Hi everyone,

I am using the AD8352 differential amplifier coupled with the AD9649 ADC.

In the AD9649 datasheet rev 0. one suggested schematic appears on figure 42, page 18/32. The unipolar load of the AD8352 appears to be 200 Ohms, the differential load is therefore 400 Ohms.

Yet, in the AD8352 datasheet rev B.the (unipolar) output impedance of the amplifier is given to be 100 Ohms on page 3.

Therefore the AD9649 schematic seems not to insure impedance matching : Is it done on purpose ? On the opposite is there an error in this schematic ?

Or in other words, what is the actual AD8352 output impedance ?

Thanks to those who can help.

Mathieu

Hello Mathieu,

The output impedance of the AD8352 is 100 ohms differential. To match the specs on the datasheet, you would want to terminate the amp with 200 ohms. This can be done by replacing the 200 ohms on page 18 of the AD9649 datasheet with 100 ohms. Since the ADC has a switch cap input the impedance will be changing in the sample and hold modes. Therefore it is best to put a fixed impedance differentially across the ADC inputs. The ADC inputs are normally modeled as 200 ohms inputs; that is why the AD8352 was characterized with a 200 ohm load.

Qui