APPLICATION INFORMATION
Figure 1 shows the basic connections required for operation
of the INA128/INA129. Applications with noisy or high
impedance power supplies may require decoupling capaci-
tors 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 8Ω in series with the Ref pin will cause a typical
device to degrade to approximately 80dB CMR (G = 1).
SETTING THE GAIN
Gain is set by connecting a single external resistor, R
G
,
connected between pins 1 and 8:
INA128:
accurate absolute values. The accuracy and temperature
coefficient of these internal resistors are included in the gain
accuracy and drift specifications of the INA128/INA129.
The stability and temperature drift of the external gain
setting resistor, R
G
, also affects gain. R
G
’s contribution to
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 INA128/INA129
achieves wide bandwidth, even at high gain. This is due to
the current-feedback topology of the input stage circuitry.
Settling time also remains excellent at high gain.
NOISE PERFORMANCE
The INA128/INA129 provides very low noise in most appli-
cations. Low frequency noise is approximately 0.2µVp-p
measured from 0.1 to 10Hz (G
≥
100). This provides
dramatically improved noise when compared to state-of-the-
art chopper-stabilized amplifiers.
V+
0.1µF
G
=
1
+
50kΩ
R
G
49. 4kΩ
R
G
(1)
INA129:
G
= 1+
(2)
Commonly used gains and resistor values are shown in
Figure 1.
The 50kΩ term in Equation 1 (49.4kΩ in Equation 2) comes
from the sum of the two internal feedback resistors of A
1
and
A
2
. These on-chip metal film resistors are laser trimmed to
INA128:
G=1+
50kΩ
R
G
INA129:
G=1+
49.4kΩ
R
G
–
V
IN
2
Over-Voltage
Protection
7
INA128, INA129
A
1
40kΩ
25kΩ
(1)
R
G
8
25kΩ
(1)
A
2
40kΩ
40kΩ
5
Ref
A
3
6
+
INA128
DESIRED
GAIN (V/V)
1
2
5
10
20
50
100
200
500
1000
2000
5000
10000
R
G
(
Ω
)
NC
50.00k
12.50k
5.556k
2.632k
1.02k
505.1
251.3
100.2
50.05
25.01
10.00
5.001
NEAREST
1% R
G
(
Ω
)
NC
49.9k
12.4k
5.62k
2.61k
1.02k
511
249
100
49.9
24.9
10
4.99
R
G
(
Ω
)
INA129
NEAREST
1% R
G
(
Ω
)
NC
49.9k
12.4k
5.49k
2.61k
1k
499
249
100
49.9
24.9
9.76
4.87
1
40kΩ
–
V
O
= G • (V
IN
– V
IN
)
+
NC
49.4k
12.35k
5489
2600
1008
499
248
99
49.5
24.7
9.88
4.94
Load V
O
–
+
V
IN
3
Over-Voltage
Protection
4
NOTE: (1) INA129: 24.7kΩ
V–
Also drawn in simplified form:
–
V
IN
R
G
+
V
IN
INA128
Ref
V
O
0.1µF
NC: No Connection.
FIGURE 1. Basic Connections.
BURR-BROWN [ BURR-BROWN CORPORATION ]
