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# AD9695- DC Offset and harmonics Coefficient Request

I referred one of the ADI article "Wide band A/D Converter Front-End Design Considerations"

https://www.analog.com/en/analog-dialogue/articles/wideband-a-d-converter-front-end-design-considerations.html

In the Theoretical analysis section, ADC is considered as symmetrical third order transfer Function.

1. a0 (dc offset)

2. a1 (1st order harmonic coefficient)

3. a2 (2nd order harmonic coefficient)

4. a3 (3rd order harmonic coefficient)

Thanks and Regards,
Sakthi

• Hi,

I'll be moving this thread to its appropriate forum. Someone here may be able to assist you.

(+) Q&A - High-Speed ADCs - EngineerZone (analog.com)

Thanks,

Jellenie

• Hi Jellenie,

Regards,
Sakthi

• Hi Sakhti,

You can use the Virtual Eval Tool to check the simulated FFT for AD9695.

Below is the sample generated AD9695 FFT using the tool. 1300Msps, IF=170.3MHz

From here, you can derive the coefficients:

a1 = 10^(F1 amplitude/20) = 10 ^ (-1.029/20) = 0.88828024

a2 = 10^(2F1/20) = 10 ^ (-81.8/20) = 0.000081283

a3 = 10^(3F1/20) = 10 ^ (-78.03/20) = 0.00012546

If you intend to try out different input frequency, keep the F1 amplitude near -1dBFS value by adjusting the input amplitude.

Best regards,
jsalenga

• Hi Jsalenga,

How to calculate the Dc offset value a0?

As per your reply, these coefficients are derived from the measured result. ie as function of second and third order harmonics amplitude.

But how to calculate these values as a function of ADC parameters.

Is this values depends on ADC Power Supply, Clock Input Power, Amplitude and Phase unbalance into ADC or any other parameter?

Regards,
Sakthi Brundha R

• Hi Sakthi,

How to calculate the Dc offset value a0?

You can obtain this from the FFT sample I shared, a0 = 10^(DC/20) = 10^(-77.25/20) = 0.00013725

But how to calculate these values as a function of ADC parameters.

Is this values depends on ADC Power Supply, Clock Input Power, Amplitude and Phase unbalance into ADC or any other parameter?

All of these affect the ADC performance, it's possible to obtain the coefficients based on the ADC parameters but it's not an easy task to do. It's easier to refer to the simulated result to get the idea on how the ADC would perform.

If you are interested on how these tools are modeled, here is an application note related to this topic. AN-737 How ADIsimADC Models an ADC Application Note (Rev. B) (analog.com)

B
est regards,
Peevee (jsalenga)

• Thank you Peevee for your immediate reply. I think the App note will be very helpful for me.

Sakthi