Specifications

AM25T Solid State Multiplexer

recommends use of Teflon, polyethylene, or polypropylene insulation around

individual conductors. Do not use PVC as conductor insulation. PVC may be

used as a cable jacket.

With long lead lengths, a delay within the measurement instruction will allow

the capacitance of the lead wires to discharge before the measurement is made.

Consult the Measurement Section of your datalogger manual for more

information.

6. Datalogger Programming

The CR23X, CR1000, and CR9000 each have a special instruction for

measuring the AM25T. While such an instruction may be added to the CR10X

operating system in the future, the CR10(X), 21X and CR7 use a program that

contains a loop in which the multiplexer is advanced to the next channel and

the datalogger makes a measurement.

An outline of the looping datalogger program appears below. Steps two

through seven are used to measure the reference temperature for

thermocouples and could be skipped for voltage measurements.

1. Turn on the AM25T

2. Measure the output of the RTD full bridge

3. Find the resistance of the RTD

4. Calculate the reference temperature

5. Pulse the CLK line once (long lead length compensation only)

6. Measure the excitation at the full bridge (long lead length compensation

only)

7. Reset the AM25T by pulsing the RES line (long lead length compensation

only)

8. Loop through the measurements

a) Pulse the CLK line twice

b) Measure the sensor

c) Convert the engineering units

d) Go to a) until all the sensors have been measured

9. Turn off the AM25T

Earlier versions of the AM25T had a zener diode for transient protection on the

RTD excitation. This diode limited the negative excitation to -400 mV.

Instruction 6 used to measure the RTD uses both a positive and negative

excitation voltage. Table 6-1 lists recommended excitation voltages and

measurement ranges to use with these early multiplexers. The newer

multiplexers will work with the excitation voltages in Table 6-1, but the higher

excitations used in the example programs provide more immunity to noise.

AM25T multiplexers starting with serial number 1839 allow an excitation

voltage up to ± 5000 mV.

6.1 Example CR10(X) Programs

The two example CR10(X) programs in this section cover measuring

thermocouples and low level voltage sensors with the differential measurement

instruction.