LMP8358 datasheet(18/32 Pages) NSC

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Application Information

INTRODUCTION

The LMP8358 is a precision programmable gain instrumen-

tation amplifier. Its gain can be programmed to 10, 20, 50,

100, 200, 500 or 1000 through an SPI-compatible serial in-

terface or through a parallel interface. Alternatively, its gain

can be set to an arbitrary value using external resistors. Note

that at low gains the dynamic range is limited by the maximum

input differential voltage of ±100mV. The LMP8358 uses

patented techniques to measure and continuously correct its

input offset voltage, eliminating offset drift over time and tem-

perature, and the effect of 1/f noise. Its ground sensing CMOS

input features a high CMRR and low input bias currents. It is

capable of sensing differential input voltages in a common-

mode range that extends from 100 mV below the negative

supply to 1.4V below the positive supply, making it an ideal

solution for interfacing with ground-referenced sensors, sup-

ply-referenced sensor bridges, and any other application re-

quiring precision and long term stability. Additionally, the

LMP8358 includes fault detection circuitry, so open and short-

ed inputs can be detected, as well a deteriorating connection

to the signal source. Other features that make the LMP8358

a versatile solution for many applications are: its rail-to-rail

output, low input voltage noise and high gain-bandwidth prod-

uct.

TRANSIENT RESPONSE TO FAST INPUTS

The LMP8358 is a current-feedback instrumentation amplifier

that consists of two auto-zeroed input stages. These two input

stages are operated in a ping-pong fashion: as one stage is

auto-zeroed the other stage provides the path between the

input pins and the output. The auto-zeroing decreases offset,

offset drift, and 1/f noise while the ping-pong architecture pro-

vides a continuous path between the input and the output. As

with all devices that use auto-zeroing, care must be taken with

the signal frequency used with the device. On-chip continu-

ous auto-zero correction circuitry eliminates the 1/f noise and

significantly reduces the offset voltage and offset voltage drift;

all of which are very low-frequency events. For slow-changing

sensor signals, below 2kHz, this correction is transparent.

Higher-frequency signals as well as fast changing edges will

show a settling and ramping time lasting about 1

μs. Like all

auto-zeroing devices, if the input frequency is above the auto-

zero frequency, aliasing will occur. This can occur both at the

auto-zeroing frequency of about 12kHz and the ping-pong

frequency of about 50kHz. If needed, a low-pass filter should

be placed on the output of the LMP8358 to filter out this dis-

turbance.

COMMUNICATION WITH THE PART AND REGISTER

DESCRIPTION

The LMP8358 supports a serial and a parallel digital interface

mode as shown in Figure 1.

Parallel user mode Gain is set using G0, G1 and G2 pins.

The shutdown mode can be activated by

asserting SHDN. Fault detection fea-

tures are unavailable.

Serial user mode

The part is SPI-programmable through

SDI, SCK, SDO and CSB. All features

are available.

30045410

LMP8358 Datasheet(PDF) 18 Page - National Semiconductor (TI)

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