DNx-AI-208 Analog Input Layer
27
Tel: 508-921-4600
www.ueidaq.com
Vers:
4.6
Date: November 2013
File:
AI-208 App B.fm
© Copyright 2009
United Electronic Industries, Inc.
B.3
Using Shunt
Resistors on
the AI-208
There are two programmable digital shunt calibration resistors on the AI-208: Ra
and Rb. The Shunt calibration resistor Ra shunts the branch R4 and Rb shunts
R3
Figure B-3. Using Shunt Resistors on the DNA-AI-208 Layer
The internal circuitry of the AI-208 makes it difficult to know the exact value of
the resistance used to shunt the Wheatstone bridge, due to the following factors:
•
The digital shunt resistor accuracy is only 30% and needs to be mea-
sured prior to doing any calculation.
•
The resistance of internal components on the AI-208 such as multiplex-
ers and switches is not negligible and needs to be measured and added
to the global shunt resistance.
•
Semiconductors involved in the shunt calibration circuitry have signifi-
cant changes in resistance with temperature change
To overcome those problems, UEI included ±25 ppm/°C 5kOhm 0.1% resistors
into shunt calibration circuitry. When PS+ is connected to S+ on the screw termi-
nal panel, internal circuitry makes it possible to measure voltage drop on one of
those precision ±25 ppm/°C resistors, thus precisely measuring current through
them. By knowing current and voltage drop in the shunt calibration circuitry, you
can calculate total resistance of the switches, resistors, and multiplexers, which
is equal to the shunt resistance.
Due to the additional resistance in the shunt calibration circuitry and the 30%
accuracy of 200kOhm digital potentiometer, the shunt calibration resistance can
be between 10k and 170k Ohms.
R4 (Rg)
R1
R2
R3
Ra
Rb
External Wiring
STP
AI-208
5k 0.1%
R
L
R
L
R
L
R
L
S+
S–
P+
Mux
PS+
S+
S–
5k 0.1%
Strain
Gauge
P+
P–
i
i
J
Disconnect normal
sense wiring from
PS+ when shunt
calibration is used.
To use the shunt
calibration feature,
remove jumper.
