Model TT231-0600
Two-Wire RTD Transmitter w/USB
Acromag, Inc. Tel: 248-295-0880
- 27 - http://www.acromag.com
- 27 -
http://www.acromag.com
BLOCK DIAGRAM
How It Works
Key Points of Operation
- Signal Path is Analog
- Unit is Loop Powered
- Input is Non-Isolated
- Conversion is Differential
- Configuration is Digital
- Calibration is Digital
- Converts RTD with a Single
Differential Measurement
- Output/Power Terminals are Not
Polarized
- Only ± Leads must be balanced
for lead compensation.
This digitally calibrated analog transmitter uses a unique, low noise, voltage to
current conversion scheme that delivers equivalent 12-bit performance, but does
not actually digitize the input signal. Instead it uses integrated Digital-to-Analog
Converters (DAC) to adjust the zero offset, control the excitation currents, and drive
linearization correction to the input. These DAC’s work together to achieve nearly
12-bits of adjustment resolution, but do not operate directly on the analog input
signal itself. Likewise, there are no microcontrollers in the I/O signal path of this
design, and no embedded firmware relative to processing the signal. Transmitter
functionality is actually hard-wired (integrated) into an application specific
component IC. The only microcontroller in this design is used to convert the
external USB signals to an internal SPI bus signal during reconfiguration. Windows
configuration software is used to write configuration parameters into non-volatile
EEPROM memory at setup. These stored parameters are auto-downloaded into the
transmitter ASIC at power-up and will define its normal operation. Setup involves
selecting the input type (Pt RTD or Resistance), input wiring (2-wire, 3-wire/4-wire),
the Pt RTD alpha coefficient, the input range zero (-50°C, 0°C, or 0°F), the input
range full-scale (up to 850°C or 900Ω), the output range zero, the output range full-
scale, specifying the output over and under-scale thresholds and alarm detents, and
setting upscale or down-scale lead break or sensor fault detection.
DAC
DAC
IREF
ILIN
DAC
ZERO
LINEARIZER
CIRCUITRY
PGA
SPI & CTRL CIRCUITS
OSC
SUB-REGULATOR
DRIVER
VOLTAGE
REFERENCE
MUX
1.193V
1
2
OUTPUT CURRENT AMP
S
C
L
K
S
D
IO
C
S
1
C
S
2
R
9-32VDC
+
-
~
~
+
-
LOOP+
LOOP-
4-20
mA
TWO-WIRE OUTPUT
POLARITY
PROTECTION
5
6
I
LOAD
EARTH
GROUND
MICRO
USB-TO-SPI
USB
+5V
+5V
PORT
IN+
IN-
2
1
3
4
NC
RTD INPUT
+
-
REF2
REF1
R
Vo
vi
Vs
Rset
Rlin
SCHOTTKY
BRIDGE
1.193V
Cflt
1
2
3
5
4
6
1
2
3
5
4
6
MUX
493uA
493uA
2W
2W
2W
2W
L
H
493uA
493uA
Rz1 Rz2 Rz3
0C 0F
-50C
Rcm
475
Rz sets input range zero
Rz forces diff input voltage near 0 at Tmin
+
-
V cm '=0.467V
Rcm sets a positive bias within common mode voltage range
3-WIRE RTD CONNECTION
Pt RTD
1
5
.8
K
1
2
.1
K
6.34K
POSITIVE AND NEGATIVE INPUT
LEADS MUST BE SAME LENGTH,
TYPE, AND SIZE FOR LEAD
COMPENSATION.
986uA
493uA
493uA
Transmitter ASIC
Iout=50*Vo/6340
EEPROM
Vs
REF1=REF2 (MATCHED)
TT231-0600 SIMPLIFIED SCHEMATIC
(FILTERING AND DETAIL OMITTED FOR CLARITY)
P
A
S
S
T
R
A
N
S
IS
T
O
R
4-20mA
4-20mA
COMMON MODE VOLTAGE OF IR DROP IN EACH LEAD IS REJECTED BY TRANSMITTER
36V
CONVERTER
BUFFER
DRIVER
(FILTERING OMITTED FOR CLARITY)
1. THIS NON-ISOLATED RTD/RESISTANCE TRANSMITTER IS INTENDED FOR UN-GROUNDED RTD PROBES.
Iref 1,2 =5*Vref/12100
x50
I=Vo/6340
9
2
.0
7
9
.6
9
8
.8
1K
1K
1K
1K
C
C
7
8
TB4
THERE ARE NO INTERNAL
C CONNECTIONS ARE USED
FOR OPTIONAL SOURCED
WIRING CONNECTIONS
CONNECTIONS TO C TERMINALS
TB3
TB1
TB2