Datasheet
INA331, INA2331
SBOS215C
10
www.ti.com
SETTING THE GAIN
The ratio of R
2
to R
1
, or the impedance between pins 1, 5,
and 6, determines the gain of the INA331. With an internally
set gain of 5, the INA331 can be programmed for gains
greater than 5 according to the following equation:
G = 5 + 5 (R
2
/R
1
)
The INA331 is designed to provide accurate gain, with gain
error less than 0.1%. Setting gain with matching TC resistors
will minimize gain drift. Errors from external resistors will add
directly to the error, and may become dominant error sources.
COMMON-MODE INPUT RANGE
The upper limit of the common-mode input range is set by the
common-mode input range of the second amplifier, A2, to
1.2V below positive supply. Under most conditions, the
amplifier operates beyond this point with reduced perfor-
mance. The lower limit of the input range is bounded by the
output swing of amplifier A1, and is a function of the refer-
ence voltage according to the following equation:
V
OA1
= 5/4 V
CM
– 1/4 V
REF
(See typical characteristics
Common-Mode Input Range vs
Reference Voltage
.)
REFERENCE
The reference terminal defines the zero output voltage level.
In setting the reference voltage, the common-mode input of
A3 should be considered according to the following equation:
V
OA2
= V
REF
+ 5 (V
IN
+ – V
IN
–)
For ensured operation, V
OA2
should be less than
V
DD
– 1.2V.
The reference pin requires a low-impedance connection. As
little as 160Ω in series with the reference pin will degrade the
CMRR to 80dB. The reference pin may be used to compen-
sate for the offset voltage (see Offset Trimming section). The
reference voltage level also influences the common-mode
input range (see Common-Mode Input Range section).
INPUT BIAS CURRENT RETURN
With a high input impedance of 10
13
Ω, the INA331 is ideal for
use with high-impedance sources. The input bias current of
less than 10pA makes the INA331 nearly independent of
input impedance and ideal for low-power applications.
For proper operation, a path must be provided for input bias
currents for both inputs. Without input bias current paths, the
inputs will “float” to a potential that exceeds common-mode
range and the input amplifier will saturate. Figure 3 shows
FIGURE 3. Providing an Input Common-Mode Path.
how the bias current path can be provided in the cases of
microphone applications, thermistor applications, ground re-
turns, and dc-coupled resistive bridge applications.
When differential source impedance is low, the bias current
return path can be connected to one input. With higher
source impedance, two equal resistors will provide a bal-
anced input. The advantages are lower input offset voltage
due to bias current flowing through the source impedance
and better high-frequency gain.
47kΩ
Microphone,
Hydrophone,
etc.
Center-tap
provides bias
current return
Bridge resistance
provides bias
current return
Transformer
Bridge
Amplifier
Bridge
Sensor
V
B
(1)
V
B
(1)
V
EX
NOTE: (1) V
B
is bias voltage within
common-mode range, dependent
on REF.
INA331
5
3
2
8
7
6
4
1
V+
Shutdown
V
IN
–
V–
V
IN
+
REF
INA331
5
3
2
8
7
6
4
1
V+
Shutdown
V
IN
–
V–
V
IN
+
REF
INA331
5
3
2
8
7
6
4
1
V+
Shutdown
V
IN
–
V–
V
IN
+
REF
V
OUT
RG
V
OUT
RG
V
OUT
RG