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APPLICATION BULLETIN
By R. Mark Stitt (602) 746-7445 and David Jones
Mailing Address: PO Box 11400 鈥?Tucson, AZ 85734 鈥?Street Address: 6730 S. Tucson Blvd. 鈥?Tucson, AZ 85706
Tel: (602) 746-1111 鈥?Twx: 910-952-111 鈥?Telex: 066-6491 鈥?FAX (602) 889-1510 鈥?Immediate Product Info: (800) 548-6132
USE LOW-IMPEDANCE BRIDGES ON 4-20mA CURRENT LOOPS
If you need more transducer excitation current than is avail-
able on a standard 4-20mA 2-wire current loop, consider a 3-
wire transmitter. A 2-wire 4-20mA transmitter uses the same
two wires for signal and power. Part of the 4mA minimum
loop current is used to power the transmitter circuitry. The
remaining current can be used for transducer excitation. In
some applications even the entire 4mA is not enough for
transducer excitation.
Exciting a low-impedance bridge (such as a 350鈩?bridge)
often requires more current than is normally available from
a 2-wire 4-20mA current loop. A 350鈩?bridge excited with
a 10V reference requires more than 28mA. An easy way to
solve this problem is to use a 3-wire transmitter such as the
XTR110.
The XTR110 3-wire transmitter is similar to a 2-wire trans-
mitter, except that one of the two wires is connected to
ground and a third (power supply) wire is added. With
external power available at the transmitter, it is easy to
interface to low-impedance bridges. The XTR110 contains
an on-board precision 10.0V reference for sensor excitation.
The reference has a sense connection so that its output can
be easily boosted.
THE COMPLETE BRIDGE TO CURRENT-LOOP
CIRCUIT IS SHOWN IN FIGURE 1
A series-pass transistor, Q
1
, boosts the XTR110鈥檚 10.0V
reference output-current to drive the bridge. Using an exter-
nal pass transistor allows high output-drive without over-
heating the XTR110. To improve reliability you may need to
heat-sink Q
1
, especially for high ambient temperatures.
An INA114 precision instrumentation amplifier is used to
amplify the bridge output to drive the XTR110 low-imped-
ance input. The INA114 is operated single-supply from the
10V reference to eliminate any error due to power-supply
changes. The XTR110 is connected so a 1V to 5V input on
pin 5 produces a 4-20mA output. A voltage divider buffered
by an OPA177 drives the INA114
reference
pin. With 3V on
the
reference
pin, the INA114 output is 3V with the bridge
in balance. This produces a 12mA (mid-scale) XTR110
output with the bridge in balance. If you want to use the
bridge in a unipolar mode, the resistor divider can be set to
put either 1V or 5V on the INA114
reference
pin to produce
either a 1V or 5V INA114 output at bridge balance. In any
case, select the INA114 gain-set resistor for 4V INA114
output change with
鹵full-scale
bridge output.
A P-channel enhancement-mode MOSFET, Q
2
, is used to
drive the 4-20mA output current. Using an external FET to
drive the output current improves precision by eliminating
漏
thermal feedback. If an internal driver were used, the signal-
dependent power change due to the 4-20mA current change
would result in relatively large nonlinearity in the transfer
function.
Burr-Brown offers a complete line of 2-wire and 3-wire
current loop transmitters and receivers.
XTR101
General purpose two-wire 4-20mA current-loop transmitter.
This transmitter has an instrumentation amplifier input and
two 1mA current sources for transducer excitation and
offsetting.
XTR103
Two-wire RTD 4-20mA current-loop transmitter with 9V
compliance. Similar to XTR101, but with internal lineariza-
tion circuitry for direct interface to RTD Resistance Tem-
perature Detectors. The XTR103, along with an RTD, forms
a precision temperature to 4-20mA current loop transmitter.
Along with an RTD, the XTR103 can achieve better than
0.1% span linearity over a 鈥?00擄C to +850擄C temperature
span.
XTR104
Two-wire bridge 4-20mA current-loop transmitter with 9V
compliance. Similar to XTR101, but with shunt regulator
and linearization circuitry for direct interface to high-im-
pedance strain-gauge and Wheatstone bridges. The XTR104
can provide better than 0.1% span linearity from bridges
with uncorrected linearity in excess of 2%.
XTR110
Three-wire 4-20mA current-loop transmitter. Essentially a
precision, single-supply voltage-to-current converter with
an internal 10.0V reference and input resistor network for
span offsetting. Various input-output ranges are available
by pin strapping so that 0 to 5V or 0 to 10V inputs can be
used to get 0 to 20mA or 4 to 20mA outputs for example.
RCV420
Self-contained 4-20mA receiver. Conditions and offsets
4-20mA input signals to give a precision 0-5V output.
Contains precision voltage reference, 75鈩?precision sense
resistor and
鹵40V
common-mode input range difference
amplifier. The RCV420 has a total combined span and zero
error of less than 0.1%鈥攁djustable to zero.
AB-043
Printed in U.S.A. September, 1992
1992 Burr-Brown Corporation
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