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2.6.2
Using the ADS126xEVM for Ratiometric Measurements
Ratiometric measurements are often used with resistive-type sensors requiring a current excitation source
(such as an RTD). The current is forced through the resistive sensor to generate a voltage signal for the
ADC input. If the current source deviates from the programmed value (because of drift or noise), an
apparent change in resistance is observed by the ADC. To correct for this error, the current source is also
forced through a precision resistor to generate the ADC reference voltage. In this ratiometric configuration,
a change in current directly affects the ADC input signal and reference voltage proportionally. Therefore,
the ratio of the input signal to reference voltage remains constant for a given sensor impedance.
A simple block diagram of a ratiometric connection, using a 3-wire RTD with the ADS126xEVM, is shown
in
Figure
11. The 3-wire RTD is connected to the J2 header on inputs IN7, IN6, and IN4. Two IDACs
output 250 µA (each) on pins AINCOM and AIN3 of the ADS126x. Jumper wires then connect IN7 to
COM, and IN6 to IN3. These jumper wires route the IDAC currents around the input filtering to prevent
voltage drops in the input signal path. IDAC current flow thought the R17 resistor, between IN4 and IN5, to
provide the ratiometric voltage reference. IDAC currents are routed to AVSS through JP3, or directly to
ground by connecting another jumper wire between IN5 and GND.
3-Wire
RTD
NOTE: The purpose of R19 is to boost up the negative reference voltage when using a single-
supply configuration. However, the ADS126x allows for the negative reference voltage to be
connected directly to AVSS (when used with a bipolar supply) or GND potential (when used
with a single supply). Short R19 or replace with a 0-Ω resistor to allow additional headroom
for the IDAC compliance voltage.
Although this example shows a 3-wire RTD, the ADS126x can also support 2- or 4-wire RTDs. For a 2-
wire RTD, use JP1 to replace the IN4 connection. For a 4-wire RTD, remove the IN6 to IN3 jumper wire,
remove the IN7 to COM jumper wire, and connect the additional RTD wire to COM.
SBAU206 – April 2015
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COM
IN7
IN6
JP1
IN3
IN4
+
500 uA
R17
1.95 V
3.9k
IN5
-
JP3
+
GND
R19
0.31 V
620
-
AVSS
Figure 11. ADS126xEVM Ratiometric Connection Example
Copyright © 2015, Texas Instruments Incorporated
AINCOM
ADS126x
AIN7
PGA
ADC1
AIN6
REF
AIN3
AIN4
AIN5
ADS126xEVM
ADS126xEVM Hardware
15
ADS126xEVM-PDK
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