DC coupled input path on ADS62PXXEVM

Tristan,

There are 2 possible ways to do this,

1)       If you have differential signal source, you can use transformer path, take transformer T1 and T2 off, then put 0 ohm to connect the trace. Replace C13 and C66 by 0 ohm. The differential signal source can be a THS4509 EVM.

2)       If you have a single ended signal source, you can use on board THS4509 path, in this case, need to take off T5 and use the space to configure the input path of THS4509 based on the circuit in the file. You also need to take off L5 and L8. Replace C62 and C28 by 0 ohm if it is DC coupled.

Regards,

Hui Qing

Thanks, this looks like it could work for us.  The only issue is that we need to be able to simultaneously measure two low frequency single-ended inputs.  It seems that we could make the modifications you suggested, use the on-board THS4509 on channel B for one signal path, and then use an additional external THS4509EVM to feed channel A, with the additional modifications of removing R67 and installing R66 and J7.  Does this sound correct?

Thanks a lot,

Tristan

Tristan,

   I think this should be ok since you are running very low frequency, please keep the cable between two boards short as possible, so you can have two channels having similar signal path.

Regards,

Hui Qing

Hmm, that's a good point.  As it happens, we also need to be able to run at much higher frequencies.  We can be careful to make sure that the differential signal lines are of the same length to avoid phase differences, but cable resonances may be a problem at the higher frequencies.  

It seems that we could properly impedance match to two 50ohm coax differential lines by installing 50ohms on R9 and R10 of the THS4509EVM, removing R7 and R8, and also replacing R5 and R6 on the ADS62PXXEVM with 50ohms.  Doing this along with the modifications previously discussed would then give us a fully matched, DC-coupled input path.  Am I thinking about this correctly?

Thanks for you help,

Tristan