of Microprocessors

268 MUSICAL ApPLICATIONS OF MICROPROCESSORS
+5V
2
~ READ RO
CHIP SELECT
CS
10 K 19 ClK R
.--_4--\ClKRC
20
L.r START WR ADC
0803
100 pF
ANALOG ::f 6
INPUT
o TO +5V.
Vref
7 Vin+
Vin-
8 Agnd
87
II
86 12
13
85 14
84 15
83 16
82 17
81 18
80
INTR
DIGITAL OUTPUT
END OF CONVERSION --'l-.
+2.5 V 9
OR lEAVE
Vref Dgnd
OPEN 10
-=-
Fig. 7-35. Application of ADC0803
low will initiate the conversion and make end of conversion (EOC) go high if
it is not already high. After conversion, EOC goes low, and the converted
value may be read by driving read low while chip select is low to enable the
internal tti-state buffer. Reading the register will also force EOC high.
Sample and Hold
There is one practical difficulty, however, with the successive approximation
algorithm; it assumes that the unknown voltage input does not
change during the course of the conversion. If it does change, significant
conversion errors can occur, although their magnitude will not exceed the
total amount of change over the conversion interval. Even though successive
approximation ADCs are quite fast, signals one might think are slow moving
can still change enough in a few microseconds to create an error of several
resolution steps in a high-resolution converter. A 20-Hz full-scale sine wave
"moves" at rates above one step (12-bit ADC) every 4 /-Lsec. If the conversion
time was a speedy 16 /-Lsec, random errors as large as four steps would result,
giving an effective resolution of only 10 bits. Such a "raw" ADC would be
worthless for converting audio signals.
The same sample-and-hold setup that was used to multiplex a DAC can
also be used to sample a fast-moving signal and then hold its instantaneous
value at a predictable point in time for accurate conversion. The performance
requirements of the sample and hold can be substantially greater than for
DAC multiplexing, however. When in the sample mode (switch closed), the
on resistance and holding capacitor size must be small enough so that a large
difference between input voltage and capacitor voltage does not develop due
to the rapidly changing input. Ideally, this difference amounts to a slight