ADC1005 National Semiconductor Corporation, ADC1005 Datasheet - Page 8

• Manufacturer Part Number: ADC1005
• Description: ADC1005 - 10-Bit Microprocessor Compatible A/D Converter, Package: Cerdip, Pin Nb=20
• Manufacturer: National Semiconductor Corporation

www.national.com
Functional Description
signal makes a high-to-low transition the one shot fires,
setting the INTR F/F. An inverting buffer then supplies the
INTR output signal.
Note that this SET control of the INTR F/F remains low for
approximately 400 ns. If the data output is continuously
enabled (CS and RD both held low) the INTR output will still
signal the end of the conversion (by a high-to-low transition).
This is because the SET input can control the Q output of the
INTR F/F even though the RESET input is constantly at a “1”
level. This INTR output will therefore stay low for the duration
of the SET signal.
When data is to be read, the combination of both CS and RD
being low will cause the INTR F/F to be reset and the
TRI-STATE output latches will be enabled.
1.4 Free-Running and Self-Clocking Modes
For operation in the free-running mode an initializing pulse
should be used, following power-up, to ensure circuit opera-
tion. 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 ensure start up.
The clock for the A/D can be derived from the CPU clock or
an external RC can be added to provide self-clocking. The
CLK IN makes use of a Schmitt trigger as shown in Figure 2 .
2.0 REFERENCE VOLTAGE
The voltage applied to the reference input of these convert-
ers defines the voltage span of the analog input (the differ-
ence between V
possible output codes apply. The devices can be used in
either ratiometric applications or in systems requiring abso-
lute accuracy. The reference pin must be connected to a
voltage source capable of driving the reference input resis-
tance of typically 4.8 k . This pin is the top of a resistor
divider string used for the successive approximation conver-
sion.
In a ratiometric system ( Figure 3 ) the analog input voltage is
proportional to the voltage used for the A/D reference. This
voltage is typically the system power supply, so the V
can be tied to V
requirements of the system references as the analog input
and A/D reference move together maintaining the same
output code for a given input condition.
For absolute accuracy ( Figure 4 ), where the analog input
varies between very specific voltage limits, the reference pin
FIGURE 2. Self-Clocking the A/D
IN(MAX)
CC
. This technique relaxes the stability
and V
IN(MIN)
) over which the 1024
(Continued)
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REF
pin
8
can be biased with a time and temperature stable voltage
source. The LM385 and LM336 reference diodes are good
low current devices to use with these converters.
The maximum value of the reference is limited to the V
supply voltage. The minimum value, however, can be small
to allow direct conversions of transducer outputs providing
less than a 5V output span. Particular care must be taken
with regard to noise pickup, circuit layout, and system error
voltage sources when operating with a reduced span due to
the increased sensitivity of the converter (1 LSB equals
V
3.0 THE ANALOG INPUTS
3.1 Analog Differential Voltage Inputs and
Common-Mode Rejection
The differential inputs of these converters reduce the effects
of common-mode input noise, which is defined as noise
common to both selected “+” and “−” inputs (60 Hz is most
typical). The time interval between sampling the “+” input
and the “−” input is half of an internal clock period. The
REF
/1024).
FIGURE 4. Absolute with a Reduced Span
FIGURE 3. Ratiometric
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CC