ADS8528: 60 Hz Input signal from Agilent 33220A signal generator is distorted after being connected to ADS8528.

Hello Stephen,

Thank you for your post.

Please provide more details about the test conditions and the behavior you're observing, such as :

1. ADC power supplies
2. Data Rate
3. Input signal amplitude
4. Input signal chain schematic
5. Scope captures validating input signal is correct
6. Output waveforms illustrating the issue.

Regards,

Hello Ryan,

I disconnected my previous setup, so I currently can't give you a scope capture.  I can only tell you that the signal was distorted when I connected it to the ADC and the signal was not distorted if I disconnected it from the ADC.

My current setup uses an Op-amp to feed the signal to the ADC. The signal does not get distorted when I use the op-amp setup.

Common specs between both setups:

Tektronix MDO3014 Mixed Domain Oscillscope (I press autoset to default the setup and I also  performed compensation on the probes before the test).

Input Signal = 2.7V 60Hz sinewave from Agilent 33220A Arbitrary waveform generator. (50Ohm  and High Impedance had the same affect)

ADC sample period=3.3us (I verified this by look at the CONVST signal)

ADC Power Supply Rail = +15/-15 (AVDD = 5V, DVDD = 3.3V, HVDD = +15V and HVSS = -15V )

Input Signal amplitude  = 2.7V

There is a 470pF capacitor to ground of each ADC change.  I removed the capacitor to see if it would make any difference and it didn't.

When I get some time, I will capture some waveforms from the previous setup.

Could you give me some ideas of what might be causing the issue?

Thanks,
Stephen

Thanks, Stephen,

stevenh said:

My current setup uses an Op-amp to feed the signal to the ADC. The signal does not get distorted when I use the op-amp setup.

Do you mean you are somehow separating the op-amp output from the ADC input to validate that the amplifier stage is not distorting the signal? How are you disconnecting the signal from the ADC?

The ADC sampling will present a dynamic load on the amplifier output. This is due to the charge injection that occurs with the internal sampling capacitor is connected and disconnected repeatedly from the input circuitry during sampling. Such a dynamic load may cause instability issues with the amplifier drive circuit.

We should rule out any grounding issues by checking that the function generator shares a proper ground connection with your circuit board.

Unless you are 50-ohm terminating the input on your board, you should be using the High-Z output impedance setting. Configuring the output amplitude and offset using the 50-ohm setting and then changing to High-Z will double the output signal voltage. You likely did not notice a difference because your amplifier supply rails are wide enough to accept both.

Best regards,

 previously verified those things (grounding, signal amplitude while at 50Ohms and High-Z).  I'll check it again when I have time.

I just wanted to connect the 33220A directly to the ADC.  I thought I had done that in the past without signal distortion, but I might not have remembered correctly or I might not have verified that was true at that time.

Previous Set up (Causes distortion at output of 33220A)

.

Current Setup (No Distortion of ADC input signal)

Ok, that is probably what it is.

It seems to be working ok with the op-amp.

Thanks,

Stephen