ADC3644: ADC3644

saritha nalluri

Part Number: ADC3644

Tool/software:

I am using the ADC3644 in my design, paired with a low-pass filter and glitch filter as per the datasheet recommendations. However, I am observing beat frequencies in the ADC output that seem to correspond to the ADC clock.

Here are the steps I’ve taken so far:

Verified the clock stability and ensured it is within the recommended range.
Designed the low-pass and glitch filters according to the datasheet specifications.
Ensured the input signal is within the specified amplitude and frequency range.

Despite these measures, the beat frequencies persist.

My questions are:

Are there specific configuration settings in the ADC3644 that could help mitigate or eliminate these beat frequencies?
Could oversampling or changing the clocking mode (if supported) reduce these artifacts?
Are there any additional filter adjustments or board-level design considerations I should explore?

ADC clock is 80Mhz and passband is 1-40Mhz .getting beat at 20Mhz .

15 days ago

0 Rob Reeder 15 days ago

TI__Mastermind 27620 points

Hi Saritha,

The ADC3644 is an unbuffered ADC and will always exhibit charge kickback onto the input sampling network or filter that you have designed.

The glitch filter helps to eliminate these clocking glitches but won't resolve these completely.

You can observe the input network differentially with an oscope for example, and these "spikes" that align with the clock frequency should be minimized.

Since the ADC is settling in time between samples from the analog input and onto the next sample, there is no reason to worry about this.

If you want to read more on this, please see this app note, pages 1 thru 3 for a good description as well:

www.analog.com/.../an-742.pdf

Regards,

Rob

0 saritha nalluri 15 days ago in reply to Rob Reeder

Prodigy 40 points

Is there any possibility to reduce these with BPF in between ADC and FDA. In current design we are using LPF and Glitch in between ADC and FDA.

0 Rob Reeder 14 days ago in reply to saritha nalluri

TI__Mastermind 27620 points

Hi Saritha,

Are you seeing, noise or spurious on the ADC capture, FFT?

If you provide a schematic on your FDA/ADC interface, I can help give some guidance as well.

Regards,

Rob

0 saritha nalluri 14 days ago in reply to Rob Reeder

Prodigy 40 points

20Mhz beat frequency at ADC. We are doing 1k FFT.

In attached images 80Mhz and 100Mhz are effecting our ADC output.

Is there any possibility to remove them?

0 Rob Reeder 13 days ago in reply to saritha nalluri

TI__Mastermind 27620 points

Hi Saritha,

What is your exact analog input frequency?

what is your exact sampling frequency?

If you are not coherently sampling properly, then you can see this type of beat frequency. I suspect this might be the case.

For more on coherent vs noncoherent sampling, please see this article:

www.planetanalog.com/.../

Regards,

Rob

0 saritha nalluri 12 days ago in reply to Rob Reeder

Prodigy 40 points

Hi,

our input frequency is 1-40Mhz

sampling frequency is 80Mhz.

and we have DDS AD9914 in our board and that is working for a sync clock f 100mhz .when we put DDS in power down mode that 20mhz is disappearing in ADC.

what might be the issue ?

0 Rob Reeder 9 days ago in reply to saritha nalluri

TI__Mastermind 27620 points

Hi Saritha,

Yes, that could be the issue. Is the AD9914 DDS the sampling clock for the ADC or something else on your board?

Is it possible to filter the 9914 with a 100MHz filter?

Regards,

Rob

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ADC3644: ADC3644