Datasheet

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SBOS051B − OCTOBER 1995 − REVISED FEBRUARY 2005
www.ti.com
9
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).
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:
G + 1)
49.4kW
R
G
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 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.
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
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.
R
G
Also drawn in simplified form:
INA128
Ref
V
O
V
IN
V
IN
+
−
G + 1)
50kW
R
G
G + 1)
49.4kW
R
G
INA128:
INA129:
DESIRED R
G
NEAREST R
G
NEAREST
GAIN (V/V) (Ω) 1% R
G
(Ω)(Ω) 1% R
G
(Ω)
1NCNCNCNC
2 50.00k 49.9k 49.4k 49.9k
5 12.50k 12.4k 12.35k 12.4k
10 5.556k 5.62k 5489 5.49k
20 2.632k 2.61k 2600 2.61k
50 1.02k 1.02k 1008 1k
100 505.1 511 499 499
200 251.3 249 248 249
500 100.2 100 99 100
1000 50.05 49.9 49.5 49.9
2000 25.01 24.9 24.7 24.9
5000 10.00 10 9.88 9.76
10000 5.001 4.99 4.94 4.87
INA128 INA129
NC: No Connection
A
1
A
2
A
3
6
7
4
3
8
1
2
V
IN
V
IN
R
G
V+
INA128, INA129
+
5
Over−Voltage
Protection
Over−Voltage
Protection
Load
+
V
O
Ref
NOTE: (1) INA129: 24.7kΩ
−
0.1µF
0.1µF
−
V−
25kΩ
(1)
25kΩ
(1)
40kΩ 40kΩ
40kΩ 40kΩ
V
O
=G (V
IN
−
V
IN
+
−
• )
Figure 1. Basic Connections
(1)
(2)