Q
I’m trying to find out the CM_input resistance value of AD8131 and AD8138. I’m
driving 10 ad8131 CM inputs with a single 2.5V reference chip. The max current
this reference module can output is 30mA. I would be grateful if you could let
me know the CM_input resistance value of an AD8131 and of AD8138.
A
This parameter is actually specified, but I agree that it is somewhat difficult
to find out where it is in the data sheet.
On pages 2 and 3 of each data sheet are the specs: +/-5V supplies on page 2 and
+5V supply on page 3.
There are two bold headings in the Parameter column. At the top is +/-Din...
and 2/3 of the way down is Vocm to +/-OUT Specs. These mean that the top
section is for the differential input operation and the bottom is for the Vocm
operation. I don’t think that this is so clear in the way these specs are dealt
with.
Under the Vocm specs is a Parameter “Input Resistance”. I believe that this is
what you want.
In case you’re wondering why there is a difference in this between 10V supplies
and 5V supplies: Vocm is biased at mid-supply by FET devices operating in their
pinch-off region. These change resistance vs. bias voltage.
Also, the AD8138 impedance is higher, but it has a higher offset voltage when
Vocm floats. This is not specified on pages 2 and 3 as for the AD8131, but it
is addressed on page 10 in the paragraph: “Setting the Output Common-Mode
Voltage”.
If it is desired to have a more accurate Vout,cm, then Vocm should be directly
biased and do not rely on the internal bias.