8

®

INA118

APPLICATION INFORMATION

Figure 1 shows the basic connections required for operation

of the INA118. Applications with noisy or high impedance

power supplies may require decoupling capacitors close to

the device pins as shown.

The output is referred to the output reference (Ref) terminal

which is normally grounded. This must be a low-impedance

connection to assure good common-mode rejection. A resis-

tance of 12

Ω in series with the Ref pin will cause a typical

device to degrade to approximately 80dB CMR (G = 1).

SETTING THE GAIN

Gain of the INA118 is set by connecting a single external

Commonly used gains and resistor values are shown in

Figure 1.

The 50k

Ω term in Equation 1 comes from the sum of the two

metal film resistors are laser trimmed to accurate absolute

values. The accuracy and temperature coefficient of these

resistors are included in the gain accuracy and drift specifi-

cations of the INA118.

(1)

G

= 1 +

50k

Ω

R

G

FIGURE 1. Basic Connections.

DESIRED

GAIN

(

Ω)(Ω)

1NC

NC

2

50.00k

49.9k

5

12.50k

12.4k

10

5.556k

5.62k

20

2.632k

2.61k

50

1.02k

1.02k

100

505.1

511

200

251.3

249

500

100.2

100

1000

50.05

49.9

2000

25.01

24.9

5000

10.00

10

10000

5.001

4.99

NC: No Connection.

The stability and temperature drift of the external gain

gain accuracy and drift can be directly inferred from the gain

equation (1). Low resistor values required for high gain can

make wiring resistance important. Sockets add to the wiring

resistance which will contribute additional gain error (possi-

bly an unstable gain error) in gains of approximately 100 or

greater.

DYNAMIC PERFORMANCE

The typical performance curve “Gain vs Frequency” shows

that, despite its low quiescent current, the INA118 achieves

wide bandwidth, even at high gain. This is due to the

current-feedback topology of the INA118. Settling time also

remains excellent at high gain.

The INA118 exhibits approximately 3dB peaking at 500kHz

in unity gain. This is a result of its current-feedback topol-

ogy and is not an indication of instability. Unlike an op amp

with poor phase margin, the rise in response is a predictable

+6dB/octave due to a response zero. A simple pole at

300kHz or lower will produce a flat passband unity gain

response.

A

1

A

2

A

3

6

60k

Ω

60k

Ω

60k

Ω

60k

Ω

7

4

3

8

1

2

V

IN

V

IN

R

G

V+

V–

INA118

G = 1 +

50k

Ω

R

G

–

+

5

Over-Voltage

Protection

25k

Ω

25k

Ω

Over-Voltage

Protection

Load

V

+

–

0.1µF

0.1µF

+

–

V

O

R

G

Also drawn in simplified form:

INA118

Ref

V

O

V

IN

–

V

IN

+

Ref