ADC0803, ADC0804
function. IC voltage regulators may be used for references if
Loads less than 50pF, such as driving up to 7 A/D converter
clock inputs from a single CLK R pin of 1 converter, are
allowed. For larger clock line loading, a CMOS or low power
TTL buffer or PNP input logic should be used to minimize the
loading on the CLK R pin (do not use a standard TTL buffer).
the ambient temperature changes are not excessive.
Zero Error
The zero of the A/D does not require adjustment. If the
minimum analog input voltage value, V
, is not ground, a
lN(MlN)
Restart During a Conversion
zero offset can be done. The converter can be made to output
0000 0000 digital code for this minimum input voltage by
If the A/D is restarted (CS and WR go low and return high)
during a conversion, the converter is reset and a new
conversion is started. The output data latch is not updated if
the conversion in progress is not completed. The data from
the previous conversion remain in this latch.
biasing the A/D V
IN(-)
input at this V value (see
lN(MlN)
Applications section). This utilizes the differential mode
operation of the A/D.
The zero error of the A/D converter relates to the location of
the first riser of the transfer function and can be measured by
Continuous Conversions
grounding the V
input and applying a small magnitude
input. Zero error is the difference
IN(-)
In this application, the CS input is grounded and the WR
input is tied to the INTR output. This WR and INTR node
should be momentarily forced to logic low following a power-
up cycle to insure circuit operation. See Figure 17 for details.
positive voltage to the V
IN(+)
between the actual DC input voltage which is necessary to
just cause an output digital code transition from 0000 0000 to
1
1
0000 0001 and the ideal / LSB value ( / LSB = 9.8mV for
2
2
V
/2 = 2.500V).
10K
REF
5V (V
)
REF
Full Scale Adjust
ADC0803 - ADC0804
150pF
The full scale adjustment can be made by applying a
1
2
3
4
5
6
7
8
9
CS
V+ 20
1
+
differential input voltage which is 1 / LSB down from the
2
RD
CLK R
19
18
17
16
15
14
10µF
desired analog full scale voltage range and then adjusting
DB
DB
DB
DB
DB
WR
0
1
2
3
4
LSB
the magnitude of the V
/2 input (pin 9) for a digital output
N.O.
REF
code which is just changing from 1111 1110 to 1111 1111.
When offsetting the zero and using a span-adjusted V
CLK IN
INTR
START
/2
MlN
REF
V
V
(+)
(-)
IN
IN
DATA
ANALOG
INPUTS
voltage, the full scale adjustment is made by inputting V
OUTPUTS
to the V
IN(+)
input of the A/D and applying a voltage to the
IN(-)
AGND
/2
DB 13
5
DB 12
6
V
input which is given by:
V
REF
(V
– V
256
)
MAX
MIN
V
f
= V
– 1.5 ----------------------------------------- ,
MSB
IN(+) SADJ
MAX
10 DGND
DB 11
7
where:
FIGURE 17. FREE-RUNNING CONNECTION
V
V
= the high end of the analog input range, and
MAX
= the low end (the offset zero) of the analog range.
(Both are ground referenced.)
MIN
Driving the Data Bus
This CMOS A/D, like MOS microprocessors and memories,
will require a bus driver when the total capacitance of the
data bus gets large. Other circuItry, which is tied to the data
bus, will add to the total capacitive loading, even in three-
state (high-impedance mode). Back plane busing also
greatly adds to the stray capacitance of the data bus.
Clocking Option
The clock for the A/D can be derived from an external source
such as the CPU clock or an external RC network can be
added to provIde self-clocking. The CLK IN (pin 4) makes
use of a Schmitt trigger as shown in Figure 16.
There are some alternatives available to the designer to
handle this problem. Basically, the capacitive loading of the
data bus slows down the response time, even though DC
specifications are still met. For systems operating with a
relatively slow CPU clock frequency, more time is available
in which to establish proper logic levels on the bus and
therefore higher capacitive loads can be driven (see Typical
Performance Curves).
CLK R
1
19
4
f
ADC0803-
ADC0804
CLK
1.1 RC
R
R
10kΩ
CLK IN
C
CLK
FIGURE 16. SELF-CLOCKING THE A/D
At higher CPU clock frequencies time can be extended for
I/O reads (and/or writes) by inserting wait states (8080) or
using clock-extending circuits (6800).
Heavy capacitive or DC loading of the CLK R pin should be
avoided as this will disturb normal converter operation.
11
INTERSIL [ Intersil ]
