ADC0808CCN/NOPB National Semiconductor, ADC0808CCN/NOPB Datasheet - Page 5

• Manufacturer Part Number: ADC0808CCN/NOPB
• Description: IC ADC 8BIT MPU 8CH MUX 28-DIP
• Manufacturer: National Semiconductor

Specifications of ADC0808CCN/NOPB

• Number Of Bits: 8
• Number Of Converters: 1
• Power Dissipation (max): 875mW
• Voltage Supply Source: Single Supply
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Through Hole
• Package / Case: 28-DIP (0.600", 15.24mm)
• Number Of Elements: 1
• Resolution: 8Bit
• Architecture: SAR
• Sample Rate: 10KSPS
• Input Polarity: Unipolar
• Input Type: Voltage
• Rated Input Volt: 5V
• Differential Input: No
• Power Supply Requirement: Single
• Single Supply Voltage (typ): 5V
• Single Supply Voltage (min): 4.5V
• Single Supply Voltage (max): 6V
• Dual Supply Voltage (typ): Not RequiredV
• Dual Supply Voltage (min): Not RequiredV
• Dual Supply Voltage (max): Not RequiredV
• Power Dissipation: 875mW
• Operating Temp Range: -40C to 85C
• Operating Temperature Classification: Industrial
• Mounting: Through Hole
• Pin Count: 28
• Package Type: PDIP
• Input Signal Type: Single-Ended
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *ADC0808CCN; *ADC0808CCN/NOPB; ADC0808; ADC0808CCN

Functional Description
MULTIPLEXER
The device contains an 8-channel single-ended analog signal
multiplexer. A particular input channel is selected by using the
address decoder. Table 1 shows the input states for the ad-
dress lines to select any channel. The address is latched into
the decoder on the low-to-high transition of the address latch
enable signal.
CONVERTER CHARACTERISTICS
The Converter
The heart of this single chip data acquisition system is its 8-
bit analog-to-digital converter. The converter is designed to
give fast, accurate, and repeatable conversions over a wide
range of temperatures. The converter is partitioned into 3 ma-
jor sections: the 256R ladder network, the successive ap-
proximation register, and the comparator. The converter's
digital outputs are positive true.
The 256R ladder network approach (Figure 1) was chosen
over the conventional R/2R ladder because of its inherent
monotonicity, which guarantees no missing digital codes.
Monotonicity is particularly important in closed loop feedback
control systems. A non-monotonic relationship can cause os-
cillations that will be catastrophic for the system. Additionally,
the 256R network does not cause load variations on the ref-
erence voltage.
SELECTED ANALOG
TABLE 1. Analog Channel Selection
CHANNEL
IN0
IN1
IN2
IN3
IN4
IN5
IN6
IN7
C
H
H
H
H
L
L
L
L
ADDRESS LINE
B
H
H
H
H
L
L
L
L
A
H
H
H
H
L
L
L
L
5
The bottom resistor and the top resistor of the ladder network
in Figure 1 are not the same value as the remainder of the
network. The difference in these resistors causes the output
characteristic to be symmetrical with the zero and full-scale
points of the transfer curve. The first output transition occurs
when the analog signal has reached +½ LSB and succeeding
output transitions occur every 1 LSB later up to full-scale.
The successive approximation register (SAR) performs 8 it-
erations to approximate the input voltage. For any SAR type
converter, n-iterations are required for an n-bit converter. Fig-
ure 2 shows a typical example of a 3-bit converter. In the
ADC0808, ADC0809, the approximation technique is extend-
ed to 8 bits using the 256R network.
The A/D converter's successive approximation register (SAR)
is reset on the positive edge of the start conversion start pulse.
The conversion is begun on the falling edge of the start con-
version pulse. A conversion in process will be interrupted by
receipt of a new start conversion pulse. Continuous conver-
sion may be accomplished by tying the end-of-conversion
(EOC) output to the SC input. If used in this mode, an external
start conversion pulse should be applied after power up. End-
of-conversion will go low between 0 and 8 clock pulses after
the rising edge of start conversion.
The most important section of the A/D converter is the com-
parator. It is this section which is responsible for the ultimate
accuracy of the entire converter. It is also the comparator drift
which has the greatest influence on the repeatability of the
device. A chopper-stabilized comparator provides the most
effective method of satisfying all the converter requirements.
The chopper-stabilized comparator converts the DC input sig-
nal into an AC signal. This signal is then fed through a high
gain AC amplifier and has the DC level restored. This tech-
nique limits the drift component of the amplifier since the drift
is a DC component which is not passed by the AC amplifier.
This makes the entire A/D converter extremely insensitive to
temperature, long term drift and input offset errors.
Figure 4 shows a typical error curve for the ADC0808 as
measured using the procedures outlined in AN-179.
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