adc12441cmj-883 National Semiconductor Corporation, adc12441cmj-883 Datasheet - Page 12

• Manufacturer Part Number: adc12441cmj-883
• Description: Dynamically-tested Self-calibrating 12-bit Plus Sign A/d Converter With Sample-and-hold
• Manufacturer: National Semiconductor Corporation

3 0 Analog Considerations
For absolute accuracy where the analog input varies be-
tween very specific voltage limits the reference pin can be
biased with a time and temperature stable voltage source
In general the magnitude of the reference voltage will re-
quire an initial adjustment to null out full-scale errors
3 2 INPUT CURRENT
Because the input network of the ADC12441 is made up of
a switch and a network of capacitors a charging current will
flow into or out of (depending on the input voltage polarity)
of the analog input pin (V
sampling period (t
pend on the actual input voltage applied
3 3 NOISE
The leads to the analog input pin should be kept as short as
possible to minimize input noise coupling Both noise and
undesired digital clock coupling to this input can cause er-
rors Input filtering can be used to reduce the effects of
these noise sources
3 4 INPUT BYPASS CAPACITORS
An external capacitor can be used to filter out any noise due
to inductive pickup by a long input lead and will not degrade
the accuracy of the conversion result
3 5 INPUT SOURCE RESISTANCE
The analog input can be modeled as shown in Figure 3
External R
voltage on C
input voltage With f
3 5 ms R
settle properly
3 6 POWER SUPPLIES
Noise spikes on the V
conversion errors as the comparator will respond to this
noise The A D is especially sensitive during the auto-zero
or auto-cal procedures to any power supply spikes Low in
ductance tantalum capacitors of 10 mF or greater paralleled
S s
S
will lengthen the time period necessary for the
REF
1 kX will allow a 5V analog input voltage to
A
to settle to within
) The peak value of this current will de-
CLK
CC
e
IN
and V
) on the start of the analog input
2 MHz t
b
A
supply lines can cause
e
FIGURE 3 Analog Input Equivalent Circuit
LSB of the analog
7 clock periods
(Continued)
e
12
with 0 1 mF ceramic capacitors are recommended for supply
bypassing Separate bypass capacitors whould be placed
close to the DV
voltage source is available in the system a separate
LM340LAZ-5 0 voltage regulator for the A-to-D’s V
other analog circuitry) will greatly reduce digital noise on the
supply line
3 7 THE CALIBRATION CYCLE
On power up the ADC12441 goes through an Auto-Cal cy-
cle which cannot be interrupted Since the power supply
reference and clock will not be stable at power up this first
calibration cycle will not result in an accurate calibration of
the A D A new calibration cycle needs to be started after
the power supplies reference and clock have been given
enough time to stabilize During the calibration cycle cor-
rection values are determined for the offset voltage of the
sampled data comparator and any linearity and gain errors
These values are stored in internal RAM and used during an
analog-to-digital conversion to bring the overall full scale
offset and linearity errors down to the specified limits Full
scale error typically changes
and linearity error changes even less therefore it should be
necessary to go through the calibration cycle only once af-
ter power up if auto-zero is used to correct the zero error
change
3 8 THE AUTO-ZERO CYCLE
To correct for any change in the zero (offset) error of the
A D the auto-zero cycle can be used It may be necessary
to do an auto-zero cycle whenever the ambient temperature
changes significantly (See the curved titled ‘‘Zero Error
Change vs Ambient Temperature’’ in the Typical Perform-
ance Characteristics ) A change in the ambient temperature
will cause the V
change which may cause the zero error of the A D to be
greater than
zero error to
g
g
1 LSB An auto-zero cycle will maintain the
1 LSB or less
CC
OS
AV
of the sampled data comparator to
CC
TL H 11017 – 21
and V
g
0 1 LSB over temperature
b
pins If an unregulated
CC
(and