QHow much is the current in REF IN when, for example, Vin+ = 15V and Vref=1V? In
the datasheet I have only found the data under test condition Vin+=0 and
Vref=0, but how can I calculate the current with other voltage value? Can I
works with simplified schematics on page 33?
AThe input current for the REF pin is not zero and will vary with Vin and Vref,
therefore the REF pin should be driven from a low impedance source ( typically
less than 100ohm source impedance ) in order to minimize errors. Referring to
the simplified schematics in figure 31, the REF input current is a combination
of the input bias current of A3 and the current due to the voltage drop across
the 10k resistor between REF and the non-inverting terminal of A3. The input
bias current of A3 is of the order of nA and can safely be ignored.
Therefore to calculate the REF input current we need to know the voltage at the
output of A2, the voltage applied to REF and the source impedance of whatever
supplying the REF voltage.
You can use our "Interactive design Tool" for the AD620 to calculate the output
of A2 for any power supply and input signal:
AD620 Interactive Design Tool
It's not allowed to apply +15V to Vin+ you must operate the AD620 from +/-18V
supplies. Assuming a common mode voltage of 15V, gain of 1 and differential
input voltage of 0V, the output of A2 is 14.5V. If we assume a reference
source impedance of zero, and REF = 1V, the input current is
( 1V - 14.5V ) / 20kohm = -675uA.
The negative sign indicates that the current flows out of the AD620.
Note that if the source impedance was 100ohms, the input current would be:
( 1V - 14.5V ) / 20.1kohm = -671uA
and the voltage applied to REF is actually ( 671uA x 100ohms ) + 1V = 1.0671V
So you can see the importance of a low source impedance.