Cirrus Logic AN75 User Manual

AN75

Application Note

Using the CS5525/CS5526 in Multiplexed Applications

By Jerome Johnston

The CS5525/ CS5526 A/D converter is o ptimized
for the measurement of thermocouples. It is de­signed with a single analog input, but includes four
digital outp ut pins to control a nalog switches and
multiplexers. The performance characteristics of
an analog switch or multiplexer in a thermocouple
digitizer ap plication are import ant because of the
typical protection circuitry which is included in the
signal path. Figure 1 illustra tes a mult iplexed ther­mocouple circ uit which include s input protectio n.
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 wh ich can get couple d into long thermo ­couple leads in an industrial env ironment, an d in­correct conn ections which c an cause +24 vol ts to
become directly connected to the digitizer input
terminals.

To achie ve sufficien t protection , the input protec­tion resistor is usually 10 KΩ or greater. This resis­tance can intro duc e m easure ment erro rs if l eaka ge
currents in the circuitry are large. Even small
changes in leakage currents can affect accuracy
since thermocouple output voltages change very
little for each degre e of tem perature c hange (ty pi-

cally betw een 6 µV/°C and 80 µV/°C). Th erefore,
leakage current from the analog switch or from the
input of the a mplifier us ed to ampl ify the th ermo­couple sign al can introduce signi ficant errors if a
large protec tion resistor is used. A large value for
the protection resistor is desirable because the com­bination of R and C also act a s a low pass filte r to
help reject 50/60 Hz. A large resistor value helps to
achieve a low freque ncy cutoff for the filter while
using a small capacitor value (many applications
use surface mounted components in which the
availabi lity of su rface mount e d capa ci tors is li mit ­ed to low values ). The capacitor is usua lly a low
leakage device with a qu ality dielec tric to ach ieve
low dielectric absorp ti on.

Figure 1. Typical Thermocouple

Digitizer Input Circuitry

Crystal Semiconductor Corporation

P.O. Box 17847, Austin, Texas 78760
[phone] FAX: [phone]
http://www.crystal.com

Figure 2 illu strates the C S5525/CS55 26 using the
p-channel J177 (or SST177 for surface mount)
JFET as an analog switch. The J177 has a maxi­mum V

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

GS(off)

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 ga te 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

Copyright  Crystal Semiconductor Corporation 1997

(All Rights Reserv ed)

MAY ‘97

AN75REV2

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Figure 2. CS5525/CS5526 Controls P-Channel

JFETs as analog switches.

gate junction. When ON, the J177 (SST177) exhib­its a resistanc e of 300 ohms maximum. If u sed to

switch signals less than ±250 mV, the leaka ge cur ­rent (25 °C) will be sub-picoamps.

AN75

the CS5525/CS5 526 is low. The -0.6V gate drive
shuts off the tran sistor with adeq uate ga te drive to
ensure very high off resistance. When the D0-D3
output drive is high, the transistor is on with a resis­tance less tha n 100 Ω. Signals of ±1 V can readily
be passed by the switch.

Some analog switches are available which are de­signed to function with supply voltages of ±5 V or
less. The Advanced Linear Devices ALD4201 quad
switch is used in Figure 4 (1/2 of dev ice is shown)
for switching the rmocouple signa ls. The switch i s
operated from a +5 V supply and from the NBV
voltage (-2.1) used for the CS5525/CS5526. This is
acceptable as the supply current requirement of the
ALD4201 is only a microampere. Leakage currents
for the ALD4201 are stated to remain below 500
pA over the industrial temperature range. On resis­tance over the industrial t emperature range with a
total supply of +5V or more is less than 300 Ω.

Figure 3 illustrates an example of using an en­hancement mode transistor switch as an analog
switch. The SD5400-2 quad DMOS switch is illus­trated (the SD5000-2, SD5200-2, and SD5300-2
series are similar). The diode-resistor drive combi­nation on the gate of the SD5400-2 allows the gate
to go to -0.6 V when the DO -D3 logic dr ive from

This application note illustrates the flexibility of
using the CS5525/CS5526 digital output pins to
control analog switches and multiplexers. While all
the circuits introduced have illustrated the CS5525/
CS5526 being used in thermoc ouple applica tions,
the device can be used for bridge transducers as
well.

Figure 3. CS5525/CS5526 Controls N-Channel DMOS transistors as analog switches.

2 AN75REV2