How to set the Cin capacitor value at an SAR ADC input ?

makdam ASFOUR

Other Parts Discussed in Thread: OPA2836, OPA890

Hello

I'm driving a 16 bit SAR ADC (AD7626 from ADI! ) with OPA890 or OPA2836 amplifiers in a multiplexing system (@10MHZ), The time constant of RC filter between the amplifier output and ADC input is about 8ns . The filter capacitor value is a the key point for good performance in ADC and driver gain linearity. Can you explain this ? I know only that this capacitor shoud be at least 20 times the CSH capacitor. in our application 820pF is the optimum value

thanks in advance for answering .

over 12 years ago

0 Kang Hsia over 12 years ago

TI__Guru* 78744 points

Hi Makdam,

You could probably try the precision data converter forum or the amplifier forum for suggestions. One forum post has discussion on SAR ADC drivers

http://e2e.ti.com/support/data_converters/precision_data_converters/f/73/p/217959/770897.aspx#770897

it mentioned the following literature from our TI engineers:

Designing SAR ADC drive circuitry, Part I: A detailed look at SAR ADC operation. Analog Zone. Download at:http://www.analogzone.com/acqt0221.pdf

Designing SAR ADC drive circuitry, Part II: Input behavior of SAR ADCs. Analog Zone. Download at:http://www.analogzone.com/acqt1003.pdf

Designing SAR ADC drive circuitry, Part III: Designing the optimal input drive circuit for SAR ADCs. Analog Zone. Download at: http://www.analogzone.com/acqt0312.pdf

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How to set the Cin capacitor value at an SAR ADC input ?