User manual

MCP355X Tiny Application Sensor Demo Board User’s Guide

FIGURE 2-2: Block Diagram.

2.3 FUNCTIONAL BLOCK DESCRIPTIONS

2.3.1 SM5420

The SM5420 device is an absolute pressure sensor in a surface mount SOIC package.

The device is a piezo resistive silicon device available from Silicon Microstructure.

With a constant excitation voltage, the output changes linearly with pressure. The zero

scale error of the device is rated at ±10 mV/V and the full scale output is rated at

60 mV/V ±20 mV. These large errors require an overall system calibration, as well as

temperature compensation for a working absolute pressure sensor design. This board

demonstrates the overall system resolution and noise performance of the ADC and

sensor with no voltage reference.

2.3.2 MCP3551

The MCP3551 is a 22-bit Delta-Sigma A/D converter. The device includes a third-order

modulator, fourth-order digital SINC filter, internal oscillator for oversampling clock and

digital logic for a simple SPI interface.

Digital SINC Filter

The MCP3551 device includes a digital decimation filter, which is a fourth-order modi-

fied SINC filter. This filter averages the incoming bitstream from the modulator and out-

puts a 22-bit conversion word in binary two's complement. When all bits have been

processed by the filter, the output code is ready for SPI communication, the RDY

flag

is set on the SDO/RDY

pin and all the internal registers are reset in order to process

the next conversion. This filter achieves greater than -80 dB of rejection at both 50 and

60 Hz. For improved performance, the MCP3550 device is available which gives

greater than -120 dB of rejection at either 50 or 60 Hz.

USB Power (Digital)

Digital Ground

Plane

Analog

Ground

Plane

Filtering

Power Supply

MCP3551

Filter Network

Analog Power & Reference

SM5420

USB Connector

JP1

USB Power & Data

PIC18F4550