OFFSET TRIM
The INA145 is laser-trimmed for low offset voltage and
drift. Most applications require no external offset adjust-
ment. Figure 2 shows an optional circuit for trimming the
offset voltage. A voltage applied to the Ref terminal will
be summed with the output signal. This can be used to null
offset voltage. To maintain good common-mode rejection,
the source impedance of a signal applied to the Ref
terminal should be less than 10Ω and a resistor added to
the positive input terminal should be 10 times that, or
100Ω. Alternatively, the trim voltage can be buffered with
an op amp such as the OPA277.
INPUT IMPEDANCE
The input impedance of the INA145 is determined by the
input resistor network and is approximately 40kΩ. The
source impedance at the two input terminals must be nearly
equal to maintain good common-mode rejection. A 5Ω
mismatch in impedance between the two inputs will cause
the typical common-mode rejection to be degraded to ap-
proximately 72dB. Figure 7 shows a common application
measuring power supply current through a shunt resistor.
The source impedance of the shunt resistor, R
S
, is balanced
by an equal compensation resistor, R
C
.
Source impedances greater than 300Ω are not recommended,
even if they are perfectly matched. Internal resistors are laser
trimmed for accurate ratios, not to absolute values. Adding
equal resistors greater than 300Ω can cause a mismatch in
the total resistor ratios, degrading CMR.
10kΩ
5
40kΩ
–
V
IN
40kΩ
10kΩ
10Ω
V
IN
40kΩ
+
A2
6
V
O
40kΩ
A1
INA145
1
V
O1
R
T
100kΩ
100kΩ
10Ω
NOTE: Increasing the trim resistor
R
T
will decrease the trim range
–15V
+15V
Offset Adjustment Range =
±15mV,
RTI
(±1.5mV at pin 1)
FIGURE 2. Optional Offset Trim Circuit.
V+
+5V
7
R
S
1Ω
2
R
G1
10.2kΩ
5
R
G2
1MΩ
G = 100
V
B
I
L
10kΩ
6
Load
3
INA145
4
1
8
V+
+5V
+7V
+10V
+15V
Max V
B
8V
12V
18V
28V
V
O
= 100 I
L
R
S
FIGURE 3. Measuring Current with Shunt Resistor.
®
9
INA145
TI [ TEXAS INSTRUMENTS ]
