TESTING THE AD CONVERTER

There are many degrees of complexity associated with test-

ing an AD converter One of the simplest tests is to apply a

known analog input voltage to the converter and use LEDs

to display the resulting digital output code as shown in

Fig-

ure 3 Note that the LED drivers invert the digital output of

the AD converter to provide a binary display A lab DVM

can be used if a precision voltage source is not available

After adjusting the zero and full-scale any number of points

can be checked as desired

for the AD converter This provides an LSB of 40 mV

(10240256) To adjust the zero of the AD an analog input

voltage of

LSB or 20 mV should be applied and the

zero adjust potentiometer should be set to provide a flicker

on the LSB LED readout with all the other display LEDs

OFF

To adjust the full-scale adjust potentiometer an analog in-

put that is 1

LSB less than the reference (10240 – 0060

the full-scale adjusted for a flicker on the LSB LED but this

time with all the other LEDs ON

A complete circuit for a simple AD tester is shown in

Figure

4 Note that the clock input voltage swing and the digital

output voltage swings are from 0V to 1024V

The

MM74C901 provides a voltage translation to 5V operation

and also the logic inversion so the readout LEDs are in bina-

ry

TLH5670 – 15

FIGURE 3 Basic AD Tester

TLH5670 – 7

FIGURE 4 Complete Basic Tester Circuit

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