines (sometimes called Kelvin sense lines) for the AD converter will take its reference at the sensor which can be a long distance away. 

 

So I am looking for options in buffering the REFIN+- inputs on the converter while maintaining ratiometric tracking with the scale excitation voltage.

 

On the AD7730 I was able to work with 7V excitation across the bridge using a simple voltage divider for the REFIN+  without any additional circuitry.   

 

I am looking for some guidance on this.

 

Thank you.

 

SM

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