Digital to OI Analog

[marcan]

Originally Posted by Mike Copioli

http://ww1.microchip.com/downloads/e...Doc/11195c.pdf

If you choose to use the digital pots, make sure you read the data sheet very carefully. They have a tolerance of +-30%. So that means you could get a pot that has a value of 70k to 130k.

Not a problem - all we need to do is calibrate to our specific parts.

Originally Posted by Mike Copioli

Also in order to use the pots you must interface the to a device with SPI or I2C. Another words you will need a processor. Processors need power, usually more than the 10mAs supplied by the digital outputs. In short you will need an external power source, which you know, is illegal. This is one of the difficulties that we were faced with when developing the Chicklet.

You are overestimating the amount of power a small MCU requires. Developing a USB Host device is much more complex than SPI which is a very simple protocol - a small PIC will do, which will definitely use less than 100mA of power. In Sleep mode (which is what the PIC should be most often in, if it doesn't have much to do and it's coded properly), the current is measured in single-digit microamps (yes, micro). With EVERYTHING enabled (not likely), running at 48Mhz (maximum, overkill) frequency, the maximum current drawn is 50mA. With a much more common 4Mhz frequency (plenty for most usages), current is 6mA. These values are for a PIC18F4550, which is a pretty big PIC. The digital pots use less than a milliamp each even under active conditions. The OI provides 100mA. You do the math.

Also, if we really wanted to use a high-current custom circuit, we could just buy the USB-Chicklet, and use it and a USB PIC to get 500mA.