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Hi experts,

I'm interested in ADA4940. My interested frequency range is DC~5kHz, and my ADC is LTC2500 with typical INL=0.5ppm and THD=-120dB in 2kHz. I've learned that the INL and THD are convertible in low frequency. The final target of my whole design is the INL<1ppm.

Since ADA4940 is the ADC driver, I don't expect it to lower the ADC's performance. (Clearly that ADA4925 & LTC6363 is better but out of stock for now)

I've got "2nd and 3rd harmonic distortion" parameters about -125dBc in 10kHz, but I don't know how to convert them into THD to evaluate their impacts.
(I know there is the -122dB THD in 50kHz on the datasheet's first page, but some other amplifiers don't show this parameter, so this question is not for this specific device only.)

Can you suggest some reference materials or tutorials? Or showing me your estimate process will be helpful.

Thank you very much!

[编辑人: RobertLiang 编辑时间: 29 Mar 2022 7:49 AM (GMT -4)]
• Hi RobertLiang,

Your question: "I've got "2nd and 3rd harmonic distortion" parameters about -125dBc in 10kHz, but I don't know how to convert them into THD to evaluate their impacts."

My response: If the 2nd and 3rd order distortion are the dominant terms are the most dominant ones, THD can be found by using this online calculator:

https://www.rfwireless-world.com/calculators/THD-Calculator-and-THD-formula.html

I've assumed the following for this example:

HD2 = -125dBc

HD3 = -118dBc

--> THD = -117.2dBc (calculated by the tool)

In this tool, you can enter up to 6 harmonics. Here I've set the other harmonics to -150dBc so that they won't affect your calculated THD.

Regards,

Hooman

• Hi Robert,

I suggest you also look at ADC driver settling time. Many SAR ADCs have a sampling glitch which the driver must recover from before the ADC starts its next conversion. This will not be an issue with the ADA4940 which has a very fast settling time but many low power ADC drivers do not have a fast settling time. As a general rule of thumb the driver settling time should be the same order of magnitude or faster as the ADC acquisition time.