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AD8138 drive AD9434

Question asked by zhoukai119 on Jul 6, 2017
Latest reply on Jul 19, 2017 by zhoukai119

Hi Experts,


I'm using AD8138 to drive the AD9434 differential inputs and met a problem.

I made the schematics exactly the same as the Figure 41 ref design on AD9434 datasheet, and put AD8138 V- to GND and V+ to 3.3V (is it valid?), and connect the AD9434 CML (~1.7V) to AD8138 VOCM as I was told, but after power on, I got 2.3V on the VOCM, and after I cut the connection between CML and VOCM, I supporising discovered that the CML from AD9434 is normal (1.74V), but VOCM is at around abnormal 2.8V level (I read from datasheet then I suppose the VOCM should output 3.3/2 = 1.75V level if I float VOCM pin, is this correct understanding??) and I think if it is because the 1.74V and 2.8V mixed together to create the 2.3V (mid level of 1.74 and 2.8). When the VOCM appears 2.3V, and I feed in the 1.2Vpp sine waveform, the OUT- has the waveform appear but the OUT+ has no waveform.

And then I made my next trial to force the AD8138 VOCM to some levels from 0V to 2.5V by an Agilent power supply, but I could never get an balanced output, and when I tune power supply to drive the VOCM to below 0.8V the OUT- is always 800mVpp sine and OUT+ is always around 480mVpp sine, but when I raise the VOCM level to over 0.8V, the OUT- is getting lower, if contine raise VOCM, the OUT+ sine waveform will disappear.

Even more surprised to me, after the whole bunch of operation, I connect CML and VOCM back again, the power level at the VOCM point is always 1.64V.

Could you please guide me to use the AD8138, I have read through the datasheet AD8138 and check the reference design schematic, and checked my application by the tool Diff-Amp Calculator, but I dont think I made any obvious mistake here. could you please help me?

PS: The input signal is 2.4Vpp and become 1.2Vpp at R214, and I expect the 1.7V common mode and Vpp = 0.6V on both VIN+ and VIN- on AD9434. What should I change to adjust my current design?