LT1990-10
12
Rev 0
For more information www.analog.com
APPLICATIONS INFORMATION
Primary Features
The LT1990-10 is a complete gain-block solution for
high input common mode voltage applications. The part
combines a low-power precision operational amplifier
with thin-film resistors trimmed to produce a gain of 10
with high accuracy. The Block Diagram shows the inter
-
nal architecture of the part. The on-chip resistors form a
modified difference-amplifier including a reference port for
introducing offset or other additive waveforms. The resistor
network is structured to produce internal common mode
voltage division of 27, enabling a very large input range.
The input range can far exceed the power supply voltage(s)
used by the LT1990-10 itself. Standard ESD clamp diodes
are included on all the I/O except the –IN and +IN pins.
The inputs are rated to ±250V and protected to ±500V. The
LT1990-10 is ideally suited to situations where relatively
small signals need to be extracted from high voltage circuits,
as is the case in many instrumentation applications. With its
wide input voltage range and greater than 1 megohm input
impedances, development of instrumentation designs is
greatly simplified with the LT1990-10 single-chip solution
over conventional discrete methods.
Classic Difference Amplifier
The basic gain of ten difference amplifier topology has the
following dc transfer function:
V
O
= 10·(V
+IN
– V
–IN
) + V
REF
By including the internal common mode division by 27,
the input common mode range capability is extended up
to ±250V according to the following relationships:
V
CM+
≤ 27 • V
+
– 26 • V
REF
– 23
V
CM–
≥ 27 • V
–
– 26 • V
REF
+ 27
For split supplies over about ±11V, the full ±250V common
mode range is normally available (with V
REF
a small fraction
of the supply). With lower supply voltages, an appropriate
selection of V
REF
can tailor the input common mode range
to a specific requirement. For single supply circuits, V
REF
should be greater than V
–
to allow bidirectional output
swing and to keep the inputs of the internal op amp within
their operating region. Note: the differential input voltage
range is reduced as V
CM
approaches its limits. The follow-
ing low supply-voltage scenarios are readily implemented
with the LT1990-10:
Table 1.
Supply V
REF
V
CM
Range
3V 1.25V –5V to 25V (e.g. 12V Automotive Environment)
5V 1.25V –5V to 80V (e.g. 42V Automotive Environment)
5V 4.00V –77V to 8V (e.g. Telecom Environment;
Use Downward Signaling)
Preserving and Enhancing Common Mode Rejection
The basic difference amplifier topology of the LT1990
-10 is
sensitive to the external resistances of circuits driving the
part. To preserve the high accuracy of the LT1990-10, the
source impedance of any signal connected to the REF pin
must be on the order of a few ohms or less, such as from
a Reference or op-amp output. The difference inputs have
nominal 1 megohm internal resistances that are matched
to within a few hundred ohms, so source resistances
should also be kept low to maximize accuracy and CMRR.
While every LT1990-10 is factory trimmed, some precision
applications with a large applied common mode voltage
may benefit from a trim method to further minimize com
-
mon mode error. This is easily accomplished as shown in
Figure 1. A series resistance is added to each input:
a fixed
1kΩ in series with one of the inputs and a 2kΩ trimmer
