The bottom resistor and the top resistor of the ladder net-
work 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 transi-
Functional Description
Multiplexer. The device contains an 8-channel single-ended
analog signal multiplexer. A particular input channel is se-
lected by using the address decoder. Table 1 shows the input
states for the address lines to select any channel. The ad-
dress is latched into the decoder on the low-to-high transition
of the address latch enable signal.
tion occurs when the analog signal has reached +1⁄
LSB
2
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.
Figure 2 shows a typical example of a 3-bit converter. In the
ADC0808, ADC0809, the approximation technique is ex-
tended to 8 bits using the 256R network.
TABLE 1.
SELECTED
ANALOG
CHANNEL
IN0
ADDRESS LINE
C
B
A
The A/D converter’s successive approximation register
(SAR) is reset on the positive edge of the start conversion
(SC) pulse. The conversion is begun on the falling edge of
the start conversion pulse. A conversion in process will be in-
terrupted by receipt of a new start conversion pulse. Con-
tinuous conversion 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 ap-
plied after power up. End-of-conversion will go low between
0 and 8 clock pulses after the rising edge of start conversion.
L
L
L
L
L
H
L
IN1
IN2
L
H
H
L
IN3
L
H
L
IN4
H
H
H
H
IN5
L
H
L
IN6
H
H
IN7
H
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 require-
ments.
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
major sections: the 256R ladder network, the successive ap-
proximation register, and the comparator. The converter’s
digital outputs are positive true.
The chopper-stabilized comparator converts the DC input
signal into an AC signal. This signal is then fed through a
high gain AC amplifier and has the DC level restored. This
technique limits the drift component of the amplifier since the
drift is a DC component which is not passed by the AC am-
plifier. This makes the entire A/D converter extremely insen-
sitive to temperature, long term drift and input offset errors.
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.
Figure 4 shows a typical error curve for the ADC0808 as
measured using the procedures outlined in AN-179.
5
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