how to find input impedance of bit-per-stage ADC?

Question

I've got this Variable Gain Amplifier (AD8369) that I'm using to drive my ADC (AD9283). The problem is that the Variable Gain Amplifier requires a load impedance of 1k.

However, I'm not able to find any information from the ADC datasheet about what the ADC input impedance is.

All I know from the datasheet is that it is a "bit-per-stage pipeline type converter utilizing switch capacitor techniques."

VGA Datasheet:

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ADC Datasheet:

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Does anyone know whether the input impedance should be specified in the datasheet? If not, how do you find/calculate the input impedance of a bit-per- stage ADC?

Thanks, Jim

Joerg · Accepted Answer

Ahm, look under "Analog Input" on page 2. Heck, they even gibe you the schematic of the analog input in figure 2 on page 5.It _is_ in the datasheet :-)-- Regards, Joerg

a7yvm109gf5d1 · Answer

Input Resistance 25=B0C I 7 10 13 7 10 13 7 10 13 k=D9Right on page 2.

john jardine · Answer

The VGA only "requires" a 1kohm ADC load to terminate that particular assymetrical, low pass, balanced filter arrangement they show (in the data sheet) feeding an ADC. The VGA otherwise will run with loads from 0ohms upwards. Offhand, that particular filter looks something like Butterworth style

80MHzish, in which case the >1k AD9283 load impedance, will have trivial effect on the filter rolloff and offers even less signal loss.

how to find input impedance of bit-per-stage ADC?

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