ADC0804LCWM datasheet(24/41 Pages) NSC

24 / 41 page

Functional Description (Continued)

For a higher speed test system, or to obtain plotted data, a

digital-to-analog converter is needed for the test set-up. An

accurate 10-bit DAC can serve as the precision voltage

source for the A/D. Errors of the A/D under test can be ex-

pressed as either analog voltages or differences in 2 digital

words.

A basic A/D tester that uses a DAC and provides the error as

an analog output voltage is shown in

Figure 8. The2op

amps can be eliminated if a lab DVM with a numerical sub-

traction feature is available to read the difference voltage,

“A–C”, directly. The analog input voltage can be supplied by

a low frequency ramp generator and an X-Y plotter can be

used to provide analog error (Y axis) versus analog input (X

axis).

For operation with a microprocessor or a computer-based

test system, it is more convenient to present the errors digi-

tally. This can be done with the circuit of

Figure 11, where the

output code transitions can be detected as the 10-bit DAC is

incremented. This provides 1⁄4 LSB steps for the 8-bit A/D un-

der test. If the results of this test are automatically plotted

with the analog input on the X axis and the error (in LSB’s)

as the Y axis, a useful transfer function of the A/D under test

results. For acceptance testing, the plot is not necessary and

the testing speed can be increased by establishing internal

limits on the allowed error for each code.

4.0 MICROPROCESSOR INTERFACING

To dicuss the interface with 8080A and 6800 microproces-

sors, a common sample subroutine structure is used. The

microprocessor starts the A/D, reads and stores the results

of 16 successive conversions, then returns to the user’s pro-

gram. The 16 data bytes are stored in 16 successive

memory locations. All Data and Addresses will be given in

hexadecimal form. Software and hardware details are pro-

vided separately for each type of microprocessor.

4.1 Interfacing 8080 Microprocessor Derivatives (8048,

8085)

This converter has been designed to directly interface with

derivatives of the 8080 microprocessor. The A/D can be

mapped into memory space (using standard memory ad-

dress decoding for CS and the MEMR and MEMW strobes)

or it can be controlled as an I/O device by using the I/O R

and I/O W strobes and decoding the address bits A0 → A7

(or address bits A8 → A15 as they will contain the same 8-bit

address information) to obtain the CS input. Using the I/O

space provides 256 additional addresses and may allow a

simpler 8-bit address decoder but the data can only be input

to the accumulator. To make use of the additional memory

reference instructions, the A/D should be mapped into

memory space. An example of an A/D in I/O space is shown

Figure 12.

DS005671-18

FIGURE 9. Basic A/D Tester

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