USB-IO Interface Adapter has 5 analog inputs connected to internal 10-bit analog to digital converter. You can use these analog inputs to measure DC and AC voltage.
USB-IO adapter sends the results of the measurement to PC software. The voltage can be measured on demand (when PC software requests to measure voltage on the specified analog input pin) or periodically (each in milliseconds).
You can use either the USB-IO adapter positive and negative supply voltage (VDD and VSS) or the voltage level on the C.6 (VRefHi) and C.5 (VRefLow) pins as analog reference voltage.
For additional information about USB-IO adapter ADC module see our article ADC Module Overview.
DC Voltage Measurement
The analog input voltage of USB-IO adapter analog to digital converter should not exceed 5V.
If the analog input voltage would not exceed the VRefHi (high reference voltage), you can connect your circuit directly to USB-IO adapter analog input pin through current-limiting resistor (1-2k).
If you want to measure voltage in wider range, add an external resistive divider:
The resistors used in this divider should meet the following requirements:
The total input resistance (R1+R2) should be at least 10 times greater than the output impedance of the connected circuit;
The total input resistance (R1 + R2) should be at least 10 times less than the input impedance of the ADC;
The ratio between R1 and R2 should comply with the equation:
where: Vadc is the maximum input voltage of the ADC (5V in our example); Vin is the maximum input voltage of the resistive divider (maximum output voltage of the connected circuit).
You need to measure the voltage from 0V up to 30V. The output impedance of the connected circuit is 1k. The total resistance of R1 and R2 should be within:
In current example we will set R1+R2 to 300k.
You can calculate the R1 and R2 values with the equation:
AC Voltage Measurement
You can measure the AC voltage by measuring the amplitude of the positive half-wave.
The possible connection to USB-IO adapter is illustrated by the following schematics:
In case of sinusoidal signal, you can get the actual voltage level with the following expression:
where: VIn - measured input voltage; ADC Code - ADC digital code received from USB-IO adapter.
We are currently developing algorithm to measure voltage level of an arbitrary AC signal. If you want to be notified when it is ready, send an e-mail to firstname.lastname@example.org.