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

[Jeepster]

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: http://www.analog.com/en/prod/0,2877,AD8369,00.html
ADC Datasheet: http://www.analog.com/en/prod/0,2877,AD9283,00.html

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]

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: http://www.analog.com/en/prod/0,2877,AD8369,00.html
ADC Datasheet: http://www.analog.com/en/prod/0,2877,AD9283,00.html

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.

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?

It _is_ in the datasheet

 

[[email protected]]

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:http://www.analog.com/en/prod/0,2877,AD8369,00.html
ADC Datasheet:http://www.analog.com/en/prod/0,2877,AD9283,00.html

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

Input Resistance 25°C I 7 10 13 7 10 13 7 10 13 kÙ

Right on page 2.

 

[john jardine]

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: http://www.analog.com/en/prod/0,2877,AD8369,00.html
ADC Datasheet: http://www.analog.com/en/prod/0,2877,AD9283,00.html

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

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.