R/E/P > Dan Lavry

SAR ADC schematic

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abirkett wrote on Mon, 20 November 2006 14:24
I would firtsly try and track down the offending switching supplies and tame their EMC.

Common mode chokes at the input often help, especially if followed by small caps to ground from the signal pair.

If this is fails, then you can make a line receiver from an AD8130/AD8129 which has a CMRR of well over 100db@100kHz. I have used these devices to great effect in video systems.
See AD8129/30 datasheet:

 http://www.analog.com/UploadedFiles/Data_Sheets/AD8129_8130. pdf

Choose Zi and Zo to suit your needs. DC offset is less than 5mV
and can be trimmed by lifting U2s + terminal and injecting an offset voltage.

The 8130 is not aimed at audio. The distortion figure is too high and it just does not fit audio.

Also, one needs to see the distinction between power supply rejection and common mode rejection.

PSRR (power supply rejection ratio) is telling us the amount of noise that will get to the device output when the power supply line has noise on it.

CMRR (common mode rejection ratio) applies to balanced drive, it tells us how much signal will get to the device output when both inputs (balanced drive) move up and down together. For testing it, we often just tie the 2 inputs together, and apply a common signal to both.

Of course, noise from a power supply may find its way to the output via either mechanism: Directly from the power supply lines (PSRR) or via coupling to the inputs.

Dan Lavry


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