INA129-EP
SBOS508 –DECEMBER 2009
www.ti.com
APPLICATION INFORMATION
Figure 23 shows the basic connections required for operation of the INA129. 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 resistance of 8 Ω in series with the Ref pin
will cause a typical device to degrade to approximately 80 dB CMR (G = 1).
Setting the Gain
Gain is set by connecting a single external resistor, RG, between pins 1 and 8.
49.4 kW
G = 1 +
RG
(1)
Commonly used gains and resistor values are shown in Figure 23.
The 49.9-kΩ term in Equation 1 comes from the sum of the two internal feedback resistors of A1 and A2. These
on-chip metal film resistors are laser trimmed to accurate absolute values. The accuracy and temperature
coefficient of these internal resistors are included in the gain accuracy and drift specifications of the 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 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
(possibly an unstable gain error) in gains of approximately 100 or greater.
V+
0.1mF
49.4kW
RG
G = 1 +
7
2
1
Over Voltage
Protection
-
DESIRED
GAIN (V/V)
NEAREST
1% RG (W)
RG
VIN
(W)
A1
40kΩ
40kΩ
-
VO
=
G · (VIN+ - VIN
)
1
2
5
10
20
50
100
200
500
1000
2000
5000
10000
NC
NC
24.7kΩ
49.4K
12.35K
5489
2600
1008
499
248
99
49.5
24.7
9.88
4.94
49.9K
12.4K
5.49K
2.61K
1K
499
249
100
49.9
24.9
9.76
4.87
6
5
A3
RG
+
8
24.74kΩ
A2
VO
-
Load
+
VIN
3
Over Voltage
Protection
Ref
40kΩ
40kΩ
4
0.1mF
NC: No Connection
-
IN
V
V-
Also drawn in simplified form:
V
R
G
O
Ref
+
IN
V
Figure 23. Basic Connections
Dynamic Performance
Figure 1 shows that, despite its low quiescent current, the 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.
10
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