Model TT231-0600
Two-Wire RTD Transmitter w/USB
Acromag, Inc. Tel: 248-295-0880
- 28 - http://www.acromag.com
- 28 -
http://www.acromag.com
How It Works…
This transmitter uses a unique signal processing method that reduces error by
converting the 3 or 4-wire sensor with a single differential measurement, including
the lead-wire compensation. During operation, a small excitation current is passed
through the positive lead of the RTD element. A matching excitation current is
passed through a zero pedestal resistor Rz and into the minus lead of the sensor
element. These currents combine and return to the unit via a third lead that is
terminated with a common-mode resistance in the unit (3-wire connection). The
voltage drop produced in the series-connected zero resistor of the minus lead has
the effect of driving the differential input voltage across the bulb and in parallel
with the input amplifier near 0V, for bulb temperatures near the minimum
temperature for the RTD range (-50°C, 0°C, or 0°F). The return current sinking
through the common-mode resistance drives a positive-bias to the differential
voltage signal that is proportional to the RTD element resistance. The differential
voltage measured by the transmitter is corrected slightly to make it linear with
temperature by modulating the sensor excitation current with a value determined
during calibration, then converted to a proportional process current at its output.
Because the currents in each lead match, and if both the positive and negative leads
to the RTD are of the same length, type, and diameter, then the IR drop in these
lines will create small common-mode voltages that are effectively rejected by the
differential instrumentation amplifier measurement. In this way, the measured
signal is compensated for the additional resistance of the ±lead wires without
making a separate measurement. Refer to the block diagram to gain a better
understanding of how this transmitter works.
A third sensor wire is used to compensate the sensor for the resistance of the lead
wires, which can affect the accuracy of the RTD bulb given its low initial resistance
(100 ohms at 0°C typical), and its small change in resistance per degree of
temperature change. Here, the third lead wire is used as the return path for both
the positive and negative sensor lead currents.
As long as both the positive and negative lead wires to the resistance bulb are the
same type and length, their individual contributions to the differential signal cancel
out (as equal IR drops in each lead), and the precise voltage across the RTD element
is measured directly proportional to its sensed temperature. Connecting without
this third lead causes the sensor excitation current to return via the minus lead,
then combining with the minus lead current in the small jumper placed between
terminals 3 & 4 of the transmitter for a 2-wire sensor connection. This unbalances
the sensor measurement preventing lead-wire compensation. The current returned
via the third sensor lead is shunted through a common-mode resistor, effectively
biasing the input signal above 0V and into the common mode input range of the
amplifier. The small resistance of this line adds a small common-mode voltage that
increases the bias and is essentially rejected by the amplifier. Note that if the
sensor is connected via two-wires, the lead-wire resistance is not compensated for.
Thus, for two-wire sensors, you must a small jump-wire between leads 3 & 4 which
allow the combined excitation currents to reach the common-mode shunt resistor
and properly bias the sensor. Note that any 2-wire sensor can be made to
compensate for its lead-wire resistance by simply adding a third lead to the sensor
(in place of the jumper), and for this unit, that third lead can be a different size and
type of material than the ±input leads to the sensor.