ADS4129: ADC Input Signal Issue on OFDM Waveform

Ok, your set-up is essentially FPGA > DAC5672 (pnt A) > THS3091 (pnt B) > cable > THS3091 (pnt C) > ADS4149.  Point A matches with point B and with point C only if the load of the ADC is not present.

I am not quite following the "matches" statement.  The plots above look very similar.  I do not know which color represents which point.  They seem pretty close with perhaps a bit of time dependent amplitude variation and a bit of noise floor difference.  To my eye it is not egregious and may be expected by the additional amplifiers and loading.

The situation sounds like a loading issue for the THS3091.  The ADC transformer input provides an AC 50 ohm impedance, but does not provide a DC path to ground.  Please ensure the THS3091 is properly terminated/loaded for optimum operation.  In the datasheet, for 50 ohm load, it recommends to add a series 50 ohm resistor.  You may also need to add a high impedance (say 1 kohm) resistor to ground prior to the AC coupling cap.

The OFDM signal has a higher peak to average ratio compared to single tone signals...roughly 10 dB.  It is possible that the amplifiers do not have enough headroom to handle the peaks of the OFDM signal without introducing distortion.  You can tray to add a 10 dB pad at point B to see if that impacts the loading issue.

--RJH

Hi

Thanks for your sooner reply. The setup that you mentioned is correct. Attaching the schematics of DAC and ADC sections for your reference -

By the word "matches", I mean to say that the waveforms should overlap each other. But as you can see, somewhere they are overlapping, and somewhere they are above/below each other. This causes the corruption of values.
Also, the "yellow" waveform is the "DAC Output" signal probed at point B, and the "blue" waveform is the ADC Input probed at Point C (RF transformer I/P).

Also, our loading issue is at Point C (i.e. the ADC I/P side). But, you have suggested to add a 10dB pad at Point B (i.e. DAC Amplifier O/P).
So, can you explain why?

Perhaps you can observe the spectrum in the frequency domain to see how the "corruption of values" manifests in the frequency domain.  I anticipate that it would look like spectrum regrowth (i.e. ACPR) due to intermodulation.  This type of distortion is expected with an OFDM signal operating through amplifiers and other active circuitry.

I recommend placing the pad a point B to protect the second TH3091 and the ADC.  The thought is that overdriving the second THS3091 with the load of the ADC may be the culprit.

--RJH

Hi,

I tried increasing the headroom for OFDM signal by applying +15V/-15V to the both the Amplifiers (THS3091). But it did not create any impact on the signal waveform. The issue was still there.

I also tried adding 10dB pad at point B (i.e. TX amplififer) in "PI" and "T" configurations. The  signal shape deteriorated even more. It did not help in resolving the issue.

So, how to proceed further in the current scenario.

Also, I would like to know whether using the Interpolation DACs will create any impact in my current application or not?

Rishabh,

Do you have ADC low latency mode enabled or disabled? Can you try enabling it to disable all digital functions.

Regards,

Jim