Datasheet

R
L
(1)
15W
R
L
(1)
15W
R
L
(1)
15W
R
BIAS
(2)
833W
R
COMP
(2)
110W
IDAC1
1.5mA
REFP0
REFN0
IDAC2
1.5mA
Modulator
MSP430
or
other
Microprocessor
ADS1247/48
PGA
Gain=128
VDD
+3.3V
+5V
RESET
AVDD
RTD
AVSS DGND
GND
SCLK
AIN0
AIN1
DIN
DOUT/DRDY
CS
START
CLK
DVDD
IN
EN
V
NR
OUT
GND
TPS79333
0.1 Fm 2.2 Fm
ADS1246
ADS1247
ADS1248
www.ti.com SBAS426G –AUGUST 2008–REVISED OCTOBER 2011
Hardware-Compensated, Three-Wire RTD
be equal to the resistance of the PT-100 sensor at
Measurement Example
+25°C (approximately 110Ω). The IDAC current is set
to 1.5mA. This setting results in a differential input
Figure 85 is an application circuit to measure
swing of ±14.7mV at the inputs of the ADC. The PGA
temperatures in the range of 0°C to +50°C using a
gain is set to 128. The full-scale input for the ADC is
PT-100 RTD and the ADS1247 or ADS1248 in a
±19.53mV. Fixing R
BIAS
at 833Ω fixes the reference at
three-wire, hardware-compensated topology. The two
2.5V and the input common-mode at approximately
onboard matched current DACs of the ADS1247/8
2.7V, ensuring that the voltage at AIN0 is far away
are ideally suited for implementing the three-wire
from the IDAC compliance voltage.
RTD topology. This circuit uses a ratiometric
approach, where the reference is derived from the
The maximum number of noise-free output codes for
IDAC currents in order to achieve excellent noise
this circuit in the 0°C to +50°C temperature range is
performance. The resistance of the PT-100 changes
(2
ENOB
)(14.7mV)/19.53mV.
from 100Ω at 0°C to 119.6Ω at +50°C. The
compensating resistor (R
COMP
) has been chosen to
(1) RTD line resistances.
(2) R
BIAS
and R
COMP
should be as close to the ADC as possible.
Figure 85. Three-Wire RTD Application with Hardware Compensation
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