Part Number: THS4551
We have designed custom PCB for video applications. In these PCB, we used THS4551 FDA to drive the ADC(AD9649).
My question is related to the Test signals to fed THS4551 through function generator for Initial testing before integrating it in the system.
Input Requirement:
VIN+ = (0.9V ~ 4.3V)
VIN-=0.9V (Used as Offset)
Configurations:
Schematics is attached for the VCM and Charge KickBack ciruicts.
Please provide the Test signals scheme to standalone test the FDA.
Morning Mohsin,
Let's discuss your ckt first.
Looking at the input side,
1. Those DNP 330ohm are shorts I am guessing, opens will open the DC feedback loop
2. Assuming those are shorts, C49 is across the summing nodes - not an RC filter element then, but a noise gain shaping element with the 46pF feedbacks
Best to start building a TINA sim to iterate on the ckt block before test issues discussed (what does the source look like?
The output filter is a simple cascaded first order sections? You can do better with a 2nd order RLC in terms of flatness to stopband transition if you would like.
Anyway, your DC input side should be midpoint on the expected signal input range. Vg1 is a bit of an odd signal, you really wan that DC level at mid scale of your expected VG2 range - which you have set to 2.1. Here I just ran a DC operating point with the supplies changed to no clip. Which is was before.
but first, what does the ADC need, Well it is pretty forgiving on input Vcm voltage, I would shift that up to 1V to give more operating headroom in the THS4551. Also, it accepts a max 2Vpp differential - which is Vcm+/-0.5V - you are exceeding that right now, You can operate the FDA on 0 and 3.3V and get this work, but you need to attenuate your Vpp input to stay at about 1.9Vpp diff max. Also, add those ADC input impedance terms to your sim.
