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LMP90080/LMP90079/LMP90078/LMP90077 Sensor AFE System: Multi-Channel, Low-Power 16-Bit Sensor AFE with True Continuous Background Calibration

LMP90080/LMP90079/

LMP90078/LMP90077

May 29, 2012

Sensor AFE System: Multi-Channel, Low-Power 16-Bit

Sensor AFE with True Continuous Background Calibration

1.0 General Description

The LMP90080/LMP90079/LMP90078/LMP90077 are highly

integrated, multi-channel, low-power 16-bit Sensor AFEs.

The devices feature a precision, 16-bit Sigma Delta Analog-

to-Digital Converter (ADC) with a low-noise programmable

gain amplifier and a fully differential high impedance analog

input multiplexer. A true continuous background calibration

feature allows calibration at all gains and output data rates

without interrupting the signal path. The background calibra-

tion feature essentially eliminates gain and offset errors

across temperature and time, providing measurement accu-

racy without sacrificing speed and power consumption.

Another feature of the LMP90080/LMP90079/LMP90078/

LMP90077 is continuous background sensor diagnostics, al-

lowing the detection of open and short circuit conditions and

out-of-range signals, without requiring user intervention, re-

sulting in enhanced system reliability.

Two sets of independent external reference voltage pins allow

multiple ratiometric measurements. In addition, two matched

programmable current sources are available in the

LMP90080/LMP90078 to excite external sensors such as re-

sistive temperature detectors and bridge sensors. Further-

more, seven GPIO pins are provided for interfacing to external

LEDs and switches to simplify control across an isolation bar-

rier.

Collectively, these features make the LMP90080/LMP90079/

LMP90078/LMP90077 complete analog front-ends for low-

power, precision sensor applications such as temperature,

pressure, strain gauge, and industrial process control. The

LMP90080/LMP90079/LMP90078/LMP90077 are guaran-

teed over the extended temperature range of -40°C to +125°

C and are available in a 28-pin TSSOP package with an ex-

posed pad.

■

Low-Noise programmable gain (1x - 128x)

■

Continuous background open/short and out of range

■

sensor diagnostics

8 output data rates (ODR) with single-cycle settling

2 matched excitation current sources from 100 µA to

1000 µA (LMP90080/LMP90078)

4-DIFF / 7-SE inputs (LMP90080/LMP90079)

2-DIFF / 4-SE inputs (LMP90078/LMP90077)

7 General Purpose Input/Output pins

Chopper-stabilized buffer for low offset

SPI 4/3-wire with CRC data link error detection

50 Hz to 60 Hz line rejection at ODR

≤

13.42 SPS

Independent gain and ODR selection per channel

Supported by Webench Sensor AFE Designer

Automatic Channel Sequencer

3.0 Key Specifications

■

ENOB/NFR

■

Offset Error (typ)

■

Gain Error (typ)

■

Total Noise

■

Integral Non-Linearity (INL max)

■

Output Data Rates (ODR)

■

Analog Voltage, VA

■

Operating Temp Range

■

Package

Up to 16/16 bits

8.4 nV

7 ppm

< 10 µV-rms

±1LSB

1.6775 SPS - 214.65 SPS

+2.85V to +5.5V

-40°C to 125°C

28-Pin TSSOP

exposed pad

2.0 Features

■

16-Bit Low-Power Sigma Delta ADC

■

True Continuous Background Calibration at all gains

■

In-Place System Calibration using Expected Value

programming

4.0 Applications

■

Temperature and Pressure Transmitters

■

Strain Gauge Interface

■

Industrial Process Control

5.0 Typical Application

30169774

TRI-STATE

is a registered trademark of National Semiconductor Corporation.

6.0 Block Diagram

30169775

FIGURE 1. Block Diagram

• True Continuous Background Calibration

The LMP90080/LMP90079/LMP90078/LMP90077 feature a

16 bit ΣΔ core with continuous background calibration to com-

pensate for gain and offset errors in the ADC, virtually elimi-

nating any drift with time and temperature. The calibration is

performed in the background without user or ADC input in-

terruption, making it unique in the industry and eliminating

down time associated with field calibration required with other

solutions. Having this continuous calibration improves perfor-

mance over the entire life span of the end product.

• Continuous Background Sensor Diagnostics

Sensor diagnostics are also performed in the background,

without interfering with signal path performance, allowing the

detection of sensor shorts, opens, and out-of-range signals,

which vastly improves system reliability. In addition, the fully

flexible input multiplexer described below allows any input pin

to be connected to any ADC input channel providing addi-

tional sensor path diagnostic capability.

• Flexible Input MUX Channels

The flexible input MUX allows interfacing to a wide range of

sensors such as thermocouples, RTDs, thermistors, and

bridge sensors. The LMP90080/LMP90079’s multiplexer sup-

ports 4 differential channels while the LMP90078/LMP90077

supports 2. Each effective input voltage that is digitized is VIN

= VINx – VINy, where x and y are any input. In addition, the

input multiplexer of the LMP90080/LMP90079 also supports

7 single-ended channels (LMP90078/LMP90077 supports 4),

where the common ground is any one of the inputs.

• Programmable Gain Amplifiers (FGA & PGA)

The LMP90080/LMP90079/LMP90078/LMP90077 contain

an internal 16x fixed gain amplifier (FGA) and a 1x, 2x, 4x, or

8x programmable gain amplifier (PGA). This allows accurate

gain settings of 1x, 2x, 4x, 8x, 16x, 32x, 64x, or 128x through

configuration of internal registers. Having an internal amplifier

eliminates the need for external amplifiers that are costly,

space consuming, and difficult to calibrate.

• Excitation Current Sources (IB1 & IB2) - LMP90080/

LMP90078

Two matched internal excitation currents, IB1 and IB2, can be

used for sourcing currents to a variety of sensors. The current

range is from 100 µA to 1000 µA in steps of 100 µA.