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AD5933 External Amplification

Question asked by richeekdey on Mar 13, 2018
Latest reply on May 4, 2018 by Snorlax

Hi Guys, 


I'm building an external amplification circuit for the AD5933 eval board. The excitation signal is 2Vpp and I am amplifying it to 17Vpp. I need to amplify the signal since the AD5933 is connected to 2 parallel copper plates which are placed apart. And in my case, these copper plates will eventually be opened up so that they are no longer parallel but adjacent rather. So my aim is to have a high enough signal amplitude such that the plates have a field around them even when placed side by side. 


The issue now is that the internal op-amp (at U1-B) is saturating. Can someone please provide some sort of guidance for this? One way for me is to reduce the gain of my external amplifier but then my electric field probably won't be strong enough to have a decent electric field with the 2 copper plates placed adjacent to each other. Is there a way for me to modify the gain of the internal op-amp (U1-B). I see that the calibration resistor (RCAL) is the feedback resistor in this case (from the schematic), so I suppose changing that would help. But since RCAL also depends on Z(max) and Z(min) (which will change as the "plates" are varied), the situation gets a bit more complex. 


The saturation could be due to the noise of course and the internal op-amp could be oscillating as well (due to phase shift etc), so I could use frequency compensation at the receive side to better filter out my signal, but at this point, any suggestions would be welcome. 


My aim eventually is to "disturb" the dielectric and observe the change.