Rainbow Electronics ADC12441 - Manual (Page 11)

2 0 Functional Description
(Continued)
Digital Control Inputs
A D 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
1 X X X 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 ADC12441 The Test
Mode where RD is high and CS and CAL are low is used
during manufacture to thoroughly check out the operation of
the ADC12441 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 re-
quiring an Auto-Cal cycle before the next conversion This is
true with or without Auto-Zero The Calibration Cycle cannot
be reset once started On power-up the ADC12441 auto-
matically goes through a Calibration Cycle that takes typi-
cally 1396 clock cycles For reasons that will be discussed
in Section 3 7 a new calibration cycle needs to be started
after the completion of the automatic one
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
IN 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 2a is an example of a very stable reference that is
appropriate for use with the ADC12441 The simple refer-
ence circuit of
Figure 2b may be used when the application
does not require low full scale errors
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
CC
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 11017 – 19
Tantalum
Ceramic
FIGURE 2a Low Drift Extremely Stable Reference Circuit
TL H 11017 – 20
Errors without any trims
25 C b 40 C to a 85 C
Full Scale g 0 075% g 0 2%
Zero g 0 024% g 0 024%
Linearity g LSB g LSB
FIGURE 2b Simple Reference Circuit
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