# AD8370: Input Stage Question

Document created by analog-archivist on Feb 23, 2016
Version 1Show Document

### Q

I have a question regarding the Input stage of the AD8370. I am not sure if I
interprate the Input right.

The datasheet gives an Common Mode of typ. 3.2Vpp.
If I have a signal around 2.5V this would mean  Common Mode is allowed to be up
to 4.1V (2.5V + 1.6V) and 0.9V (2.5V – 1.6V)

The maximum Input is now given with 3.2Vpp
Maximum Input  Gain Code LG2, 1 dB compression  3.2 V p-p

What does this mean now?

I am missing a spec for the differential Input e.g. ((Vin+) – (Vin-))= 1.2 V or
so.

Otherwise in the max Ratings
Differential Input Voltage, VINHI – VINLO 2 V
Meaning ((Vin+) – ( Vin-)) = 2 V
Would mean +-1V swing around Common Mode – but this is not specified

### A

I believe that the 3.2V spec for common mode input range is incorrect, should
be vsupply/2. I will verify this with the designer. I believe that the way to
interpret the specs is that an input signal of3.2V p-p (1.6V peak) will put the
part at its 1dB compression point and that a differential voltage of +/-2V
between the two inputs will damage the device.

I have verified the following statement to be correct;

“an input signal of3.2V p-p (1.6V peak) will put the part at its 1dB
compression point and that a differential voltage of +/-2V between the two
inputs will damage the device.”

I have also verified with the designer that a dc coupled input signal should
have a dc bias of 2.5V (assuming a 5V supply). If this is done, then each input
can swing +/-1.6V for a total signal amplitude of 3.2V p-p.