With the IntuiFace Interface Asset, the Analog Inputs measurement is represented as a numeric value between 0 and 1000. 1000 corresponds to a high input value (5V), and 0 corresponds to zero input value (0V). So we have a conversion of 1 unit for 0,005V.
As describe here, when you use the 1135 Phidget Precision Voltage Sensor, you need to apply an additional formula to convert the input 1135 voltage (voltage that you want to measure), and the voltage returned by the 1135 (and read by the 8/8/8 card).
Vdiff = (Vsens - 2.5) / 0.0681
Vdiff = ((Analog input provide by IA * 0.005) - 2.5) / 0.0681
You can easily calculate this conversion with a "Custom Script" Binding Converter. See example below.
I hope this will help you.