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SBOS051B − OCTOBER 1995 − REVISED FEBRUARY 2005
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 INA128/INA129.
APPLICATIONS 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 capacitors close to the device pins as shown.
The stability and temperature drift of the external gain
setting resistor, R , also affects gain. R ’s contribution
G
G
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 (possibly 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 resistance of 8Ω in series
with the Ref pin will cause a typical device to degrade
to approximately 80dB CMR (G = 1).
DYNAMIC PERFORMANCE
SETTING THE GAIN
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.
Gain is set by connecting a single external resistor, R ,
G
connected between pins 1 and 8:
INA128:
50kW
RG
G + 1)
(1)
(2)
INA129:
49.4kW
NOISE PERFORMANCE
G + 1)
RG
The INA128/INA129 provides very low noise in most
applications. Low frequency noise is approximately
Commonly used gains and resistor values are shown in
Figure 1.
0.2µV measured from 0.1 to 10Hz (G ≥ 100). This
PP
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 A and A . These on-chip metal film
1
2
V+
INA129:
INA128:
0.1µF
50kW
RG
49.4kW
RG
G + 1)
G + 1)
7
INA128, INA129
2
1
Over−Voltage
Protection
−
INA128
INA129
VIN
A1
25kΩ
DESIRED
R
G
NEAREST
R
G
NEAREST
40kΩ
40kΩ
GAIN (V/V)
(Ω)
1% R (Ω)
(Ω)
1% R (Ω)
G
G
−
+
(1)
•
−
)
VO = G (VIN VIN
1
NC
NC
NC
NC
2
5
10
20
50.00k
12.50k
5.556k
2.632k
1.02k
505.1
251.3
100.2
50.05
25.01
10.00
5.001
49.9k
12.4k
5.62k
2.61k
1.02k
511
249
100
49.9
24.9
10
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
A3
RG
+
8
(1)
25kΩ
A2
VO
Load
50
−
100
200
500
1000
2000
5000
10000
5
+
VIN
3
Over−Voltage
Protection
Ref
40kΩ
40kΩ
4
0.1µF
4.99
NOTE: (1) INA129: 24.7kΩ
NC: No Connection
−
IN
V
V−
Also drawn in simplified form:
INA128
V
R
G
O
Ref
+
V
IN
Figure 1. Basic Connections
9
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