The CS5525/CS5526 A/D converter is optimized
for the measurement of thermocouples. It is designed
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 thermocouple
circuit which includes input protection.
Resistor R, capacitor C, and diodes D1 and D2 provide
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 thermocouple
leads in an industrial environment, and incorrect
connections which can cause +24 volts to
become directly connected to the digitizer input
terminals.