Rockwell Automation Liqui-Flo DBT Reach Drive - Manual (Page 24)

2-18 DBT Reach Drive User Manual
RTD/NTC Sourcing
A 2-wire RTD or NTC temperature sensor may be used. A chassis connection is
available for each sensor for use with shielded cable. Each channel requires up to
three connections.
Multiplexer and Amplifier
An analog multiplexer sequentially connects the temperature sensors one at a time to
the analog output. The active sensor channel is indicated by a 3-bit output (A0-A2).
When using an RTD or NTC, a simple voltage divider is formed and the device is
probed differentially.
The sensor voltages are amplified using a high-quality, rail-to-rail instrumentation
amplifier with gain adjustment. To accomodate a variety of sensors and temperature
ranges, an amplifier boosts the analog sensor voltage in order to increase the
temperature resolution. Discrete gains of x10 through x80 are selectable through DIP
switch settings. A 4th order low pass filter rejects normal-mode noise at the analog
input with a cutoff frequency of approximately 16 Hz.
To determine the proper gain setting:
1. Determine R
MAX
, the maximum resistance of the sensor over the temperature
range to be measured.
2. Determine the maximum voltage across R
MAX
using the following equation:
V
MAX
= 1.00V X R
MAX
/ (R
MAX
+ 998)
3. Select the largest gain that results in V
MAX
* Gain (x10, x20, or x40), being less
than 10 volts.