Cirrus-logic AN75 Manual do Utilizador descarregar pdf

MAY ‘97

AN75REV2



Crystal Semiconductor Corporation 1997

Crystal Semiconductor Corporation

P.O. Box 17847, Austin, Texas 78760

(512) 445 7222 FAX: (512) 445 7581

http://www.crystal.com

AN75

Application Note

Using the CS5525/CS5526 in Multiplexed Applications

By Jerome Johnston

The CS5525/CS5526 A/D converter is optimized

for the measurement of thermocouples. It is de-

signed with a single analog input, but includes four

digital output pins to control analog switches and

multiplexers. The performance characteristics of

an analog switch or multiplexer in a thermocouple

digitizer application are important because of the

typical protection circuitry which is included in the

signal path. Figure 1 illustrates a multiplexed ther-

mocouple circuit which includes input protection.

Resistor R, capacitor C, and diodes D1 and D2 pro-

vide input protection for the analog switch and the

A/D converter. Input protection guards against

ESD (Electro-Static Discharge), high voltage

glitches which can get coupled into long thermo-

couple leads in an industrial environment, and in-

correct connections which can cause +24 volts to

become directly connected to the digitizer input

terminals.

To achieve sufficient protection, the input protec-

tion resistor is usually 10 K

Ω

or greater. This resis-

tance can introduce measurement errors if leakage

currents in the circuitry are large. Even small

changes in leakage currents can affect accuracy

since thermocouple output voltages change very

little for each degree of temperature change (typi-

cally between 6 µV/°C and 80 µV/°C). Therefore,

leakage current from the analog switch or from the

input of the amplifier used to amplify the thermo-

couple signal can introduce significant errors if a

large protection resistor is used. A large value for

the protection resistor is desirable because the com-

bination of R and C also act as a low pass filter to

help reject 50/60 Hz. A large resistor value helps to

achieve a low frequency cutoff for the filter while

using a small capacitor value (many applications

use surface mounted components in which the

availability of surface mounted capacitors is limit-

ed to low values). The capacitor is usually a low

leakage device with a quality dielectric to achieve

low dielectric absorption.

Figure 2 illustrates the CS5525/CS5526 using the

p-channel J177 (or SST177 for surface mount)

JFET as an analog switch. The J177 has a maxi-

mum V

GS(off)

of 2.25 V, therefore a +5 V drive sig-

nal from one of the digital outputs from the

CS5525/CS5526 can turn it off (Driving the gate of

the JFET with only +3.0 volts will shut the J177 off

but it can exhibit more leakage current). When the

logic drive to the gate of the JFET is 0 V, the FET

will be turned on. It can safely handle input signals

of ±250 mV without forward biasing the channel to

Figure 1. Typical Thermocouple

Digitizer Input Circuitry

1 2 3 4

Resumo do Conteúdo

Página 1 - Application Note

Página 2 - 2 AN75REV2

AN752 AN75REV2gate junction. When ON, the J177 (SST177) exhib-its a resistance of 300 ohms maximum. If used toswitch signals less than ±250 mV, the le

Página 3 - AN75REV2 3

AN75AN75REV2 3Figure 5 illustrates the CS5525/CS5526 configuredto interface to a bridge transducer. The bridge is ex-cited with +5 V. The voltage refe

Página 4 - APPENDIX

AN754 AN75REV2APPENDIXJ177/SST177 SuppliersCalogic, Fremont, CaliforniaSiliconix, Santa Clara, CaliforniaInterfet, Garland, TexasPhilips, Eindhoven, T

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Cirrus-logic AN75 Manual do Utilizador

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