You will need to check for "USB On The Go" (USB OTG) support in both your device (iPad) and the connector/interface you are using. iPad supports OTG (I'm not sure if iPhones all do). Android support for OTG is also spotty.
OTG support depends on both the hardware platform (your iDevice) as well as software drivers to allow the device to act as a USB host rather then a peripheral. Most phones and tablets were originally intended to be USB peripherals.