APPLICATION INFORMATION
Figure 1 shows the basic connections required for operation
of the INA118. 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, RG, also affects gain. RG’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.
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 12Ω in series with the Ref pin will cause a typical
device to degrade to approximately 80dB CMR (G = 1).
DYNAMIC PERFORMANCE
The typical performance curve “Gain vs Frequency” shows
that, despite its low quiescent current, the INA118 achieves
wide bandwidth, even at high gain. This is due to the
current-feedback topology of the INA118. Settling time also
remains excellent at high gain.
SETTING THE GAIN
Gain of the INA118 is set by connecting a single external
resistor, RG, connected between pins 1 and 8:
(1)
50kΩ
G = 1+
The INA118 exhibits approximately 3dB peaking at 500kHz
in unity gain. This is a result of its current-feedback topol-
ogy and is not an indication of instability. Unlike an op amp
with poor phase margin, the rise in response is a predictable
+6dB/octave due to a response zero. A simple pole at
300kHz or lower will produce a flat passband unity gain
response.
RG
Commonly used gains and resistor values are shown in
Figure 1.
The 50kΩ 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
resistors are included in the gain accuracy and drift specifi-
cations of the INA118.
V+
0.1µF
7
–
VIN
INA118
2
1
Over-Voltage
Protection
A1
DESIRED
GAIN
RG
(Ω)
NEAREST 1% RG
V
O = G • (VI+N – VI–N
)
60kΩ
60kΩ
(Ω)
50kΩ
RG
25kΩ
25kΩ
G = 1 +
1
2
5
10
20
NC
NC
49.9k
12.4k
5.62k
2.61k
1.02k
511
249
100
49.9
24.9
10
50.00k
12.50k
5.556k
2.632k
1.02k
505.1
251.3
100.2
50.05
25.01
10.00
5.001
6
5
A3
RG
+
8
VO
Load
50
–
100
200
500
1000
2000
5000
10000
A2
+
VIN
3
Over-Voltage
Protection
Ref
60kΩ
60kΩ
4
0.1µF
4.99
NC: No Connection.
V–
Also drawn in simplified form:
–
VIN
VO
RG
INA118
Ref
+
VIN
FIGURE 1. Basic Connections.
®
8
INA118
TI [ TEXAS INSTRUMENTS ]
