LT1990CS8 datasheet(13/16 Pages) LINER

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LT1990

1990f

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.

–

LT1990

Figure 1. Optional CMRR Trim

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:

VO = 10 • (V+IN – V–IN + VGAIN2 – VGAIN1) + VREF

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– – 0.2V

to V– + 36V. If the GAIN inputs are brought beyond the

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 RS varies with operating current

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.

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