Getting a USB DAQ Device to Output a Higher Voltage or Current

Issue Details

My external device needs 5V minimum voltage and my USB DAQ Device supply voltage is often below 5V, why does this happen and how can I increase this and the current to allow connection to my low impedance circuit?

Solution

A USB DAQ device's power is typically dependent on the USB bus voltage. This may vary from one motherboard manufacturer to another, but the USB 2.0 specifications state that the valid range is from 4.4-5.25 VDC.

To solve this issue, you can incorporate an external resistor between the 5 V bus and the digital output line that will allow the digital output line to be pulled up to the 5 V rail. This user supplied resistor allows the USB device to drive up to 8.5 mA of current to a load. As seen in the diagram below, the external resistor is in parallel with the 4.7k on-board resistor.

Generally speaking, a way of having more control over the voltage and current is Using a Unity Gain Buffer (Voltage Follower) with a DAQ Device and the load as picture below; this allows the load circuit to be powered externally so there are no current/voltage limitations implied by the DAQ device.

Additional Information

A voltage follower transfer’s voltage from a high impedance circuit to a low impedance level circuit, the low impedance circuit is prevented from loading the 1st circuit. This is done without drawing current from the initial circuit, but allows varying currents to output from the operational amplifier, dependent on the operational amplifier properties. This will also allow the target circuit to receive more current then the USB device produces.