Datasheet

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SBOS168D − DECEMBER 2000 − REVISED JANUARY 2006
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12
OUTPUT BUFFERING
The INA321 is optimized for a load impedance of 10kΩ or
greater. For higher output current the INA321 can be
buffered using the OPA340, as shown in Figure 4. The
OPA340 can swing within 50mV of the supply rail, driving
a 600Ω load. The OPA340 is available in the tiny MSOP-8
package.
OPA340
V
OUT
+5V
0.1
µ
F
0.1
µ
F
INA321
5
3
2
V
OUT
8
7
6
4
1
V+
Shutdown
RG
V
IN
−
V
−
V
IN
+
REF
Figure 4. Output Buffering Circuit. Able to drive
loads as low as 600Ω.
SHUTDOWN MODE
The shutdown pin of the INA321 is nominally connected to
V+. When the pin is pulled below 0.8V on a 5V supply, the
INA321 goes into sleep mode within nanoseconds. For
actual shutdown threshold, see the Typical Characteristic
curve, Shutdown Voltage vs Supply Voltage. Drawing less
than 1µA of current, and returning from sleep mode in
microseconds, the shutdown feature is useful for portable
applications. Once in sleep-mode, the amplifier has high
output impedance, making the INA321 suitable for
multiplexing.
RAIL-TO-RAIL OUTPUT
A class AB output stage with common-source transistors
is used to achieve rail-to-rail output for gains of 10 or
greater. For resistive loads greater than 25kΩ, the output
voltage can swing to within a few millivolts of the supply rail
while maintaining low gain error. For heavier loads and
over temperature, see the Typical Characteristic curve,
Output Voltage Swing vs Output Current. The INA321’s
low output impedance at high frequencies makes it
suitable for directly driving Capacitive Digital-to-Analog
(CDAC) input A/D converters, as shown in Figure 5.
ADS7818
or
ADS7822
12−Bits
+5V
INA321
5
3
2
V
OUT
8
7
6
4
1
V+
Shutdown
RG
V
IN
−
V
−
V
IN
+
REF
f
S
<100kHz
Figure 5. INA321 Directly Drives a
Capacitive-Input, A/D Converter
OFFSET TRIMMING
The INA321 is laser-trimmed for low offset voltage. In the
event that external offset adjustment is required, the offset
can be adjusted by applying a correction voltage to the
reference terminal. Figure 6 shows an optional circuit for
trimming offset voltage. The voltage applied to the REF
terminal is added to the output signal. The gain from REF
to V
OUT
is +1. An op-amp buffer is used to provide low
impedance at the REF terminal to preserve good
common-mode rejection.
OPA336
Adjustable
Voltage
INA321
5
3
2
V
OUT
8
7
6
4
1
V+
Shutdown
RG
V
IN
−
V
−
V
IN
+
REF
(1)
NOTE: (1) REF should be adjusted for the desired output level.
The value of REF affects the common−mode input range.
Figure 6. Optional Offset Trimming Voltage