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.
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, RG, also affects gain. RG’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.
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, RG,
connected between pins 1 and 8:
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.
INA128:
(1)
50kΩ
G = 1+
G = 1+
RG
INA129:
(2)
49.4kΩ
RG
NOISE PERFORMANCE
Commonly used gains and resistor values are shown in
Figure 1.
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.
The 50kΩ term in Equation 1 (49.4kΩ in Equation 2) comes
from the sum of the two internal feedback resistors of A1 and
A2. These on-chip metal film resistors are laser trimmed to
V+
0.1µF
INA128:
INA129:
7
49.4kΩ
50kΩ
G = 1 +
G = 1 +
RG
RG
INA128, INA129
–
VIN
2
1
Over-Voltage
Protection
INA128
INA129
A1
25kΩ(1)
40kΩ
40kΩ
DESIRED
GAIN (V/V)
RG
NEAREST
RG
NEAREST
VO = G • (VI+N – VI–N
)
(Ω)
1% RG (Ω)
(Ω)
1% RG (Ω)
1
2
5
10
20
NC
NC
49.9k
12.4k
5.62k
2.61k
1.02k
511
249
100
49.9
24.9
10
NC
49.4k
12.35k
5489
2600
1008
499
248
99
49.5
24.7
9.88
4.94
NC
49.9k
12.4k
5.49k
2.61k
1k
499
249
100
49.9
24.9
9.76
4.87
6
5
A3
RG
50.00k
12.50k
5.556k
2.632k
1.02k
505.1
251.3
100.2
50.05
25.01
10.00
5.001
+
8
25kΩ(1)
VO
Load
–
50
A2
+
VIN
3
Over-Voltage
Protection
100
200
500
1000
2000
5000
10000
Ref
40kΩ
40kΩ
4
0.1µF
NOTE: (1) INA129: 24.7kΩ
4.99
V–
NC: No Connection.
Also drawn in simplified form:
–
VIN
INA128
VO
RG
Ref
+
VIN
FIGURE 1. Basic Connections.
BB [ BURR-BROWN CORPORATION ]
