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AD8436 Output buffer, gain in non-inverting configuration ?

Question asked by Lagaffe on Feb 28, 2013
Latest reply on Apr 4, 2013 by Lagaffe

Wonder if anyone can help me?

 

I'm trying to find a way to configure the output buffer of the AD8436 RMS-DC-Converter in a non inverting configuration, such as the gain can be set greater than unity. For a testing stage, I would be considering to use a trimmer to test some gains between 5x and 10x ... problem is, if I try to connect 2 resistors, like for instance with the input buffer to set a gain of x2 .. nothing happens, the gain continues to be unity. Some questions:

 

I'm not using any resistor in series with the OUT <----> OBUFIN+ ... is it really necessary to compensate for bias current? ... and the value of such resistor should be 16k ?

 

Have also noticed that if I try to configure the input buffer for 2x gain, using the circuit of the Fig39 of the datasheet, immediately after powering up the circuit, it works fine (using a sine wave 500mV Vp 50Hz) ...but seconds after the output RMS voltage slowly drops to the same value as if I was using a unity gain in the input buffer (?) ... if I remove the 10M resistor in the input, it seams to work fine ... is it normal?

 

I also believe that on page 14 of the datasheet, where it says:

"An external 10 Ω resistor connected from IBUFIN− to ground sets the gain to 101"

 

Shouldn't it be a 100ohm resistor ? Av= 1 + 10k/100

 

Any help would be greatly appreciated.

Regards

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