Level shift

+

-

Preamplifier

Gain = 10

+IN

-IN

Output Buffer

Gain = K2

A1 A2

OFFSET

100 k:

OUT

GND

8

1

3

4

5

7

2

6

18

LMP8601, LMP8601-Q1

LMP8602, LMP8602-Q1

LMP8603, LMP8603-Q1

SNOSAR2H – SEPTEMBER 2008 – REVISED APRIL 2016

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Product Folder Links: LMP8601 LMP8601-Q1 LMP8602 LMP8602-Q1 LMP8603 LMP8603-Q1

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7 Detailed Description

7.1 Overview

The LMP860x and LMP860x-Q1 are fixed gain differential voltage precision amplifiers, with a –22-V to +60-V

input common-mode voltage range when operating from a single 5-V supply, or a –4-V to +27-V input common-

mode voltage range when operating from a single 3.3-V supply. The LMP8601 and LMP8601-Q1 have a gain of

20x, the LMP8602 and LMP8602-Q1 have a gain of 50x, and the LMP8603 and LMP8603-Q1 have a gain of

100x.

The LMP860x and LMP860x-Q1 are members of the LMP family and are ideal parts for unidirectional and

bidirectional current sensing applications. Because of the proprietary chopping level-shift input stage, the

LMP860x and LMP860x-Q1 achieve very low offset, very low thermal offset drift, and very high CMRR. The

LMP860x and LMP860x-Q1 amplify and filter small differential signals in the presence of high common-mode

voltages.

The LMP860x and LMP860x-Q1 use level shift resistors at the inputs. Because of these resistors, the LMP860x

and LMP860x-Q1 can easily withstand very large differential input voltages that may exist in fault conditions

where some other less protected high-performance current sense amplifiers might sustain permanent damage.

7.1.1 Theory of Operation

The schematic shown in the Functional Block Diagram gives a basic representation of the internal operation of

the LMP860x and LMP860x-Q1.

The signal on the input pins is typically a small differential voltage developed across a current sensing shunt

resistor. The input signal may also appear at a high common-mode voltage. The input signals are accessed

through two input resistors that change the voltage into a current. The proprietary chopping level-shift current

circuit pulls or pushes current through the input resistors to bring the common-mode voltage behind these

resistors within the supply rails.

Subsequently, the signal is gained up by a factor of 10 and brought out on the A1 pin through a trimmed 100-k

Ω

resistor. In the application, additional gain adjustment or filtering components can be added between the A1 and

A2 pins as explained in subsequent sections. The signal on the A2 pin is further amplified by a factor of 2

(LMP8601 and LMP8601-Q1), 5 (LMP8602, LMP8602-Q1), or 10 (LMP8603, LMP8603-Q1), and brought out on

the OUT pin.

7.2 Functional Block Diagram

NOTE: K2 = 2 for LMP8601, LMP8601-Q1; 5 for LMP8602, LMP8602-Q1; or 10 for LMP8603, LMP8603-Q1.