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| OPA378 Datasheet, PDF (11/14 Pages) Texas Instruments – Low Noise, 900kHz, 50mV, Rail-to-Rail Input/Output Precision OPERATIONAL AMPLIFIER Zerø-Drift Series | |||
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Figure 27 shows a high side current monitor. The
load current develops a drop across RSHUNT, and the
low (or load side, most negative side) of RSHUNT is
connected to the noninverting input of the op amp.
The op amp feedback forces a replica of this voltage
on its inverting input, yielding a drop across RG that is
identical to the drop across RSHUNT. RG can be sized
to provide whatever current is most convenient to the
designer based on this voltage drop. The MOSFET
conveys this current out of its drain to ground, where
a resistor RL converts the current that flows through
RG and the MOSFET back into a voltage. In this way
RL/RG sets the voltage gain of this circuit. The op
amp is powered by the zener diode, which forms a
OPA378
OPA2378
SBOS417 â JANUARY 2008
small power supply window of voltage. For the
OPA378, ±VS but must be between 1.8V and
5.5V.Two possible methods are shown to bias the
zener, the customary resistor bias and a current
mirror. To operate over at the lowest possible voltage
for the shunt supply, current mirror biasing could also
be used. In either case, note that the voltage at the
output terminal of this circuit is restricted by whatever
voltage is available at the shunt. Resistor R1 and the
diode on the noninverting input provide protection in
situations where the load might be shorted out. These
components clamp the noninverting input to within a
diode drop of the negative rail of the op amp under
short-circuit conditions.
RG
RSHUNT
zener(1)
R1(2)
10kW
V+
OPA378
CBYPASS
MOSFET rated to
stand-off supply voltage
such as BSS84 for
up to 50V.
+5V
V+
Two zener
biasing methods
are shown.(3)
Load
RBIAS
Output
RL
NOTES: (1) zener rated for op amp supply capability (that is, 5.1V for OPA378).
(2) Current-limiting resistor.
(3) Choose zener biasing resistor or dual NMOSFETs (FDG6301N, NTJD4001N, or Si1034)
Figure 27. High-Side Current Monitor
1MW
3V
1MW
60kW
100kW
NTC
Thermistor OPA378
Figure 28. Thermistor Measurement
V1
-In
OPA378
INA152
2
5
R2
R1
R2
6
VO
3
1
OPA378
V2
+In
VO = (1 + 2R2/R1) (V2 - V1)
Figure 29. Precision Instrumentation Amplifier
Copyright © 2008, Texas Instruments Incorporated
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Product Folder Link(s): OPA378 OPA2378
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