Datasheet
8
®
INA121
APPLICATION INFORMATION
Figure 1 shows the basic connections required for operation
of the INA121. 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 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 of the INA121 is set by connecting a single external
resistor, R
G
, connected between pins 1 and 8:
Commonly used gains and resistor values are shown in
Figure 1.
(1)
G =1+
50kΩ
R
G
The 50kΩ term in Equation 1 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
resistors are included in the gain accuracy and drift specifi-
cations of the INA121.
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 (possi-
bly 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 INA121 achieves
wide bandwidth, even at high gain. This is due to the
current-feedback topology of the INA121. Settling time also
remains excellent at high gain.
A
1
A
2
A
3
6
40kΩ40kΩ
40kΩ40kΩ
7
4
3
8
1
2
V
IN
V
IN
R
G
V+
V–
INA121
G = 1 +
50kΩ
R
G
–
+
5
Over-Voltage
Protection
25kΩ
25kΩ
Over-Voltage
Protection
Load
V
O
= G • (V
IN
– V
IN
)
+
–
0.1µF
0.1µF
+
–
V
O
R
G
Also drawn in simplified form:
INA121
Ref
V
O
V
IN
–
V
IN
+
Ref
DESIRED R
G
NEAREST 1% R
G
GAIN (Ω)(Ω)
1NC NC
2 50.00k 49.9k
5 12.50k 12.4k
10 5.556k 5.62k
20 2.632k 2.61k
50 1.02k 1.02k
100 505.1 511
200 251.3 249
500 100.2 100
1000 50.05 49.9
2000 25.01 24.9
5000 10.00 10
10000 5.001 4.99
NC: No Connection.
FIGURE 1. Basic Connections.