I am using the ADS62P24EVM with an external THS4509 EVM because I have a low frequency response requirement (1kHz). I was able to get the ADS to work with the on board 4509 based on the app notes and other posts to this forum in AC coupled mode. However, I was seeing -10dB rather than +10dB gain. My input signal is a single ended, 50ohm, 5kHz, 50mVpp sine wave for testing. I decided to go to DC coupled interface and use the CM control of the 4509. I connected the ADS CM pin to the 4509 pin directly with the ADS in internal reference mode. When I connect the CM pin of the external 4509, it kills the output, without it I am not seeing the proper swing around 1.5v.
I removed everything in the path on the ADS board for channel A and left the series resistors on the 4509 eval board. My Tina simulation looks ok, but my ADC model is rough, just a 7pF cap. It was working ok with on board 4509 ac coupled through a 10uF cap (inverting amp), but the attenuation was a problem in that I am only working with a 20-50mVpp signal. I went to the external dc coupled 4509 based on another post in this forum, but the CM setting was not discussed.
Thanks for any help you can offer.
Anthony
