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F
F
CM _ 1
CM _ 2
1
2 RS1
C
1
CM1
2 RS2
CCM2
Theory of Operation
(6)
F F
CM _ 1 CM _ 2 3.2 KHz
(7)
5.1.6 PGA Gain and Common Mode Voltage
The K-Type thermocouple has a temperature range of –270°C to 1372°C. The K-Type thermocouple
produces a voltage input to the PGA ranges from –6.458 mV to 54.886 mV. Therefore, the thermocouple
full scale input range (V TC_IN_FS ) is as shown in and Equation 9.
V TC_IN_FS = [54.886 mV – (–6.458mV) ] = 61.344 mV (8)
VREF
2.048 V
PGA Gain 33.4 V / V
VIN_FS
61.344 mV
(9)
The closest PGA gain supported by ADS1220 is 32 V/V.
The differential full scale input voltage (V FS ) range for the PGA at a gain of 32 V/V is ±64 mV. The
thermocouple full scale input voltage (V TC_IN_FS ) lies within the PGA full scale input.
K-type thermocouple temperature range:
K-type thermocouple voltage output overlapped
on PGA full scale input range:
PGA output overlapped on ADC full scale input
range:
The ADC can measure inputs differentially, which means the ADC measures one input with respect to
another input with both inputs' active signals. For 5-V single power supply operation, AIN_P and AIN_N
signals should not be allowed to swing below ground. Therefore, it is important to set common mode
voltage of the differential signals equal to the mid-supply so that the differential signals swing above and
below the common mode voltage and always remain within the valid ADC range as shown below in
Figure 20. A1 and A2 are not rail-to-rail output amplifiers. Therefore ADC_IN_P and ADC_IN_N have to
be within (AVSS + 0.2 V) and (AVDD – 0.2 V). Equation 10 shows the most confusing common mode
voltage requirement for the inputs:
VIN
PGA VIN
PGA
CM
AVSS 0.2 V V AVDD 0.2 V
2
2
For example, AVDD = 5 V, AVSS = 0 V, PGA Gain = 32 V/V and
Internal ADC Reference (V REF ) = 2.048 V.
Therefore, the maximum possible differential input voltage that can be applied is
VREF
2.048 V
0.064 V
PGA Gain
32 V/V
So, the allowed common mode voltage range is 1.224 V ≤ V CM ≤ 3.776 V.
(10)
TIDU271–May 2014
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Temperature Sensor Interface Module for Programmable Logic Controllers
(PLC)
Copyright © 2014, Texas Instruments Incorporated
19