Download note1448_20mA Analog Input

Application Note #1448
Using 0-20ma Device with Analog Inputs of Galil Controllers
This note discusses 3 basic options for connecting devices which provide 0-20ma of
current output to the analog inputs of the Galil Controllers. The analog inputs of the
Galil Controller except a voltage which ranges between –10V to +10V. This requires
that the current be converted to a voltage. This simplest method for conversion is to
provide a simple resistor to ground, connected in parallel to the analog input:
Analog Input of
Controller
0-20ma Analog
Device
500 ohms
~47K ohms
The main disadvantage is the effect of the analog input impedance of 47K ohms. This
resistance is not constant or precise. Therefore, this voltage may vary significantly over
operating conditions and from one controller to the next. This method will provide a
voltage that will vary from 0 to 9.89 V over the full current range of 0-20ma. This is a
simple, easy solution for rough measurements.
A more precise method of conversion from 0-20ma to voltage would require a buffer
amplifier:
Simple Buffer Circuit for
0-20 ma Device
+12V
.1uf
0-20ma Analog
Device
To Analog Input
Provide 0 - 10V
500 ohms
.1uf
-12V
General Purpose Voltage
Follower such as LM310
-1Galil Motion Control, Inc. • 3750 Athert on Road • Rocklin, CA 95765 USA • 800-377-6329 • Ph: 916-626-0101 • Fax: 916-626-0102 • www.galilmc.com
This circuit provides for a more precise conversion which will not be affected by the
analog input circuitry. This will provide a voltage that will vary from 0 to 10V over the
full current range of 0-20ma. The precision and drift of the voltage will depend on the
quality of the 500 ohm resistor, therefore, a precision resistor is recommended.
To get more full operation over the complete voltage range, a simple opamp circuit can
be used:
Analog Buffer Circuit for
0-20 ma Device
-10V
1000 ohms
1000 ohms
+12V
0-20ma Analog
Device
.1uf
500 ohms
-
To Analog Input
Provides -10V to +10V
+
.1uf
-12V
Use General Purpose Op-Amp
such as LM748
This circuit is not affected by the analog input circuitry and provides a voltage range of
+10V to -10V for the current output of 0-20ma. The precision and drift of the voltage
will depend on the quality of the resistors, therefore, precision resistors are
recommended.
-2Galil Motion Control, Inc. • 3750 Athert on Road • Rocklin, CA 95765 USA • 800-377-6329 • Ph: 916-626-0101 • Fax: 916-626-0102 • www.galilmc.com