10

®

INA148

Preferably, the offset trim voltage applied to the Ref pin

should be buffered with an amp such as an OPA237

(see Figure 3). In this case, the op amp output impedance is

low enough that no external resistor is needed to maintain

the INA148’s excellent CMR.

INPUT IMPEDANCE

The input resistor network determines the impedance of

each of the INA148’s inputs. It is approximately 1M

Ω.

Unlike an instrumentation amplifier, signal source imped-

ances at the two input terminals must be nearly equal to

maintain good common-mode rejection.

A mismatch between the two inputs’ source impedances will

cause a differential amplifier’s common-mode rejection to

be degraded. With a source impedance imbalance of only

500

Ω, CMR can fall to approximately 66dB.

Figure 4 shows a common application—measuring power

supply current through a shunt resistor (RS). A shunt resistor

creates an unbalanced source resistance condition that can

degrade a differential amplifier’s common mode rejection.

Unless the shunt resistor is less than approximately 100

Ω, an

additional equal compensating resistor (RC) is recommended

to maintain input balance and high CMR.

Source impedances (or shunts) greater than 5k

Ω are not

recommended, even if they are “perfectly” compensated.

This is because the internal resistor network is laser-trimmed

for accurate voltage divider ratios, but not necessarily to

absolute values. Input resistors are shown as 1M

Ω, however,

this is only their nominal value.

In practice, the input resistors’ absolute values may vary by

as much as 30 percent. The two input resistors match to

about 5 percent, so adding compensating resistors greater

than 5k

Ω can cause a serious mismatch in the resulting

resistor network voltage divider ratios, thus degrading CMR.

Attempts to extend the INA148 input voltage range by

adding external resistors is not recommended for the reasons

just described in the last paragraph. CMR will suffer a

serious degradation unless the resistors are carefully trimmed

for CMR and gain. This is an iterative adjustment and can be

tedious and time consuming.

FIGURE 4. Shunt-Resistor Current Measurement Circuit.

FIGURE 5. AC-Coupled Difference Amplifier.

1M

Ω

50k

Ω

52.6316k

Ω

50k

Ω

2.7778k

Ω

1M

Ω

R

C

R

S

I

L

V

CM

7

3

200V

2

6

41

A1

INA148

–15V

V

O

+15V

LOAD

V

O = IL • RS

Make R

C = RS if RS ≥ 100Ω

1M

Ω

50k

Ω

52.6316k

Ω

50k

Ω

2.7778k

Ω

1M

Ω

7

3

2

6

41

A1

INA148

–15V

V

O

+15V

Typical CMR: 50Hz = 59dB

60Hz = 61dB

400Hz = 78dB

NOTE: (1) Metallized polypropylene,

±5% tolerance.

C

1

4.7

µF(1)

250V

C

2

4.7

µF(1)

250V

V

IN

+

V

IN

–

V

CM = 200Vpk

V

O = (VIN – VIN)

–

+