LT1789-1/LT1789-10
20
1789fc
APPLICATIONS INFORMATION
Output Voltage vs Input Common Mode Voltage
All instrumentation amplifiers have limiting factors that
can cause an output to be invalid (the output is not equal
to the input differential voltage multiplied by the gain)
even though the output appears to be operating in a linear
region. Limiting factors such as input voltage range and
output swing can be easily measured, however, there are
also internal nodes that can limit. These internal nodes
cannot be measured externally and can lead to erroneous
output readings.
To ensure a valid output for a given input common mode
voltage and input differential voltage, the following four
limiting factors must be taken into consideration (refer to
the block diagram):
1) The input voltage ranges of the input amplifiers A1 and
A2.
2) The output swings of the input amplifiers A1 and A2
(internal nodes).
3) The input voltage range of the output amplifier A3
(internal node).
4) The output swing of the output amplifier A3.
These limits can be determined using the relationships
below.
1) The input voltage range limits can be found in the
electrical tables.
2) The output voltages of the input amplifiers A1 and A2
can be found by the following formulas:
V
OUT
A1 = (V
D
/2)(G)(R1/R2) + V
CM
+ 0.6V
V
OUT
A2 = (–V
D
/2)(G)(R1/R2) + V
CM
+ 0.6V
Where V
D
is the input differential voltage and V
CM
is the
input common mode voltage.
The typical output swing limits for A1 and A2 can be found
in the Output Swing vs Load Current typical performance
curve, using R1 + R2 as the load resistance.
This limitation usually becomes dominant when gain is
taken in the input stage and the common mode input
voltage is close to either supply rail.
The LT1789-10 is less susceptible to this limiting factor
because the gain is taken in the output stage.
3) The voltage on the inputs to the output amplifier A3
can be determined by the following formula:
V
IN
A3 = (V
OUT
A1 – V
REF
)(R2/(R1 + R2))
The input voltage range of A3 has the same input limits as
the LT1789-1. This limiting factor is more prevalent with
single supplies, where both the reference voltage and input
common mode voltage are near V
+
. This is also more of
a concern with the LT1789-10 because the ratio of R1:R2
is 1:10 instead of 1:1.
4) The output voltage swing limits are also found in the
electrical tables.
The Output Voltage vs Input Common Mode Voltage typical
performance curves show the regions of operation for the
three supply voltages specified.
Single Supply Operation
There are usually two types of input signals that need
to be processed; differential signals, like the output of a
bridge or single ended signals, such as the output from
a thermistor. Both signals require special consideration
when operating with a single supply.
When processing differential signals , REF (Pin 5) must
be brought above the negative supply (Pin 4) to allow the
output to process both the positive and negative going input
signal. The maximum output operating range is obtained
by setting the voltage on the REF pin to half supply. This
must be done with a low impedance source to minimize
CMRR and gain errors.
For single ended input signals, the REF pin can be at the
same potential as the negative supply provided the output
of the instrumentation amplifier remains inside the specified
operating range. This maximizes the output range, however
the smallest input signal that can be processed is limited
by the output swing to the negative supply.
