The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Chapter 9
line. Digital outputs DB0-DB7 connect to the data bus (D0-D7). DB0-DB4 also function as
control inputs MA0-MA4.
Up to eight analog inputs can connect to pins 2-9 on the ADC0848. The voltage at VREF
determines the converter’s full-scale voltage, which is the input that results in an output of
11111111, or FFh. For maximum range, connect VREF to the +5V supply or to a more precise
5-volt reference like an LM336-5.0 reference diode. The analog inputs can then range from
0 to +5 volts.
Adjusting the Range
If your sensor’s output is much less than 5 volts, you can increase the resolution of the
converter by connecting VREF to a voltage slightly larger than the highest voltage you expect
to measure.
To illustrate, consider a sensor whose output ranges from 0 to 0.5 volt. The 8-bit digital
output of the converter represents a number from 0 to 255. If VREF is 5 volts, each count
equals 5/255, or 19.6 millivolts. A 0.2-volt analog input results in a count of 10, while a
0.5-volt input results in a count of 26. If your input goes no higher than 0.5 volt, your count
will never go higher than 26, and the measured values will be accurate only to within 20
millivolts, or 1/255 of full-scale.
But if you adjust VREF down to 0.5 volts, each count now equals 0.5/255, or 2 millivolts. A
0.2-volt input gives a count of 102, a 0.5-volt input gives a count of 255, and the measured
values can be accurate to within 2 millivolts.
However, if you decrease VREF as described above, you also increase the converter’s
sensitivity to noise. With VREF at 5 volts, a 20-millivolt noise spike will cause at most a 1-bit
error in the output. If you decrease VREF to 0.5 volt, the same spike can cause an error of
10 bits, since each bit now represents 2 millivolts, not 20.
Minimizing Noise
The rapid switching of digital circuits can cause voltage spikes in the ground lines, and these
can cause errors in analog measurements. Good routing of ground wires or pc-board traces
can minimize noise in circuits that mix analog and digital circuits.
To minimize noise, provide separate ground paths for analog and digital signals. In Figure
9-4, this means that AGND and any ground connections related to the analog inputs or VREF
should be wired together, but kept separate from the ground connections for the digital
circuits, including logic chips, the 8052-BASIC, and memory chips. The two grounds are
tied together at one place only, as near to the power supply as possible. The schematic uses
160 The Microcontroller Idea Book