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2.6. Build Digital to Analog Converter Using 0808

The first task this week is to build a simple D/A convertor and evaluate its performance. You will be using a DAC0808 (DAC0808 datasheet).Your design should take 8 bits from the microcontroller (use the 8 8bits on Port 0 of the AT89s51 as the input to the D/A convertor). The D/A output should range from 0 to 5 volts. The lower 8 bits from the AT89s51 should go into the 8 bits from the DAC0808. Note: I expect you to try to get the device to work by reading the datasheet and trying to understand it, if you have trouble you should come see me, but I expect you to have fairly specific questions that indicate you have put some thought into the problem.

DAC 0808 Schematic Diagram

Figure 2.6.1. Pin Configuration DAC0808

Things to consider with the this design:

Figure 2.6.1 from page 4 of the DAC0808 datasheet is a good place to start your design.
The pins are labeled A1 through A8, but note that A1 is the Most Significant Bit, and A8 is the Least Significant Bit (the opposite of the normal convention). Ground the two least significant bits.
The D/A convertor has an output current, instead of an output voltage. The output pin should stay at about 0 volts. The op-amp on the “Typical Application” on the datasheet converts the current to a voltage. How does it do this?
The output current from pin 4 ranges between 0 (when the inputs are all 0) to Imax*255/256 when all the inputs are 1. The current, Imax, is determined by the current into pin 14 (which is at 0 volts). Note: Since we are using 8 bits, the maximum value is Imax*255/256.
You’ll need to modify the circuit given in the datasheet to get a full scale range of 0 to 5 volts. Again, our output will be just under 5 volts. The output of the D/A convertor takes some time to settle. You may need to take this in consideration when planning the timing of the A/D conversion in later sections of this lab. Check the DAC0808 datasheet for specs. The code below shows an easy way to send 8 bits to the output of the microcontrollerr. You should probably test your code without the D/A convertor separately to ensure that the microcontroller is behaving as you expect. ( Download Complete Circuit File : DAC0808.pdf

Microcontroller Application DAC

Figure 2.6.2. Typical Application DAC0808


    Where, Rf = Feedback Resistor of Current to Voltage 
Converter circuit