Watlow 8PID Controller - Manual (Page 18)

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4.0 ANALOG INPUTS
Connecting analog signals to the ANAFAZE 8 PID is normally straightforward. Most
signals, including thermocouples can be directly connected and mixed in any order.
However, some problems may occur that could reduce accuracy and possibly damage
the unit. Sections 4.1 through 4.4 indicate some of the potential areas for concern. [See
typical input DIAGRAM in section 4.13]
4.1 COMMON MODE VOLTAGE
Common mode voltage is the voltage between the ground at the sensor and the ground at
the ANAFAZE 8 PID. It can be an AC or DC voltage and appears equally at the high
and low input terminals. Frequently it is caused by large currents flowing in the ground
path between the ANAFAZE 8 PID and the sensors. The effects are minimized by
locating the ANAFAZE 8 PID as close as possible to the sensors. Do not exceed the
maximum common mode voltage of 125 volts AC.
4.2 NORMAL MODE VOLTAGE
Normal mode voltage appears across the terminals of the input and is the signal from the
sensor plus any undesirable noise. The major cause of this noise is AC power line pick-
up. The effects are reduced by the ANAFAZE 8 PID's capacity to integrate the signal
over a multiple of the power line frequency. Further reduction can be achieved by
locating the ANAFAZE 8 PID near the sensors and by using twisted and shielded sensor
wires. Do not exceed the maximum normal mode voltage of 2 millivolts.
4.3 GROUNDING
For best accuracy, observe the grounding recommendations for connecting each input
and output signal. The analog signal grounds should be connected to the ground
terminals on the analog input terminals. The communication and control outputs should
also be connected with their respective grounds. Do not mix the grounds or connect them
together. The analog input section is optically isolated from the processing and control
section. Connecting the grounds together will negate this feature and could damage the
unit. If possible, route the analog signal cables separately from the communication,
control and power cables.
4.4 SOURCE IMPEDANCE
Each sensor has a certain output impedance which is effectively connected across the
ANAFAZE 8 PID input amplifier when a measurement is made. To reach the rated
accuracy, the maximum source impedance should be 20 ohms. Consult ANAFAZE for
operation with higher source impedance.