of Microprocessors

282 MUSICAL ApPLICATIONS OF MICROPROCESSORS
Semiconductor Analog Switches
Semiconductor switches are likely to be of more interest. The "contact"
resistance of the switch can become a problem, however, because the voltage
drop across this resistance will have an effect on the apparent sensitivity of
control inputs. For example, a 300-ohm switch resistance and WOK control
input impedance will cause an error of 30 mV on a 10-V control signal or
nearly half a semitone. Op-amp buffers at some outputs of the switching
matrix can easily eliminate this error. Signal overload may be a problem with
analog switches. The most inexpensive CMOS switches are limited to 8 V
peak and severely distort any signals beyond that.
Several common analog switch ICs are useful in switching matrix
construction. For maximum flexibility in organizing the matrix, the 4066
type of CMOS analog switch offers four independent switches in a single 16­
pin DIP package. The on resistance is in the 50-ohm range, and the "B"
version can handle ± 8-V signal swings with a ± 8-V power supply. The
4051 l-of-8 analog multiplexor mentioned earlier gives eight switches in a
package, of which only one at a time may be on, also with a 50-ohm
resistance. These can be very effective in small matrices such as 4 x 8 or 8 x 8
in which a 4051 is used for each row. Also available is the 4052, which is a
dual l-of-4 multiplexor, and the 4053, which is a triple l-of-2 unit. One
significant advantage of the 405X series is that the control voltage to the
switches need swing only between ground and the positive supply voltage; it
need not go negative as required by the 4066. Although these switches are
inexpensive (less than $1. 50), all will require external latches to keep them in
the desired state and will require logic level shifters to be driven from TTL
logic levels. When a number of them are used in a large switching matrix,
these latches and level shifters can easily constitute half the circuitry.
A more recent IC that is tailor-made for switching matrices is the
MC142100 (Motorola) or CD22100 (RCA). This is an array of 16 CMOS
analog switches that is internally arranged into a 4 x 4 matrix and costs under
$3.00. Besides getting 16 switches from a 16-pin IC package, there is an
internal flip-flop to control each switch. The device interfaces to the driving
logic or computer just like a 16-bit write-only memory. There are four
address inpurs that are used to identify one of the 16 switches and a data and
write pulse input. To turn the addressed switch on, a one would be placed on
the data line and a positive pulse delivered to the write line. To turn a switch
off, the data would be zero instead. Anyon-offcombination is possible and a
special internal circuit turns all the switches off when power is first applied.
The on resistance is somewhat higher (100 ohms) than the 405X series, but
the error incurred (Y6 semitone for a WOK load) may be small enough to
ignore. About the only drawback of this device is the need for level shifters
on the address and control inputs than can swing between the negative and
positive supply voltages.