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AD8348 DC Operation

Question asked by Kevin.G Employee on Apr 12, 2010
Latest reply on Apr 26, 2010 by Kevin.G

I'd like the AD8348 to work down at DC.  The datasheet gives some information, but it's not entirely clear.  Is there an application note or an example circuit somewhere that goes into more details?  Specifically I want to know what to do with the IOFS and QOFS pins.  All the datasheet says is...

 

"Alternatively, the user can externally adjust the dc offset by driving IOFS (QOFS) with a digital-to-analog converter or other voltage source. In this case, the baseband circuit operates all the way down to dc (fPASS = 0 Hz). The integrator output current is only 50 μA and can be easily overridden with an external voltage source. The nominal voltage level applied to IOFS (QOFS) to produce a 0 V differential offset at the baseband outputs is 900 mV."

 

I've tried using a 4.7k and 1k resistor divider to create approximately 900mV (0.1uF capacitors are still in place), but the outputs are stuck at fixed voltages.  One is above 2.8v by a few mV and the other below 2.8v by a few mV (VCMO = 2.8V).  This biasing may not be the issue, but I'd like to rule it out as a possibility.

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