9

®

INA145

FIGURE 2. Optional Offset Trim Circuit.

FIGURE 3. Measuring Current with Shunt Resistor.

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 10

Ω and a resistor added to

the positive input terminal should be 10 times that, or

100

Ω. 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 40k

Ω. The

source impedance at the two input terminals must be nearly

equal to maintain good common-mode rejection. A 5

Ω

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, RS, is balanced

by an equal compensation resistor, RC.

Source impedances greater than 300

Ω 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 300

Ω can cause a mismatch in

the total resistor ratios, degrading CMR.

V

O = 100 ILRS

G = 100

+5V

8V

+7V

12V

+10V

18V

+15V

28V

V+

+5V

1

4

8

Load

R

G2

1M

Ω

10k

Ω

INA145

5

7

6

2

3

R

S

1

Ω

V

B

I

L

V+

Max V

B

R

G1

10.2k

Ω

V

IN

V

O

1

A1

V

IN

10

Ω

10k

Ω

Offset Adjustment Range =

±15mV, RTI

(

±1.5mV at pin 1)

R

T

100k

Ω

10

Ω

100k

Ω

+15V

V

O1

–15V

40k

Ω

40k

Ω

40k

Ω

10k

Ω

A2

40k

Ω

5

6

NOTE: Increasing the trim resistor

R

T will decrease the trim range

INA145

–

+