adc0803-1lcd NXP Semiconductors, adc0803-1lcd Datasheet - Page 6

• Manufacturer Part Number: adc0803-1lcd
• Description: Cmos 8-bit A/d Converters
• Manufacturer: NXP Semiconductors

resistances at or below 1 k , a 0.1 F bypass capacitor at the inputs
supply voltage (Pin 20). Figure 6 indicates how this is accomplished.
Philips Semiconductors
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 active filter (low pass). For applications with source
will prevent pickup due to series lead inductance or a long wire. A
100
resistor and capacitor should be placed out of the feedback loop)
from the output of the op amp, if used.
Analog Differential Voltage Inputs and
Common-Mode Rejection
These A/D converters have additional flexibility due to the analog
differential voltage input. The V
subtract a fixed voltage from the input reading (tare correction). This
is also useful in a 4/20 mA current loop conversion. Common-mode
noise can also be reduced by the use of the differential input.
The time interval between sampling V
periods. The maximum error due to this time difference is given by:
where:
For example, with a 60 Hz common-mode frequency, f
1 MHz A/D clock, f
would allow a common-mode voltage, V
or
The allowed range of analog input voltages usually places more
severe restrictions on input common-mode voltage levels than this,
however.
An analog input span less than the full 5 V capability of the device,
together with a relatively large zero offset, can be easily handled by
use of the differential input. (See Reference Voltage Span Adjust).
Noise and Stray Pickup
The leads of the analog inputs (Pins 6 and 7) should be kept as
short as possible to minimize input noise coupling and stray signal
pick-up. Both EMI and undesired digital signal coupling to these
inputs can cause system errors. The source resistance for these
inputs should generally be below 5 k to help avoid undesired noise
pickup. Input bypass capacitors at the analog inputs can create
errors as described previously. Full scale adjustment with any input
bypass capacitors in place will eliminate these errors.
Reference Voltage
For application flexibility, these A/D converters have been designed
to accommodate fixed reference voltages of 5V to Pin 20 or 2.5 V to
Pin 9, or an adjusted reference voltage at Pin 9. The reference can
be set by forcing it at V
2002 Oct 17
V(max) = (V
V = error voltage due to sampling delay
V
f
V
V
CMOS 8-bit A/D converters
CM
P
P
P
= peak value of common-mode voltage
= common mode frequency
series resistor can be used to isolate this capacitor (both the
[V(max) (f
(5 x 10
(6.28) (60) (4.5)
(2f
CM
P
) (2f
)(4.5)
CLK
3
CM
CLK
) (10
, keeping this error to 1/4 LSB (about 5 mV)
) (4.5/f
REF
)
4
)
/2 input, or can be determined by the
CLK
IN(–)
2.95V
),
input (Pin 7) can be used to
IN(+)
P
, which is given by:
and V
IN(–)
is 4.5 clock
cm
, and a
6
temperature stability of the reference voltage are important factors in
gain of 2 at the V
Reference Voltage Span Adjust
Note that the Pin 9 (V
to the V
forced at the V
references and full span voltages, this also allows for a ratiometric
voltage reference. The internal gain of the V
the full-scale differential input voltage twice the voltage at Pin 9.
For example, a dynamic voltage range of the analog input voltage
that extends from 0 to 4 V gives a span of 4 V (4–0), so the V
voltage can be made equal to 2 V (half of the 4 V span) and full
scale output would correspond to 4 V at the input.
On the other hand, if the dynamic input voltage had a range of
0.5 to 3.5 V, the span or dynamic input range is 3 V (3.5–0.5). To
encode this 3 V span with 0.5 V yielding a code of zero, the
minimum expected input (0.5 V, in this case) is applied to the V
pin to account for the offset, and the V
span, or 1.5 V. The A/D converter will now encode the V
between 0.5 and 3.5 V with 0.5 V at the input corresponding to a
code of zero and 3.5 V at the input producing a full scale output
code. The full 8 bits of resolution are thus applied over this reduced
input voltage range. The required connections are shown in
Figure 7.
Operating Mode
These converters can be operated in two modes:
In absolute mode applications, both the initial accuracy and the
the accuracy of the conversion. For V
errors of 10 mV will cause conversion errors of 1 LSB due to the
value and stability of the V
important as the same error is a larger percentage of the V
nominal value. See Figure 8.
In ratiometric converter applications, the magnitude of the reference
voltage is a factor in both the output of the source transducer and
the output of the A/D converter, and, therefore, cancels out in the
final digital code. See Figure 9.
Generally, the reference voltage will require an initial adjustment.
Errors due to an improper reference voltage value appear as
full-scale errors in the A/D transfer function.
ERRORS AND INPUT SPAN ADJUSTMENTS
There are many sources of error in any data converter, some of
which can be adjusted out. Inherent errors, such as relative
accuracy, cannot be eliminated, but such errors as full-scale and
zero scale offset errors can be eliminated quite easily. See Figure 7.
Zero Scale Error
Zero scale error of an A/D is the difference of potential between the
ideal 1/2 LSB value (9.8 mV for V
voltage which just causes an output transition from code 0000 0000
to a code of 0000 0001.
If the minimum input value is not ground potential, a zero offset can
be made. The converter can be made to output a digital code of
0000 0000 for the minimum expected input voltage by biasing the
V
that minimum value expected at the V
differential mode of the converter. Any offset adjustment should be
done prior to full scale adjustment.
IN
1) absolute mode
2) ratiometric mode
(–) input to that minimum value expected at the V
CC
supply pin, or is equal to the voltage which is externally
REF
REF
/2 pin. In addition to allowing for flexible
/2 input. In reduced span applications, the initial
REF
/2) voltage is either 1/2 the voltage applied
REF
/2 input voltage become even more
REF
/2=2.500 V) and that input
REF
IN
REF
ADC0803/0804
(+) input. This uses the
/2 voltages of 2.5 V, initial
/2 pin is set to 1/2 the 3 V
REF
/2 input is 2, making
IN
(–) input to
Product data
IN
(+) signal
REF
REF
/2
IN
(–)
/2