LT1990 Linear Technology, LT1990 Datasheet - Page 13

• Manufacturer Part Number: LT1990
• Description: Difference Amplifier
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
impedance source. Also the source impedance of a signal
connected to the REF pin must be on the order of a few
ohms or less to preserve the high accuracy of the LT1990.
While the LT1990 comes from the factory with an excellent
CMRR, some precision applications with a large applied
common mode voltage may require a method to trim out
residual common mode error. This is easily accomplished
by adding series resistance to each input, +IN and –IN,
such that an adjustable resistance difference of ±1kΩ is
provided. This is most easily realized by adding a fixed
1kΩ in series with one of the inputs, and a 2kΩ trimmer in
series with the other as shown in Figure 1. The trim range
of this configuration is ±0.1% for the internal gain resistor
matching, so a much more finely resolved correction is
available using the LT1990 than is realizable with ordinary
discrete solutions. In applications where the input
common mode voltage is relatively constant and large
(perhaps at or beyond the supply range), this same
configuration can be treated as an offset adjustment.
Figure 1. Optional CMRR Trim
U
1k
2k
U
–
+
LT1990
W
U
Dual Differential-Input Arithmetic Block
The internal resistor network topology of the LT1990
allows the GAIN1 and GAIN2 pins to be used as another
differential input in addition to the normal +IN and –IN
port. This can be a very useful function for implementing
servo-loop differential error amplifiers, for example. In
this mode of operation, the output is governed by the
following relationship:
Unlike the main inputs, the GAIN1 and GAIN2 pins are
clamped by substrate diodes and ESD structures, thus the
operating voltage range of these pins is limited to V
to V
operating input range, care must be taken to limit the input
currents to less than 10mA to prevent damage to the
device. Also, since the gain setting resistors associated
with the GAIN1 and GAIN2 inputs are in the 10kΩ area, low
source impedances are particularly important to preserve
the precision of the LT1990.
This dual differential input mode of operation is used in the
circuit as shown in Figure 2.
This circuit is a high efficiency H-bridge driver that is PWM
modulated to provide a controlled current to an electro-
magnet coil. Since the common mode voltage of the
current sense resistor R
and the coil properties, a differential feedback is required.
In this application, it was desirable to allow the control
input to utilize the wide common mode range port (+IN and
–IN) so that constraints on input referencing are elimi-
nated. The GAIN1 and GAIN2 pins always operate within
the supply range and both ports operate with a gain of 10
to develop the loop error. The LTC1923 provides the loop
integrator and PWM functions of the servo.
V
O
–
= 10 • (V
+ 36V. If the GAIN inputs are brought beyond the
+IN
– V
–IN
S
+ V
varies with operating current
GAIN2
– V
GAIN1
LT1990
) + V
REF
–
13
– 0.2V
1990f