Datasheet
LT1789-1/LT1789-10
19
1789fb
APPLICATIONS INFORMATION
Setting the Gain
The gain of the LT1789-1 and LT1789-10 is set by the
value of resistor R
G
, applied across pins 1 and 8. For the
LT1789-1, the gain G will be:
G = 1+ 200k/R
G
and R
G
can be calculated from the desired gain by
R
G
= 200k/(G – 1)
For the LT1789-10, the gain G will be
G =10 • (1 + 200k/R
G
)
and R
G
can be calculated from the desired gain by
R
G
= 200k/(0.1 • G – 1)
For the lowest achievable gain, R
G
may be set to infi nity
by leaving Pins 1 and 8 open.
Input and Output Offset Voltage
The offset voltage of the LT1789-1/LT1789-10 has two
components: the output offset and the input offset. The
total offset voltage referred to the input (RTI) is found by
dividing the output offset by the programmed gain (G) and
adding it to the input offset. At high gains the input offset
voltage dominates, whereas at low gains the output offset
voltage dominates. The total offset voltage is:
Total input offset voltage (RTI)
= input offset + (output offset/G)
Total output offset voltage (RTO)
= (input offset • G) + output offset
Reference Terminal
The output voltage of the LT1789-1/LT1789-10 (Pin 6) is
referenced to the voltage on the reference terminal (Pin
5). Resistance in series with the REF pin must be mini-
mized for best common mode rejection. For example, a
22Ω resistance from the REF pin to ground will not only
increase the gain error by 0.02% but will lower the CMRR
to 80dB.
Output Offset Trimming
The LT1789-1/LT1789-10 is laser trimmed for low offset
voltage so that no external offset trimming is required for
most applications. In the event that the offset needs to be
adjusted, the circuit in Figure 2 is an example of an optional
offset adjust circuit. The op amp buffer provides a low
impedance to the REF pin where resistance must be kept
to a minimum for best CMRR and lowest gain error.
Figure 2. Optional Trimming of Output Offset Voltage
–
+
10k
100Ω
100Ω
–10mV
1789 F02
V
–
V
+
10mV
5
6
OUTPUT
2
3
1
LT1880
±10mV
ADJUSTMENT RANGE
–
+
2
–IN
+IN
1
8
3
R
G
REF
LT1789-1/-10