ADC1241 Datasheet, PDF (11/14 Pages) National Semiconductor (TI) – Self-Calibrating 12-Bit Plus Sign mP-Compatible A/D Converter with Sample-and-Hold

2 0 Functional Description (Continued)
Digital Control Inputs
CS WR RD CAL AZ
A D Function
1
1
1
1
1XX
0X1
1 1 Start Conversion without Auto-Zero
1 1 Read Conversion Result without Auto-Zero
1 0 Start Conversion with Auto-Zero
1 0 Read Conversion Result with Auto-Zero
X Start Calibration Cycle
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_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 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_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
Tantalum
FIGURE 2 Low Drift Extremely Stable Reference Circuit
TL H 10554 – 17
11