OP90 Datasheet, PDF (11/12 Pages) Analog Devices – Precision Low-Voltage Micropower Operational Amplifier

Single-Supply Current Monitor
Current monitoring essentially consists of amplifying the voltage
drop across a resistor placed in a series with the current to be
measured. The difficulty is that only small voltage drops can be
tolerated and with low precision op amps this greatly limits the
overall resolution. The single supply current monitor of Figure 12
has a resolution of 10 µA and is capable of monitoring 30 mA of
current. This range can be adjusted by changing the current
sense resistor R1. When measuring total system current, it may
be necessary to include the supply current of the current moni-
tor, which bypasses the current sense resistor, in the final result.
This current can be measured and calibrated (together with the
residual offset) by adjustment of the offset trim potentiometer,
R2. This produces a deliberate offset that is temperature
dependent. However, the supply current of the OP90 is also
proportional to temperature and the two effects tend to track.
Current in R4 and R5, which also bypasses R1, can be accounted
for by a gain trim.
OP90
+
TO CIRCUIT
UNDER TEST
–
I_TEST
R1
1⍀
V+
3
7
6
OP90EZ
2
4
5
1
R2
100k⍀
V_OUT = 100mV/mA (I_TEST)
R4
9.9k⍀
R5
R3
100⍀ 100k⍀
Figure 12. Single-Supply Current Monitor
REV. A
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