tidu271

www.ti.com
Theory of Operation
I1
R I1
C I_CM1
R RTD
R I2
C I_DIFF
GAIN
ΔΣ
ADC
R ZERO
I2
REFP
REFN
C I_CM2
C R_CM1 C R_CM2
C R_DIFF
R R1
R R2
The –3 dB cutoff frequency of differential input filter at 186 Ω RTD resistance (at mid-scale temperature)
can be calculated as given in Equation 30.
1
F 3 dB _ I _ DIFF
2 CIDIFF RI1 RRTD RI2
F
3 dB _ I _ DIFF
402.1Hz
The –3 dB cutoff frequency of differential reference filter can be calculated as given in Equation 31.
1
F 3 dB _ R _ DIFF
2 CR DIFF RR1 RREF RR2
R REF
Figure 45. Common Mode and Differential Mode Filters on RTD Input and Reference
R I1 = R I2 = 4.12 K
C I_DIFF = 0.047 µF
To ensure that mismatch of the common-mode filtering capacitors is not translated to a differential voltage,
the common-mode capacitors (CI_CM1 and CI_CM2) are chosen to be 10 times smaller than the
differential capacitor. This common-mode capacitors' size results in a common-mode cutoff frequency that
is roughly 10 times larger than the differential filter, making the matching of the common-mode cutoff
frequencies less critical.
C I_CM1 = C I_CM2 = 4700 pF
Although it is not always possible to exactly match the corner frequencies of all the filters, a good
compromise is to attempt to balance the corner frequencies of the input path differential filter and the
reference path differential filter, because these filters have a dominant effect in the performance.
RR1 = RR2 = 4.7 K
CR_DIFF = 0.033 µF
(30)
F
3 dB _ R _ DIFF
405.83 Hz
(31)
TIDU271–May 2014
Submit Documentation Feedback
Temperature Sensor Interface Module for Programmable Logic Controllers
(PLC)
Copyright © 2014, Texas Instruments Incorporated
37