Watlow Electric Revision 5 - Manual (Page 27)

22
5.0 ANALOG INPUTS
Connecting analog signals to the ANAFAZE SYSTEM 32 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 5.1 through 5.4 indicate
some of the potential areas for concern. [See typical input DIAGRAM in section
5.13].
5.1 COMMON MODE VOLTAGE
Common mode voltage is the voltage between the ground at the sensor and
the ground at the ANAFAZE SYSTEM 32. 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 SYSTEM 32 and
the sensors. The effects are minimized by locating the SYSTEM 32 as close as
possible to the sensors. Do not exceed the maximum common mode voltage of
150 volts AC.
5.2 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 SYSTEM 32
capacity to integrate the signal over a multiple of the power line frequency.
Further reduction can be achieved by locating the SYSTEM 32 near the sensors
and by using twisted and shielded sensor wires. To ensure accurate readings, the
maximum of normal mode plus signal should not exceed -10mv to +65mv.
5.3 GROUNDING
For best accuracy, observe the grounding recommendations for connecting each
input and output signal. The analog signal grounds should be connected to the
low 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.
5.4 SOURCE IMPEDANCE
Each sensor has a certain output impedance which is effectively connected across
the ANAFAZE SYSTEM 32 input amplifier when a measurement is made. To
reach the rated accuracy, the maximum source impedance should not exceed 300
ohms. Consult ANAFAZE for operation with higher source impedance.