adc1225ccd-1 National Semiconductor Corporation, adc1225ccd-1 Datasheet - Page 13

• Manufacturer Part Number: adc1225ccd-1
• Description: 12-bit Plus Sign Mp Compatible A/d Converters
• Manufacturer: National Semiconductor Corporation

Functional Description
ADC1205
Case 1 would be the only one that would appy to the
ADC1205 since two RD strobes are necessary to retrieve
the 13 bits of information on the 8 bit data bus Simulta-
neously strobing WR and RD low will enable the most signif-
icant byte on DB0 – DB7 and start a conversion Pulsing
WR RD low before the end of this conversion will enable
the least significant byte of data on the outputs and restart a
conversion
4 0 REFERENCE VOLTAGE
The voltage applied to the reference input of the converter
defines the voltage span of the analog inputs (the difference
between V
put codes and 4096 negative output codes exist The
A-to-D can be used in either ratiometric or absolute refer-
ence applications V
source capable of driving the reference input resistance
(typically 4 kX)
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
tied to V
of the system reference as the analog input and A D refer-
ence move together maintaining the same output code for a
given input condition
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
5 0 THE ANALOG INPUTS
5 1 DIFFERENTIAL VOLTAGE INPUTS AND COMMON
MODE REJECTION
The differential inputs of the ADC1225 and ADC1205 actu-
ally reduce the effects of common-mode input noise i e
signals common to both V
most typical) The time interval between sampling the ‘‘
and ‘‘
common-mode voltage during this short time interval may
cause conversion errors For a sinusoidal common-mode
signal the error would be
where f
V
clock frequency In most cases V
cant For a 60 Hz common-mode signal to generate a
LSB error (300 mV) with the converter running at 1 MHz its
peak value would have to be 200mV
5 2 INPUT CURRENT
Due to the sampling nature of the analog inputs short dura-
tion spikes of current enter the ‘‘
input at the leading clock edges during the actual conver-
sion These currents decay rapidly and do not cause errors
as the internal comparator is strobed at the end of a clock
period
5 3 INPUT BYPASS CAPACITORS
Bypass capacitors at the inputs will average the current
spikes mentioned in 5 2 and cause a DC current to flow
PEAK
V
ERROR(MAX)
b
is its peak voltage value and f
‘‘ input is 4 clock periods Therefore a change in the
CM
CC
IN(
is the frequency of the common-mode signal
This technique relaxes the stability requirement
a
)
e
and V
V
REF
PEAK
IN(
b
must be connected to a voltage
IN(
(2
)
q
a
over which 4096 positive out-
f
)
CM
and V
a
ERROR
)
’’ input and exit the ‘‘
f
CLK
4
CLK
IN(
(Continued)
b
will not be signifi-
is the converter’s
)
REF
inputs (60 Hz is
pin can be
a
b
’’
’’
13
through the output resistance of the analog signal source
This charge pumping action is worse for continuous conver-
sions with the V
ous conversions with a 1 MHz clock frequency and the
V
ly 5 mA For this reason bypass capacitors should not be
used at the analog inputs for high resistance sources
(R
If input bypass capacitors are necessary for noise filtering
and high source resistance is desirable to minimize capacitor
size the detrimental effects of the voltage drop across this
input resistance due to the average value of the input cur-
rent can be minimized with a full-scale adjustment while the
given source resistance and input bypass capacitor are both
in place This is effective because the average value of the
input current is a linear function of the differential input volt-
age
5 4 INPUT SOURCE RESISTANCE
Large values of source resistance where an input bypass
capacitor is not used will not cause errors as the input cur-
rents settle out prior to the comparison time If a low pass
filter is required in the system use a low valued series resis-
tor (R
RC active low pass filter For low source resistance applica-
tions (R
the inputs will prevent pickup due to series lead inductance
of a long wire A 100 X series resistor can be used to isolate
this capacitor – both the R and C are placed outside the
feedback loop – from the output of an op amp if used
5 5 NOISE
The leads to the analog inputs should be kept as short as
possible to minimize input noise coupling Both noise and
undesired digital clock coupling to these inputs can cause
errors Input filtering can be used to reduce the effects of
these sources but careful note should be taken of sections
5 3 and 5 4 if this route is taken
6 0 POWER SUPPLIES
Noise spikes on the V
errors as the comparator will respond to this noise Low
inductance tantalum capacitors of 1 mF or greater are rec-
ommended for supply bypassing Separate bypass caps
should be placed close to the DV
unregulated voltage source is available in the system a sep-
arate LM340LAZ-5 0 voltage regulator for the A-to-D’s V
(and other analog circuitry) will greatly reduce digital noise
on the supply line
7 0 ERRORS AND REFERENCE VOLTAGE
7 1 ZERO ADJUST
The zero error of the A D converter relates to the location
of the first riser of the transfer function and can be mea-
sured by grounding the V
magnitude positive voltage to the V
the difference between the actual DC input voltage neces-
sary to just cause an output digital code transition from all
zeroes to 0 0000 0000 0001 and the ideal
LSB
ed as shown in Figure 15 V
V
the digital output code changes from all zeroes to
0 000 0000 0001
IN(
IN(
SOURCE
ADJUSTMENTS
a
b
e
s
)
)
0 61 mV for V
input at 5V the average input current is approximate-
is forced to 0V The potentiometer is adjusted until
100 X) for a passive RC section or add an op amp
SOURCE s
100 X)
IN(
a
100 X) a 0 001 mF bypass capacitor at
)
REF
input voltage at full-scale For continu-
CC
e
supply line can cause conversion
IN(
5 V
IN(
b
DC
)
a
input and applying a small
) Zero error can be adjust-
)
is forced to 0 61 mV and
CC
IN(
a
and AV
)
input Zero error is
LSB value (
CC
pins If an
CC