Imagine a journey of a signal from its origin at the guitar making its way to the microprocessor. After a long journey along a cable, it reaches a simple mono-1/4" input jack soldered to the board. From there, the signal enters a unity-gain follower into another op-amp through a voltage divider and low-pass filter connected to the inverting input and the output in a feedback. This causes a 10x gain in the signal and it cuts off nasty high-frequency noise. In order to have the signal go through the full 0 to 5 V swing, the non-inverting input recieves 2.5 Volts, which level-shifts the signal. See picture below.
From this circuit, the second-stage output is wired to the Channel 0 pin on a Maxim 1110 ADC. The Refin Voltage was set to 5V, while the Refout was bypassed to ground with a 1uF capacitor. Both Agrnd and Dgrnd were wired to the common ground for the power supply and STK200 board. The rest of the ADC pins were wired as shown on the graph below, so they can be used by the 8515.
Getting the ADC to work was one of the more challenging things since the code to do the serial transfer is not immediately obvious. Code was borrowed from Daanish Khan, but modified so it would work, and also for a much different pin and register assignments. Once the ADC code get run through, there is an 8-bit value waiting in the register, in our case "AnaLo" which we then manipulate for the special effects.
Output (DAC) Circuitry
For this lab, we could have used some fancy-schmancy DAC, but we went with the tried and true R-2R ladder method. Since the LSB is traditionally garbage and not a significant part of the output signal, and since we were using bit 0 for the ADC anyway, we went with a 7-bit DAC. This was a pretty economical feature, as it saved us some resistors and soldering. The output it generates is a weighted sum of the outputs, where bit7 is 1/2 the pin voltage (in this case 5V, so the contribution is 2.5V). Likewise bit6 gives 1.25V, bit5 - 0.625V, and so on down to bit1. The resistor network sums them up and passes the signal out to where we have a waiting low pass filter.
From the DAC, we pass the signal through a low-pass filter to eliminate high frequency noise introduced by the processor. We also use a voltage divider to bump the signal back down to millivolt levels so the guitar amp is at manageable levels.
EE 476 - Final Project - Sp '00