ADC0804LCN/NOPB National Semiconductor, ADC0804LCN/NOPB Datasheet - Page 20

• Manufacturer Part Number: ADC0804LCN/NOPB
• Description: IC ADC 8BIT MPU COMPAT 20-DIP
• Manufacturer: National Semiconductor

Specifications of ADC0804LCN/NOPB

• Number Of Bits: 8
• Number Of Converters: 1
• Power Dissipation (max): 875mW
• Voltage Supply Source: Single Supply
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Through Hole
• Package / Case: 20-DIP (0.300", 7.62mm)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *ADC0804LCN; *ADC0804LCN/NOPB; ADC0804

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Functional Description
2.2 Analog Differential Voltage Inputs and
Common-Mode Rejection
This A/D has additional applications flexibility due to the
analog differential voltage input. The V
be used to automatically subtract a fixed voltage value from
the input reading (tare correction). This is also useful in 4
mA–20
common-mode noise can be reduced by use of the differen-
tial input.
The time interval between sampling V
clock periods. The maximum error voltage due to this slight
time difference between the input voltage samples is given
by:
where:
As an example, to keep this error to
operating with a 60 Hz common-mode frequency, f
using a 640 kHz A/D clock, f
the common-mode voltage, V
or
which gives
The allowed range of analog input voltages usually places
more severe restrictions on input common-mode noise lev-
els.
An analog input voltage with a reduced span and a relatively
large zero offset can be handled easily by making use of the
differential input (see section 2.4 Reference Voltage).
2.3 Analog Inputs
2.3 1 Input Current
Normal Mode
Due to the internal switching action, displacement currents
will flow at the analog inputs. This is due to on-chip stray
capacitance to ground as shown in Figure 5 .
V
f
V
cm
V
P
P
.1.9V.
e
is the peak value of the common-mode voltage
is the common-mode frequency
is the error voltage due to sampling delay
mA
current
loop
CLK
P
, which is given by:
, would allow a peak value of
conversion.
IN
1
IN
⁄
4
(+) and V
(Continued)
(−) input (pin 7) can
LSB ( 5 mV) when
In
IN
(−) is 4-
addition,
cm
, and
1
⁄
2
20
r
r=r
The voltage on this capacitance is switched and will result in
currents entering the V
input which will depend on the analog differential input volt-
age levels. These current transients occur at the leading
edge of the internal clocks. They rapidly decay and do not
cause errors as the on-chip comparator is strobed at the end
of the clock period.
Fault Mode
If the voltage source applied to the V
exceeds the allowed operating range of V
input currents can flow through a parasitic diode to the V
pin. If these currents can exceed the 1 mA max allowed
spec, an external diode (1N914) should be added to bypass
this current to the V
this diode, the voltage at the V
voltage by the forward voltage of this diode).
2.3.2 Input Bypass Capacitors
Bypass capacitors at the inputs will average these charges
and cause a DC current to flow through the output resis-
tances of the analog signal sources. This charge pumping
action is worse for continuous conversions with the V
input voltage at full-scale. For continuous conversions with a
640 kHz clock frequency with the V
current is at a maximum of approximately 5 µA. Therefore,
bypass capacitors should not be used at the analog inputs or
the V
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, which is due to the average value of the input
current, can be eliminated with a full-scale adjustment while
the given source resistor and input bypass capacitor are
both in place. This is possible because the average value of
the input current is a precise linear function of the differential
input voltage.
2.3.3 Input Source Resistance
Large values of source resistance where an input bypass
capacitor is not used, will not cause errors as the input
currents settle out prior to the comparison time. If a low pass
filter is required in the system, use a low valued series
resistor ( 1 k ) for a passive RC section or add an op amp
RC active low pass filter. For low source resistance applica-
tions, ( 1 k ), a 0.1 µF bypass capacitor at the inputs will
prevent noise pickup due to series lead inductance of a long
ON
ON
of SW 1 and SW 2 . 5 k
C
REF
STRAY
/2 pin for high resistance sources (
FIGURE 5. Analog Input Impedance
. 5 k
x 12 pF = 60 ns
CC
IN
pin (with the current bypassed with
(+) input pin and leaving the V
IN
(+) pin can exceed the V
IN
(+) input at 5V, this DC
IN
(+) or V
CC
>
+50 mV, large
1 k ). If input
DS005671-14
IN
(−) pin
IN
IN
(−)
(+)
CC
CC