Watlow 12LS Controller - Manual (Page 24)

3-10
3.3 ANALOG INPUTS
Connecting analog signals to the ANAFAZE 12LS is normally straightforward. Most thermocouples can be directly
connected and mixed in any order. Other types of analog signals such as mAdc or Vdc require scaling resistors
installed on the 12LS inputs. However, some problems may occur that could reduce accuracy and possibly damage the
unit. Sections 3.1 through 3.4 indicate some of the potential areas for concern. [See typical input DIAGRAM in
section 3.13].
3.3.1COMMON MODE VOLTAGE
Common mode voltage is the voltage between the ground at the sensor and the ground at the ANAFAZE 12LS. 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 12LS and the sensors. The effects are minimized by using isolated
sensors or ungrounded thermocouples. Since the 12LS uses common ground inputs, if grounded sensors are used, any
common mode voltage will cause current flow in the sensor leads and reading errors.
3.3.2NORMAL 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 12LS
capacity to integrate the signal over a multiple of the power line frequency. Further reduction can be achieved by
locating the 12LS 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 +60mv.
3.3.3GROUNDING
For best accuracy, observe the grounding recommendations for connecting each input and output signal. The analog
signal grounds should be connected to the analog ground terminals. The communication and control outputs should
be connected to their respective grounds. Do not mix the grounds or connect them together. If possible, route the
analog signal cables separately from the communication, control and power cables.
3.3.4 SOURCE IMPEDANCE
Each sensor has a certain output impedance which is effectively connected across the ANAFAZE 12LS input amplifier
when a measurement is made. To reach the rated accuracy, the maximum source impedance should not exceed 500
ohms. Consult ANAFAZE for operation with higher source impedance.