Cirrus Logic AN31 - Manual

Introduction
Bridge transducers are common in
instrumentation. This application note illustrates
some bridge transducer digitizer circuits which
use the CS5504/5/6/7/8/9 A/D converters and the
CS5516/20 A/D converters.
The CS5504/5/6/7/8 converters can be operated
with a variety of power supply arrangements;
including operating from a single +5 V supply;
operating from +5 and -5 analog supplies with
+3.3 V or +5 V on the digital supply; or
operating with an analog supply from +5 to
+11 V and a digital supply of +5 V.
The CS5509 can operate with +5 V on its analog
and digital supplies; or with +5 V analog and
+3.3 V digital.
The CS5516 and CS5520 are A/D converters
optimized for bridge transducer applications and
are designed to operate from +5 and -5 V
supplies. Several circuits which utilize these
ADCs will be presented.
The application note is divided into two sections:
1. DC-excited bridge circuits.
2. AC-excited bridge circuits with a discussion
of the benefits of AC excitation.
Bridge Transducers
Bridge transducers are manufactured with
various technologies. The strain-sensing
elements which make up the bridge may be
made of diffused silicon, bonded silicon bars,
deposited thin film, or bonded foil materials.
The choice of technology will determine the
performance of the transducer, including the
sensitivity, the linearity, and the thermal
stability. Silicon-based gages have good linearity
with sensitivities between 3 mV/V and
20 mV/V, but tend to exhibit more drift as
temperature changes. Metal foil or thin film
gages have good linearity with sensitivities
between 1 mV/V and 4 mV/V. Precision bridge
transducers include some type of temperature
compensation as part of the bridge.
Most bridge circuits are excited with a dc
voltage, 10 volts being very common. With 10 V
excitation, the full scale signals from the various
transducers, can be as low as 10 mV to as high
as several hundred millivolts. When digitizing
these signals to high resolution (for discussion in
this application note, high resolution means
greater than 10,000 counts), one count can
represent a very small voltage. It can be difficult
to amplify and digitize these low level bridge
transducer signals. Measurement performance
can be hindered by such things as amplifier
JAN ’95
AN31REV3
1
Crystal Semiconductor Corporation
P.O. Box 17847, Austin, TX 78760
(512) 445-7222 FAX 445-7581
http://www.crystal.com
Application Note
A Collection of Bridge Transducer Digitizer Circuits
by
Jerome Johnston
Copyright  Crystal Semiconductor Corporation 1996
(All Rights Reserved)
AN31