ANALOG RESISTANCE MEASUREMENT (OHMS)
APPLICATIONS > RESISTANCE MEASUREMENT >

Measurement Hardware

The following instruNet hardware supports Resistance Measurements with the help of an external shunt resistor:

i420 / i423 / i430	i60x	i100

Wiring Diagrams

a) Resistance Measurement with i4xx/i60x Hardware, Differential Wiring

b) Resistance Measurement with i4xx/i60x Hardware, Single-Ended Wiring

c) Resistance Measurement with i51x Wiring Box Pads, Differential Wiring

d) Resistance Measurement with i100 Box, Differential Wiring

Quick Setup Instructions

For quick setup instructions, click here.

Detailed Setup Instructions

Resistance measurement using a voltage divider involves connecting a resistor of unknown value in series with an external user supplied shunt resistor of known value, applying a voltage across the voltage divider circuit, and measuring the voltage across R_unknown (i.e. the external sensor), as illustrated in the above figures. The voltage across R_unknown is measured between a pair of instruNet Vin+ and Vin- input terminals while the excitation voltage is supplied by the instruNet Vout terminal. instruNet then calculates the value of R_unknown using the following equation, and returns a value in "ohms" units:

R_unknown (Ohms) = R_shunt * (Vin+ - Vin-) / ((Vin+ - Vin-) - Vout)

To do a Resistance measurement using a Voltage Divider, you must wire your sensor per the above diagram and then set up your software via the Interview process (started after selecting sensor type in Channel Setup dialog) or by manually running through the below steps:

1.   Set the Sensor field in the Hardware settings area to Resistance.

2.   Set the Wiring field in the Hardware settings area to Voltage Divider.

3.   Set the Rshunt field in the Constants settings area to the value of your external user supplied R_shunt resistor, in ohms units. 1, 3, 6

4.   If working with hardware that has variable internal excitation (e.g. i100 ± 5V), then set the Vout field in the Constants settings area to the desired excitation voltage 2, 11. Alternatively, if applying an external excitation voltage, enter -R_o value in the Ro edit field (e.g.  -100 instead of 100 ohms) to tell the software that the excitation is external, and then enter the external excitation voltage in the Vout field.

5.   Set the Measurement Range in the Hardware settings area. For details, click here.

6.   Wire your sensor per the above figures. Click here if you need more guidance setting up the software, and click here if the measured value is not correct 10. If you want a detailed report on your setup, press the Sensor Report button in the Channel Setup dialog.

Resistance Measurement using Bridge Circuit

An alternative to the above technique is to use a Bridge circuit to measure small resistance changes from a nominal value (e.g. accurately measure a device that ranges between 340 and 360 Ω). Resistance measurement using a bridge circuit involves connecting a resistor of unknown value as one leg of a full-bridge circuit, applying a voltage across the bridge, and measuring the voltage across the two intermediate nodes.  The intermediate node voltage is measured between a pair of instruNet Vin+ and Vin- input terminals while the bridge excitation voltage is supplied either by the instruNet or by an external voltage source.  In the below figure, R_unknown is a resistor whose value is being measured and R_o is a similar valued resistor of known value.  This technique is only accurate if R_unknown stays in the range of R_o , ± 50%.  If you need to measure a resistance with more range, please use the Resistance Measurement using a Voltage Divider, described above.  instruNet calculates the value of R_unknown using the following equation, and returns a value in "ohms" units:

R_unknown (ohms)
   = R_o * (Vout - 2.0 * (Vin+ - Vin-)) / (Vout + 2.0*(Vin+ - Vin-))

   
Resistance Measurement - Bridge Circuit

To do a Resistance measurement using a Bridge circuit you must:

1.   Set the Sensor field in the Hardware settings area to Resistance.

2.   Set the Wiring field in the Hardware settings area to Bridge.

3.   Set the Ro field in the Constants settings area to the value of one R_o bridge completion resistor, in ohms units. 1, 3, 4

4.   Set the Measurement Range in the Hardware settings area. For details, click here.

5.   If working with hardware that has variable internal excitation (e.g. i100 ± 5V), then set the Vout field in the Constants settings area to the desired excitation voltage 2, 11. Alternatively, if applying an external excitation voltage, enter -R_o value in the Ro edit field (e.g.  -100 instead of 100 ohms) to tell the software that the excitation is external, and then enter the external excitation voltage in the Vout field.

6.   Wire your voltage source per the above Bridge figure. Click here if you need more guidance setting up the software, and click here if the measured value is not correct. 5, 10