Datasheet

LT1789-1/LT1789-10
20
1789fb
APPLICATIONS INFORMATION
Input Bias Current Return Path
The low input bias current of the LT1789-1/LT1789-10
(19nA) and the high input impedance (1.6GΩ) allow the
use of high impedance sources without introducing sig-
nifi cant offset voltage errors, even when the full common
mode range is required. However, a path must be provided
for the input bias currents of both inputs when a purely
differential signal is being amplifi ed. Without this path the
inputs will fl oat high and exceed the input common mode
range of the LT1789-1/LT1789-10, resulting in a saturated
input stage. Figure 3 shows three examples of an input bias
current path. The fi rst example is of a purely differential
signal source with a 10kΩ input current path to ground.
Since the impedance of the signal source is low, only one
resistor is needed. Two matching resistors are needed for
higher impedance signal sources as shown in the second
example. Balancing the input impedance improves both
common mode rejection and DC offset. The need for input
resistors is eliminated if a center tap is present as shown
in the third example.
Figure 3. Providing an Input Common Mode Current Path
Output Voltage vs Input Common Mode Voltage
All instrumentation amplifi ers 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 amplifi ers A1
and A2.
2) The output swings of the input amplifi ers A1 and A2
(internal nodes).
10k
R
G
R
G
R
G
1789 F03
THERMOCOUPLE
200k
MICROPHONE,
HYDROPHONE,
ETC
200k
CENTER-TAP PROVIDES
BIAS CURRENT RETURN
–
+
LT1789-1/
LT1789-10
–
+
LT1789-1/
LT1789-10
–
+
LT1789-1/
LT1789-10