ANALOG VOLTAGE VIN/VOUT RATIO MEASUREMENT (V/V)
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Voltage Ratio Measurement with the i100

The i100xx box provides 8 differential analog input voltage channels (14bit) that can be used for voltage ratio measurement. For specifications, click here.

Bridge Ratio Voltage measurement involves measuring the ratio of the voltage measured across a device (e.g. a bridge) to the excitation voltage applied to the device, as illustrated in the below figure.  This involves applying a voltage across the device and measuring the voltage across the two intermediate nodes via a pair of instruNet Vin+ and Vin- input terminals.  The excitation voltage for the bridge is supplied by either the instruNet Vout terminal or by an external voltage source.  instruNet calculates the ratio, returning "V/V" engineering units, using the equations:

Vratio (V/V) = ((Vin+ - Vin-) - Vinit) / Vout

Fig 3.2b- Bridge Ratio Voltage Measurement

Vinit and Vout are fixed values that are specified by the user in the Constants Settings area, whereas (Vin+ - Vin-) are measured in realtime by instruNet.  For more details, please see Maximizing Strain Gauge Accuracy, Load Cells, and Voltage Ratio Measurements.

To do Voltage ratio measurement you must:

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

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 Ro bridge resistor (i.e. or approx resistance across the device), in ohms units. 1, 3

4.   Set the Vout field in the Constants settings area to specify an excitation voltage to be applied to the bridge.  If you are applying an external excitation voltage, enter -Ro value in the Ro edit field (e.g.  -100 instead of 100ohms) to tell the software that the excitation is external, and then enter the external excitation voltage in the Vout. 2  In high current cases (e.g. >2mA), it is often helpful to alternate the polarity of the excitation voltages to evenly burden the +/-12V supplies.11 

5.   Set the Vinit field in the Constants settings area to the voltage measured when the bridge is not stimulated, in Volts units (to null the bridge). 8

6.   Wire your voltage source per figure 3.2b, click here for a review of the instruNet sensor software, and click here if the measured value is not correct. 5, 10

7.   To eliminate susceptibility to RFI noise, it may be necessary to place a capacitor between the Vin+ and Vin- screw terminals, and between the Vin- and Gnd screw terminals, as noted in Maximizing Strain Gauge Accuracy.  Typical values are .01uF.

Field Settings Summary

The following summarized the settings used with voltage ratio experiments.