Datasheet

INA322
10
SBOS174B
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 INA322. With an internally
set gain of 5, the INA322 can be programmed for gains
greater than 5 according to the following equation:
G = 5 + 5(R
2
/R
1
)
The INA322 is designed to provide accurate gain, with gain
error specified to be less than 0.4%. 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.
INPUT COMMON-MODE 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
reference voltage according to the following equation:
V
OA1
= 5/4 V
CM
– 1/4 V
REF
(See Typical Characteristic Curves for Input Common-
Mode 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 equa-
tion:
V
OA2
= V
REF
+ 5(V
IN
+ – V
IN
–)
V
OA2
should be less than V
DD
– 1.2V.
The reference pin requires a low-impedance connection.
Any resistance in series with the reference pin will degrade
the CMRR. The reference pin may be used to compensate
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 INA322 is ideal
for use with high-impedance sources. The input bias current
of less than 10pA makes the INA322 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-
FIGURE 3. Providing an Input Common-Mode Path.
mode range and the input amplifier will saturate. Figure 3
shows how bias current path can be provided in the cases of
microphone applications, thermistor applications, ground
returns, 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.
Center-tap
provides bias
current return
Bridge resistance
provides bias
current return
Transformer
Bridge
Amplifier
Bridge
Sensor
V
B
(1)
V
EX
NOTE: (1) V
B
is bias voltage within
common-mode range, dependent
on REF.
INA322
5
3
2
8
7
6
4
1
V+
Shutdown
V
IN
–
V–
V
IN
+
REF
INA322
5
3
2
8
7
6
4
1
V+
Shutdown
V
IN
–
V–
V
IN
+
REF
47kΩ
Microphone,
Hydrophone,
etc.
V
B
(1)
INA322
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