AD573
CONVERT Pulse Generation
This mode is particularly useful for bench-testing of the AD573,
and in applications where dedicated I/O ports of peripheral in-
terface adapter chips are available.
The AD573 is tested with a CONVERT pulse width of 500 ns
and will typically operate with a pulse as short as 300 ns.
However, some microprocessors produce active WR pulses
which are shorter than this. Either of the circuits shown in Fig-
ure 13 can be used to generate an adequate CONVERT pulse
for the AD573.
In both circuits, the short low going WR pulse sets the
CONVERT line high through a flip-flop. The rising edge of
DR (which signifies that the internal logic has been reset) resets
the flip-flop and brings CONVERT low, which starts the
conversion.
Figure 15. AD573 in “Stand-Alone“ Mode
(Output Data Valid 500 ns After DR Goes Low)
Note that tDSC is slightly longer when the result of the previous
conversion contains a Logic 1 on the LSB. This means that the
actual CONVERT pulse generated by the circuits in Figure 13
will vary slightly in width.
Apple II Microcomputer Interface
The AD573 can provide a flexible, low cost analog interface for
the popular Apple II microcomputer. The Apple II, based on a
1 MHz 6502 microprocessor, meets all timing requirements for
the AD573. Only a few TTL gates are required to decode the
signals available on the Apple II’s peripheral connector. The
recommended connections are shown in Figure 16.
Figure 13a. Using 74LS00
Figure 13b. Using 1/2 74LS74
Output Data Format
The AD573 output data is presented in a left justified format.
The 8 MSBs (DB9–DB2, Pins 10 through 3) are enabled by
HBE (Pin 20) and the 2 LSBs (DB1, DB0—Pins 2 and 1) are
enabled by LBE (Pin 19). This allows simple interface to 8-bit
system buses by overlapping the 2 MSBs and the 2 LSBs. The
organization of the data is shown in Figure 14.
When the least significant bits are read (LBE brought low), the
six remaining bits of the byte will contain meaningless data.
These unwanted bits can be masked by logically ANDing the
byte with 11000000 (C0 hex), which forces the 6 lower bits to
Logic 0 while preserving the two most significant bits of the byte.
Note that it is not possible to reconfigure the AD573 for right
justified data.
Figure 16. AD573 Interface to Apple ll
The BASIC routine listed here will operate the AD573 circuit
shown in Figure 16. The conversion is started by POKEing to
the location which contains the AD573. The relatively slow ex-
ecution speed of BASIC eliminates the need for a delay routine
between starting and reading the converter. This routine as-
sumes that the AD573 is connected for a ±5 volt input range.
Variable I represents the integer value (from 0 to 1023) read
from the AD573. Variable V represents the actual value of the
input signal (in volts).
Figure 14. AD573 Output Data Format
In systems where all 10 bits are desired at the same time, HBE
and LBE may be tied together. This is useful in interfacing to
16-bit bus systems. The resulting 10-bit word can then be
placed at the high end of the 16-bit bus for left justification or at
the low end for right justification.
It is also possible to use the AD573 in a “stand-alone” mode,
where the output data buffers are automatically enabled at the
end of a conversion cycle. In this mode, the DR output is wired
to the HBE and LBE inputs. The outputs thus are forced into
the high impedance state during the conversion period, and
valid data becomes available approximately 500 ns after the DR
signal goes low at the end of the conversion. The 500 ns delay
allows propagation of the least significant bit through the inter-
nal logic.
100 PRINT “WHICH SLOT IS THE A/D IN”;:INPUT S
110 A=49280 + 16*S
120 POKE A,0
130 L=PEEK(A) :H=PEEK(A+1)
140 I =(4*H) + INT(L/64)
150 V=(I/1024)*10-5
160 PRINT “THE INPUT SIGNAL IS”;V;“VOLTS.”
REV. B
–7–
ADI [ ADI ]
