Problem using EVAL-7768FMCZ + Datasorm DAQ

Question

Hello, 
 we are using EVAL-7768FMCZ demo board with DataStorm DAQ setup as per this guide github.com/.../Run-the-Demonstration-Remotely-via-Ethernet-(AD7768) with the only difference that we are using a single channel function generator connected to 0+ and GND (0- is shorted to GND). The generator output a 1KHz sinusoid 100 mVpp and when we read data from the remote client (ie your IIO oscilloscope, Python script ...) we see a sinusoid with the right frequency but with some noise then, when we raise to 500 mVpp, the sinusoid get clipped on the lower side and distorted on the top (as you can see on the attached image). The problem is not present when we add a 1V offset on the generator. Where are we going wrong ? 
 Thanx

mthoren_adi · Answer

Hi Scardig, 
 It looks like your signal is actually clipping - the fact that it's "squishy" clipping (rather than being cleanly shopped off) indicates that it's the amplifier (although similar effects can happen with ADC inputs, especially if they include onboard buffer amplifiers.) 
 The EVAL-AD7768 user guide and schematic is a little hard to follow, but it looks like the inputs are by default routed through the ADA4896-2 amplifiers, with a default negative rail of ground. This means you must observe the amplifier's input and output headroom requirements - refer to its datasheet. 
 But it looks like an easy solution is to insert a large (1 uF or 10 uF) in series with your input signal. The inputs are biased to VCOM (which is set by the AD7768) through 10 k resistors. (I know this works, I've tried it before, I am pretty sure it was an unmodified EVAL-AD7768.) 
 I'm moving this to the Precision ADCs forum, Reference Designs is mainly for Circuits from the Lab, the EVAL-AD7768 is a standard eval board. 
 -Mark