ADS1118_13 Datasheet, PDF (29/38 Pages) Texas Instruments – Ultra-Small, Low-Power, SPI™-Compatible, 16-Bit Analog-to-Digital Converter and Temperature Sensor with Internal Reference

ADS1118
www.ti.com
SBAS457C – OCTOBER 2010 – REVISED FEBRUARY 2013
THERMOCOUPLE MEASUREMENT WITH COLD JUNCTION COMPENSATION
For an independent, two-channel thermocouple system, Figure 51 shows the basic connections. This circuit
contains a simple low-pass, anti-aliasing filter, mid-point bias, and open detection. While the digital filter of the
ADS1118 strongly attenuates high-frequency components of noise, TI generally recommends providing a first-
order passive RC filter to further improve this performance. The differential RC filter formed by the 500 Ω
resistors (R_DIFFA and R_DIFFB) and the 1 µF (C_DIFF) capacitor offers a cutoff frequency of approximately 320 Hz.
Additional filtering can be achieved by increasing the differential capacitor or the resistance values. However,
avoid increasing the filter resistance beyond 1 kΩ because the effects of the interaction with ADCs input
impedance begin to affect the linearity and gain error of the ADS1118. Because of the high sampling rates
supported by the ADS1118, simple post digital filtering in a microcontroller can alleviate the requirements of the
analog filter and can also offer the flexibility to implement filter notches at 50 Hz or 60 Hz. Two 0.1 µF (C_CMA and
C_CMB) capacitors are also added to offer attenuation of high-frequency common-mode noise components.
Because mismatches in the common-mode capacitors cause differential noise, TI recommends that the
differential capacitor be at least an order of magnitude (10x) larger than the common-mode capacitors.
3.3 V
3.3 V
RPU = 1 M
RDIFFA
500
RPD = 1 M
RDIFFB
500
GND
CCMA = 0.1 PF
AIN0
CDIFF = 1 PF
AIN1
CCMB = 0.1 PF
GND
GND
3.3 V
RPU = 1 M
RDIFFA
500
RPD = 1 M
RDIFFB
500
GND
CCMA = 0.1 PF
AIN2
CDIFF = 1 PF
AIN3
CCMB = 0.1 PF
0.1 PF
GND
VDD
Voltage Reference
(PGA Gain = 16)
±256mV FS
MUX
PGA
16-Bit
û ADC
Digital Filter
and
Interface
GND
Oscillator
Temperature
Sensor
SCLK
CS
DOUT/DRDY
DIN
GND
GND
GND
Figure 51. Two-Channel Thermocouple System
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