OP90

–11–

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.

R1

1

R4

9.9k

R2

100k

R3

100k

3

7

6

4

2

5

1

V+

R5

100

TO CIRCUIT

UNDER TEST

–

+

OP90

Figure 12. Single-Supply Current Monitor

REV. C

+

−