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AD8336 VGA problems

Question asked by andrefleitao on May 7, 2011
Latest reply on Sep 14, 2011 by SpiceMan

Hello people. I am an electronic student and i have been developing a test board for the AD8336 to control the gain of a signal. I am using a pre-amplifier set to 12 dB (4x) and a +-12 V supply. To control the gain it is supposed to use the GPOS and GNEG pins (VGAIN=VGPOS-VGNEG). One of the problems is that, even though i have for example 163 mV of VGAIN the amplifier does not amplify like Figure 78 of the datasheet says (in fact it doesn't amplify at all :S). You can look at the two images: one before powering the amplifier where the input of the amplifier is a perfect sinewave on yellow and the output on blue (not yet amplified); and the other image with the VGA powered where the input is distorted and with a DC component (not a sinewave) and the output has some small waves but no amplification is noticed.


The other problem is that i am using the configuration present on Figure 3 of the attachments (using a DAC to control the voltage of GPOS and with a fixed VGNEG, both of them preceded by voltage divider with 1% precision MOhms resistors) -> it's clear if you look at the image. The voltages on the control pins are not how i expected. Do this 2 pins (GPOS and GNEG) pull any current? Cause that is bad for the voltage divider topology i chose.


I have also checked if the VGA is properly powered with +-12V and GND.


Could you look at the my board schematics to see if you find any possible mistake?