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14.1.6 Resistance

 

Index: 4

AIN Extended Feature Resistance will calculate the resistance of a specific circuit element.

 

Operation:

When AIN_EF_READ_A is read the T7 reads the analog input and calculates the resistance. The calculated resistance is returned and the voltage and current used in the calculation are then available to be read.

 

Excitation Circuits:

There are six supported excitation circuits. Three based on a current source and three based on a voltage divider.

Current source: Current source circuits are the simplest of the supported excitation circuits. The RTD is connect to the current source and to GND. The top of the RTD is then connected to an analog input. More than one RTD can be connected in series. When using a series of RTDs both sides of the RTD need to be connected to AINs and those AINs need to be set to differential, care must also be taken to ensure that the total voltage drop across the RTDs can not exceed the current source's maximum output. Following, are the 3 supported current source excitation circuit indices. 

Circuit index 0: 200 microamp current source.  This index uses the stored factory calibration value of the 200 microamp current source. 
Circuit index 1: 10 microamp current source.  This index uses the stored factory calibration value of the 10 microamp current source. 
Circuit index 2: X microamp current source.  This index uses a supplied value for the  current source (CONFIG_D).

Voltage divider:  Voltage divider circuits rely on combinations of supplied voltage and a fixed resistor in series with the unknown resistor.

Circuit index 3: Shunt resistor. In this circuit the unknown resister is connected to an unknown voltage source and a fixed resistor. The fixed resistor is then connected to GND. The analog input matching the AIN_EF number to be used is connected to the positive side of the unknown resister and a second AIN is connected to the negative side of the unknown resister. Specify shunt resistance in ohms (CONFIG_E) and a second AIN# from which to measure the shunt voltage (CONFIG_C). This circuit can be affected by noise on the voltage supply. 
Circuit index 4: Fixed voltage source. Circuit 4 is similar to 3 except it only requires one AIN and the voltage supplied must be specified (CONFIG_D). The voltage source must be low-noise. Using a Vs terminal will result in a lot of noise. The shunt resistance need to be specified as well (CONFIG_E).  This circuit can be affected by noise on the voltage supply. 
Circuit index 5: 

 

Configuration:

CONFIG_A: Reserved
CONFIG_B: Excitation Type
CONFIG_C: Second AIN
CONFIG_D: Excitation Detail Volts / Amps
CONFIG_E: Excitation Detail Ohms

 

Result Registers:

READ_A: Resistance
READ_B: Voltage
READ_C: Current

 

Example:

An unknown resistor to be measured (a 10 kΩ resistor is used in this test) is connected in series with a 200 µA current source.

Index = 4
CONFIG_B = 0 (Excitation circuit 0, 200 µA current source)

Reading from READ_A returns 10089.7 Ω. READ_B returns 2.012 V and READ_C returns 199 µA.