Datasheet

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SBOS168D − DECEMBER 2000 − REVISED JANUARY 2006
www.ti.com
11
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 INA321. With an
internally set gain of 5, the INA321 can be programmed for
gains greater than 5 according to the following equation:
G = 5 + 5 (R
2
/R
1
)
The INA321 is designed to provide accurate gain, with gain
error assured to be less than 0.1%. Setting gain with
matching TC resistors will minimize gain drift. Errors from
external resistors will add directly to the gain 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
performance. 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 Characteristics 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
equation:
V
OA2
= V
REF
+ 5 (V
IN
+ − V
IN
−)
For optimal 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 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 INA321 is ideal
for use with high-impedance sources. The input bias
current of less than 10pA makes the INA321 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 how bias current path can be provided in
the cases of microphone applications, thermistor
applications, ground returns, and dc-coupled resistive
bridge applications.
47k
Ω
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.
INA321
5
3
2
8
7
6
4
1
V+
Shutdown
V
IN
−
V
−
V
IN
+
REF
INA321
5
3
2
8
7
6
4
1
V+
Shutdown
V
IN
−
V
−
V
IN
+
REF
INA321
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
Figure 3. Providing an Input Common-Mode Path
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 balanced input. The advantages are lower input offset
voltage due to bias current flowing through the source
impedance and better high-frequency gain.