INA128
INA129
www.ti.com
SBOS051B − OCTOBER 1995 − REVISED FEBRUARY 2005
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 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).
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.
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 (possibly an unstable
gain error) in gains of approximately 100 or greater.
SETTING THE GAIN
Gain is set by connecting a single external resistor, R
G
,
connected between pins 1 and 8:
INA128:
G
+
1) 50kW
R
G
INA129:
(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
INA128:
INA129:
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.
(1)
G
+
1) 49.4kW
R
G
NOISE PERFORMANCE
The INA128/INA129 provides very low noise in most
applications. Low frequency noise is approximately
0.2µV
PP
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)
50kW
R
G
G
+
1)
49.4kW
R
G
V−
2
IN
7
INA128, INA129
Over−
Voltage
Protection
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
A
1
40kΩ
25kΩ(1)
A
3
25kΩ(1)
A
2
40kΩ
4
40kΩ
5
Ref
6
+
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
R
G
8
Load V
O
−
+
V
IN
3
Over−
Voltage
Protection
NOTE: (1) INA129: 24.7kΩ
−
V IN
0.1µF
NC: No Connection
V−
Also drawn in simplified form:
+
V IN
RG
INA128
Ref
VO
Figure 1. Basic Connections
9
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