ADC0803, ADC0804

FN3094 Rev 4.00

Page 12 of 17

August 2002

Finally, if time is short and capacitive loading is high, external

bus drivers must be used. These can be three-state buffers

(low power Schottky is recommended, such as the 74LS240

series) or special higher-drive-current products which are

designed as bus drivers. High-current bipolar bus drivers with

PNP inputs are recommended.

Power Supplies

Noise spikes on the V+ supply line can cause conversion

errors as the comparator will respond to this noise. A

low-inductance tantalum filter capacitor should be used close

to the converter V+ pin, and values of 1

F or greater are

recommended. If an unregulated voltage is available in the

system, a separate 5V voltage regulator for the converter (and

other analog circuitry) will greatly reduce digital noise on the

V+ supply. An lCL7663 can be used to regulate such a supply

from an input as low as 5.2V.

Wiring and Hook-Up Precautions

Standard digital wire-wrap sockets are not satisfactory for

breadboarding with this A/D converter. Sockets on PC boards

can be used. All logic signal wires and leads should be

grouped and kept as far away as possible from the analog

signal leads. Exposed leads to the analog inputs can cause

undesired digital noise and hum pickup; therefore, shielded

leads may be necessary in many applications.

A single-point analog ground should be used which is

separate from the logic ground points. The power supply

bypass capacitor and the self-clockIng capacitor (if used)

capacitors, analog input filter capacitors, or input signal

shielding should be returned to the analog ground point. A

test for proper grounding is to measure the zero error of the

be traced to improper board layout and wiring (see Zero Error

for measurement). Further information can be found in

Application Note AN018.

Testing the A/D Converter

There are many degrees of complexity associated with testing

an A/D 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 Figure 18.

2.560V and a V+ supply voltage of 5.12V should be used. This

provides an LSB value of 20mV.

If a full scale adjustment is to be made, an analog input

output code is just changing from 1111 1110 to 1111 1111.

The digital-output LED display can be decoded by dividing

the 8 bits into 2 hex characters, one with the 4 most-

significant bits (MS) and one with the 4 least-significant bits

(LS). The output is then interpreted as a sum of fractions

times the full scale voltage:

.

For example, for an output LED display of 1011 0110, the MS

character is hex B (decimal 11) and the LS character is hex

(and decimal) 6, so:

.

Figures 19 and 20 show more sophisticated test circuits.

MS

16

---------

LS

256

----------

+



 5.12

V

=

START

DGND

2.560V

AGND

10

F

150pF

N.O.

0.1

F

0.1

F

TANTALUM

5.120V

5V

1.3k

 LEDs

(8)

(8)

MSB

LSB

10k



+

11

12

13

14

15

16

17

18

20

19

10

9

8

7

6

5

4

3

2

1

ADC0803-

ADC0804

FIGURE 18. BASIC TESTER FOR THE A/D

VOUT

11

16

------

6

256

----------

+



 5.12



3.64V

==

ANALOG

INPUTS

“A”

R

“B”

R

R

R

“C”

100R

-

+

A2

8-BIT

A/D UNDER

TEST

10-BIT

DAC

100X ANALOG

-

+

A1

ERROR VOLTAGE

FIGURE 19. A/D TESTER WITH ANALOG ERROR OUTPUT. THIS

CIRCUIT CAN BE USED TO GENERATE “ERROR

PLOTS” OF FIGURE 11.

A/D UNDER

TEST

10-BIT

DAC

DIGITAL

INPUTS

DIGITAL

OUTPUTS

FIGURE 20. BASIC “DIGITAL” A/D TESTER