Rainbow-electronics ADC0838 Manual do Utilizador descarregar pdf (Página 13)

Functional Description (Continued)

Since the input configuration is under software control, it

can be modified, as required, at each conversion. A channel

can be treated as a single-ended, ground referenced input

for one conversion; then it can be reconfigured as part of a

differential channel for another conversion.

Figure 1

illus-

trates the input flexibility which can be achieved.

The analog input voltages for each channel can range from

50 mV below ground to 50 mV above V

(typically 5V)

without degrading conversion accuracy.

2.0 THE DIGITAL INTERFACE

A most important characteristic of these converters is their

serial data link with the controlling processor. Using a serial

communication format offers two very significant system im-

provements; it allows more function to be included in the

converter package with no increase in package size and it

can eliminate the transmission of low level analog signals by

locating the converter right at the analog sensor; transmit-

ting highly noise immune digital data back to the host proc-

essor.

To understand the operation of these converters it is best to

refer to the Timing Diagrams and Functional Block Diagram

and to follow a complete conversion sequence. For clarity a

separate diagram is shown of each device.

1. A conversion is initiated by first pulling the CS

(chip se-

lect) line low. This line must be held low for the entire con-

version. The converter is now waiting for a start bit and its

MUX assignment word.

2. A clock is then generated by the processor (if not provid-

ed continuously) and output to the A/D clock input.

3. On each rising edge of the clock the status of the data in

(DI) line is clocked into the MUX address shift register. The

start bit is the first logic ‘‘1’’ that appears on this line (all

leading zeros are ignored). Following the start bit the con-

verter expects the next 2 to 4 bits to be the MUX assign-

ment word.

8 Single-Ended

4 Differential

8 Pseudo-Differential

Mixed Mode

TL/H/5583–9

FIGURE 1. Analog Input Multiplexer Options for the ADC0838

1 2 ... 8 9 10 11 12 13 14 15 16 17 18 ... 31 32

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