Datasheet

10
®
INA155
SETTING THE GAIN
Gain of 10 is achieved simply by leaving the two gain pins
(1 and 8) open. Gain of 50 is achieved by connecting the
gain pins together directly. In the G = 10 configuration, the
gain error is less than 0.1%. In the G = 50 configuration, the
gain error is less than 0.25%.
Gain can be set to any value between 10 and 50 by connect-
ing a resistor R
G
between the gain pins according to the
following equation:
10 + 400kΩ/(10kΩ + R
G
) (1)
This is demonstrated in Figure 1 and is shown with the com-
monly used gains and resistor R
G
values. However, because the
absolute value of internal resistors is not guaranteed, using the
INA155 in this configuration will increase the gain error and
gain error drift with temperature, as shown in Figure 3.
FIGURE 3. Typical Gain Error and Gain Error Drift with
External Resistor.
OFFSET TRIMMING
The INA155 is laser trimmed for low offset voltage. In most
applications, no external offset adjustment is required. How-
ever, if necessary, the offset can be adjusted by applying a
correction voltage to the reference terminal. Figure 4 shows
an optional circuit for trimming the output offset voltage.
The voltage applied to the Ref terminal is added to the
output signal. An op amp buffer is used to provide low
impedance at the Ref terminal to preserve good common-
mode rejection.
INPUT BIAS CURRENT RETURN
The input impedance of the INA155 is extremely high—
approximately 10
13
Ω, making it ideal for use with high-imped-
ance sources. However, a path must be provided for the input
bias current of both inputs. This input bias current is less than
10pA and is virtually independent of the input voltage.
Input circuitry must provide a path for this input bias current
for proper operation. Figure 5 shows various provisions for
an input bias current path. Without a bias current path, the
inputs will float to a potential that exceeds the common-
mode range and the input amplifier will saturate.
If the differential source resistance is low, the bias current
return path can be connected to one input (see the thermo-
couple in Figure 5). With higher source impedance, using
two equal resistors provides a balanced input with advan-
tages of lower input offset voltage due to bias current and
better high-frequency common-mode rejection.
FIGURE 4. Optional Trimming of Output Offset Voltage. FIGURE 5. Providing an Input Common-Mode Current Path.
Gain (V/V)
Gain Error (%)
10
15 20
3025
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
Gain Error Drift (ppm/°C)
400
360
320
280
250
200
160
120
80
40
0
35 40 5045
Gain Error Drift
Gain Error
OPA336
INA155
Ref
(1)
3
6
2
5
V
O
Adjustable
Voltage
V
IN
+ (2)
V
IN
– (2)
NOTES: (1) V
REF
should be adjusted for the desired output
level. The value of V
REF
affects the common-mode input
range. (2) For best performance, common-mode input voltage
should be less than (V+) – 1.8V or greater than (V+)
– 0.8V.
1
8
INA155
3
6
6
6
6
25
V
REF
47kΩ
Microphone,
Hydrophone, etc.
INA155
3
2
3
2
5
V
REF
Center-tap
provides bias
current return
Low-resistance
thermocouple
provides bias
current return.
Bridge resistance
provides bias
current return
INA155
3
2
10kΩ
5
V
REF
V
B
(1)
Thermocouple
INA155
5
V
REF
Bridge
Sensor
1
8
1
8
1
8
1
8
V
B
(1)
V
B
(1)
V
EX
NOTE: (1) V
B
is bias voltage within
common-mode range, dependent
on V
REF
.