Model TT231-0600
Two-Wire RTD Transmitter w/USB
Acromag, Inc. Tel: 248-295-0880
- 29 - http://www.acromag.com
- 29 -
http://www.acromag.com
How It Works…
The zero point of the calibrated input range is set via a zero resistor Rz, connected
in series in the minus input lead. From the factory, three resistance values are
installed in three separate minus lead paths, and are at ohm values just below that
of a 100Ω Pt RTD corresponding to temperatures -50°C, 0°C, and 0°F. For two-wire
sensor connections, only a 0°C input range zero may be selected. The voltage drop
produced in Rz drives the differential voltage measured across the sensor to be near
zero at Tmin of the RTD range, as the excitation current in each lead is matched.
The combined excitation current of each lead is then shunted into a 475Ω common
mode resistor Rcm, producing a positive bias for the input sensor within the input
common mode range of the differential amplifier, as it ensures that the lowest
common mode input voltage is greater than the minimum range limit of the
amplifier.
The units excitation currents are digitally adjustable via the Iref DAC. From the
factory, this current is set to a nominal value of 493uA via the 12.1K Rset resistor
(480uA to 510uA range). It can be digitally adjusted to other levels during
calibration. The excitation current values are also influenced by the linearity DAC.
All RTD’s have a nonlinear response over temperature that is approximated by a
quadratic equation. The linearity DAC uses positive feedback from the input signal
to produce a system response that is also nearly quadratic, but curving in the
opposite direction, producing a net response that is very linear. This DAC allows the
nonlinearity error to be calibrated out by modulating the excitation current with the
input signal of the RTD during calibration, producing a nearly 40:1 improvement in
linearity. The adjustment range of this linearity correction is set via the 15.8K Rlin
resistor, which has been optimized for increased accuracy for the most common
spans that occur between -50°C and +500°C.
The PGA includes a zero DAC that allows the magnitude of the zero output current
to be precisely adjusted near 4mA. The output voltage of the PGA voltage amplifier
is converted to current through a 6.34K Rvi resistor at its output, just prior to the
current amplifier that drives the output loop. The current gain of this output
current amplifier is 50x. Note that the output loop is bridge-coupled to the
transmitter, making the transmitter output polarity insensitive.
The USB port ground is common to the circuit ground. The USB port ground of most
PC’s is common to the USB cable shield and earth ground. The output current loop
is typically earth grounded at the loop supply minus connection. For this reason, it
is recommended that USB signals be isolated when connected to a PC to prevent a
ground loop from occurring between the PC earth ground and the traditional
current loop earth ground.