ADC's low frequency region noise, ADS62C17

Hi Duan,

Can you share your schematic?

Thanks,
Matt Guibord 

my schematic is quit simple, like this. the clock is from a AD9516 LVPECL output.

THX!

Hi Duan,

Thanks. This is a little strange and I'm not sure where it is coming from. To help narrow it down, can you provide the following measurements?

1. Common-mode voltage at the ADC input (both pins), with and without signal applied
2. Common-mode voltage at the ADC clock input (both pins)
3. How does the noise change when no input is applied?
4. How does the noise change when a higher frequency is applied?

Thanks,
Matt Guibord

thanks for your attention, you will be my saver!

First, I post my signal generator spectrum, since its specs is relavant with measurement result:

5MHZ:

14MHZ:

44MHZ:

you can see the performance is not so good, but it is my only.

Secondly, I post the spectrum without input signal:

 then, 5 MHZ signal with input level 2Vpp, 1.4Vpp and 1Vpp:

then 7 MHZ input, 2Vpp, 1.4Vpp and 1Vpp:

then 14MHZ, 2Vpp, 1.4Vpp and 1Vpp:

then 44MHZ:

As for OCM, the ADS62C17's input OCM is 1.49V, both P and N pins and with and without input signal. it's from the ADS62C17's VCM pin.

And the clock input pin's bias is 1.43V, both P and N pins.

I hope to get your help!

THX!

Donald

Hi Donald,

There doesn't appear to be any logical pattern for the noise. For instance, if it was related to the analog input level, you would expect the noise to be largest when the input signal is at full scale. Instead, we see that sometimes the noise is largest at full scale input and other times it's larger when the signal is backed off.

Is the noise always present or does it vary from capture to capture? Have you made any other progress?

Thanks,
Matt Guibord 

Hi Matt;

You are probably right. Actually I noticed those pattern, and I will double check it. By the way, could those noises relative to some amplifier unstability? we have a op amp before the ADC, which has a very wide bandwith, could some PCB stray capacitances led to those noises?

THX!

Donald

Hi Donald,

It is possible, but I would expect amplifier oscillations to occur at a higher frequency, not at DC.

It could be coming from a noisy power supply, have you checked your amplifier power supplies?

Regards,
Matt Guibord 

Hi, Matt;

I will check it but I doubt it, if the noise is from power system, it would have nothing to do with the input signal so it will show up even there is no input signal. Am I right?

Donald