Rainbow-electronics ADC1241 Uživatelský manuál stáhnout pdf (Strana 11)

2.0 Functional Description (Continued)

Digital Control Inputs

A/D Function

CS WR RD CAL AZ

ßß 1 1 1 Start Conversion without Auto-Zero

ß 1 ß 1 1 Read Conversion Result without Auto-Zero

ßß 1 1 0 Start Conversion with Auto-Zero

ß 1 ß 1 0 Read Conversion Result with Auto-Zero

1XXßX Start Calibration Cycle

0 X 1 0 X Test Mode (DB2, DB3, DB5 and DB6 become active)

FIGURE 1. Function of the A/D Control Inputs

The table in

Figure 1

summarizes the effect of the digital

control inputs on the function of the ADC1241. The Test

Mode, where RD

is high and CS and CAL are low, is used by

the factory to thoroughly check out the operation of the

ADC1241. Care should be taken not to inadvertently be in

this mode, since DB2, DB3, DB5, and DB6 become active

outputs, which may cause data bus contention.

2.2 RESETTING THE A/D

All internal logic can be reset, which will abort any conver-

sion in process. The A/D is reset whenever a new conver-

sion is started by taking CS

and WR low. If this is done when

the analog input is being sampled or when EOC is low, the

Auto-Cal correction factors may be corrupted, therefore

making it necessary to do an Auto-Cal cycle before the next

conversion. This is true with or without Auto-Zero. The Cali-

bration Cycle cannot be reset once started. On power-up

the ADC1241 automatically goes through a Calibration Cy-

cle that takes typically 1396 clock cycles.

3.0 Analog Considerations

3.1 REFERENCE VOLTAGE

The voltage applied to the reference input of the converter

defines the voltage span of the analog input (the difference

between V

and AGND), over which 4095 positive output

codes and 4096 negative output codes exist. The A-to-D

can be used in either ratiometric or absolute reference ap-

plications. The voltage source driving V

REF

must have a

very low output impedance and very low noise. The circuit in

Figure 2

is an example of a very stable reference that is

appropriate for use with the ADC1241.

In a ratiometric system, the analog input voltage is propor-

tional to the voltage used for the A/D reference. When this

voltage is the system power supply, the V

REF

pin can be

tied to V

. This technique relaxes the stability requirement

of the system reference as the analog input and A/D refer-

ence move together maintaining the same output code for

given input condition.

TL/H/10554–17

*Tantalum

FIGURE 2. Low Drift Extremely Stable Reference Circuit

1 2 ... 6 7 8 9 10 11 12 13 14

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