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SNOSBI1B – NOVEMBER 2009 – REVISED FEBRUARY 2013

Figure 53. Basic A/D Tester

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 expressed 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 52.

The 2 op amps can be eliminated if a lab DVM with a numerical subtraction 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

digitally. This can be done with the circuit of Figure 55, 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 under 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.

MICROPROCESSOR INTERFACING

To dicuss the interface with 8080A and 6800 microprocessors, 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 program. 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.

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 address 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 in Figure 56.

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