Digital Control Inputs

AD Function

CS

WR

RD

CAL

AZ

1

1

1

Start Conversion without Auto-Zero

1

1

1

Read Conversion Result without Auto-Zero

1

1

0

Start Conversion with Auto-Zero

1

1

0

Read Conversion Result with Auto-Zero

1X

X

X

Start Calibration Cycle

0

X

1

0

X

Test Mode (DB2 DB3 DB5 and DB6 become active)

FIGURE 1 Function of the AD 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

22 RESETTING THE AD

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

sion in process The AD 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

30 Analog Considerations

31 REFERENCE VOLTAGE

The voltage applied to the reference input of the converter

defines the voltage span of the analog input (the difference

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

can be used in either ratiometric or absolute reference ap-

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 AD reference When this

of the system reference as the analog input and AD refer-

ence move together maintaining the same output code for

given input condition

TLH10554 – 17

Tantalum

FIGURE 2 Low Drift Extremely Stable Reference Circuit

11