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Lessen in-amp RFI rectification errors

Posted: 29 Dec 2008     Print Version  Bookmark and Share

Keywords:rectification  in-amp  AD8221 

One potential problem that is frequently overlooked is that of radio frequency rectification inside the in-amp. When strong RF interference is present, it may become rectified by the IC and then appear as a dc output offset error. Common-mode signals present at an in-amp's input are normally greatly reduced by the amplifier's common-mode rejection.

The best practical solution to this problem is to provide RF attenuation ahead of the in-amp by using a differential low-pass filter. The filter needs to do three things: remove as much RF energy as possible from the input lines, preserve the ac signal balance between each line and ground (common), and maintain a high enough input impedance over the measurement bandwidth to avoid loading the signal source.

Component values shown in this article were selected for the Analog Devices AD8221, which has a typical –3 dB BW of 1 MHz and a typical voltage noise level of 7 nV/÷Hz. In addition to RFI suppression, the filter provides additional input overload protection, as resistors R1a and R1b help isolate the in-amp's input circuitry from the external signal source.

View the PDF document for more information.

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