Datasheet

Digital Filter
and
Interface
Oscillator
Voltage
Reference
Temp Sensor
MUX
PGA
12-Bit
DS
ADC
GND
3.3 V
500 W
500 W
C
CMB
= 0.1 Fm
C
DIFF
= 1 Fm
C
CMA
= 0.1 Fm
GND
R = 1 M
PU
W
GND
3.3 V
0.1 Fm
GND
(PGA Gain = 16)
±256 mV FS
AIN0
AIN1
AIN2
AIN3
SCLK
CS
VDD
GND
DOUT/DRDY
DIN
R
DIFFA
R
DIFFB
3.3 V
500 W
500 W
C
CMB
= 0.1 Fm
C
DIFF
= 1 Fm
C
CMA
= 0.1 Fm
GND
GND
R
DIFFA
R
DIFFB
GND
GND
R = 1 M
PU
W
R = 1 M
PD
W
R = 1 M
PD
W
ADS1018
www.ti.com SBAS526B –NOVEMBER 2012–REVISED OCTOBER 2013
THERMOCOUPLE MEASUREMENT WITH COLD-JUNCTION COMPENSATION
For an independent, two-channel thermocouple system, Figure 22 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
ADS1018 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 ADS1018. Because of the high sampling rates
supported by the ADS1018, 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.
Figure 22. Two-Channel Thermocouple System
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