ADC0808/ADC0809
8-Bit µP Compatible A/D Converters with 8-Channel
Multiplexer
General Description
The ADC0808, ADC0809 data acquisition component is a
monolithic CMOS device with an 8-bit analog-to-digital con-
verter, 8-channel multiplexer and microprocessor compatible
control logic. The 8-bit A/D converter uses successive ap-
proximation as the conversion technique. The converter fea-
tures a high impedance chopper stabilized comparator, a
256R voltage divider with analog switch tree and a succes-
sive approximation register. The 8-channel multiplexer can
directly access any of 8-single-ended analog signals.
The device eliminates the need for external zero and
full-scale adjustments. Easy interfacing to microprocessors
is provided by the latched and decoded multiplexer address
inputs and latched TTL TRI-STATE outputs.
The design of the ADC0808, ADC0809 has been optimized
by incorporating the most desirable aspects of several A/D
conversion techniques. The ADC0808, ADC0809 offers high
speed, high accuracy, minimal temperature dependence,
excellent long-term accuracy and repeatability, and con-
sumes minimal power. These features make this device
ideally suited to applications from process and machine
control to consumer and automotive applications. For
16-channel multiplexer with common output (sample/hold
port) see ADC0816 data sheet. (See AN-247 for more infor-
mation.)
Features
nEasy interface to all microprocessors
nOperates ratiometrically or with 5 V
DC
or analog span
adjusted voltage reference
nNo zero or full-scale adjust required
n8-channel multiplexer with address logic
n0V to 5V input range with single 5V power supply
nOutputs meet TTL voltage level specifications
nADC0808 equivalent to MM74C949
nADC0809 equivalent to MM74C949-1
Key Specifications
nResolution 8 Bits
nTotal Unadjusted Error ±
1
⁄
2
LSB and ±1 LSB
nSingle Supply 5 V
DC
nLow Power 15 mW
nConversion Time 100 µs
Block Diagram
00567201
See Ordering
Information
October 2002
ADC0808/ADC0809 8-Bit µP Compatible A/D Converters with 8-Channel Multiplexer
© 2002 National Semiconductor Corporation DS005672 www.national.com
Connection Diagrams
Dual-In-Line Package
00567211
Order Number ADC0808CCN or ADC0809CCN
See NS Package J28A or N28A
Molded Chip Carrier Package
00567212
Order Number ADC0808CCV or ADC0809CCV
See NS Package V28A
Ordering Information
TEMPERATURE RANGE −40˚C to +85˚C
Error ±
1
⁄
2
LSB Unadjusted ADC0808CCN ADC0808CCV
±1 LSB Unadjusted ADC0809CCN ADC0809CCV
Package Outline N28A Molded DIP V28A Molded Chip Carrier
ADC0808/ADC0809
www.national.com 2
Absolute Maximum Ratings (Notes 2,
1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage (V
CC
) (Note 3) 6.5V
Voltage at Any Pin −0.3V to
(V
CC
+0.3V)
Except Control Inputs
Voltage at Control Inputs −0.3V to +15V
(START, OE, CLOCK, ALE, ADD A, ADD B, ADD C)
Storage Temperature Range −65˚C to +150˚C
Package Dissipation at T
A
=25˚C 875 mW
Lead Temp. (Soldering, 10 seconds)
Dual-In-Line Package (plastic) 260˚C
Molded Chip Carrier Package
Vapor Phase (60 seconds) 215˚C
Infrared (15 seconds) 220˚C
ESD Susceptibility (Note 8) 400V
Operating Conditions (Notes 1, 2)
Temperature Range (Note 1) T
MIN
≤T
A
≤T
MAX
ADC0808CCN,ADC0809CCN −40˚C≤T
A
≤+85˚C
ADC0808CCV, ADC0809CCV −40˚C≤T
A
≤+85˚C
Range of V
CC
(Note 1) 4.5 V
DC
to 6.0 V
DC
Electrical Characteristics
Converter Specifications: V
CC
=5 V
DC
=V
REF+
,V
REF(−)
=GND, T
MIN
≤T
A
≤T
MAX
and f
CLK
=640 kHz unless otherwise stated.
Symbol Parameter Conditions Min Typ Max Units
ADC0808
Total Unadjusted Error 25˚C ±
1
⁄
2
LSB
(Note 5) T
MIN
to T
MAX
±
3
⁄
4
LSB
ADC0809
Total Unadjusted Error 0˚C to 70˚C ±1 LSB
(Note 5) T
MIN
to T
MAX
±1
1
⁄
4
LSB
Input Resistance From Ref(+) to Ref(−) 1.0 2.5 kΩ
Analog Input Voltage Range (Note 4) V(+) or V(−) GND−0.10 V
CC
+0.10 V
DC
V
REF(+)
Voltage, Top of Ladder Measured at Ref(+) V
CC
V
CC
+0.1 V
Voltage, Center of Ladder V
CC
/2-0.1 V
CC
/2 V
CC
/2+0.1 V
V
REF(−)
Voltage, Bottom of Ladder Measured at Ref(−) −0.1 0 V
I
IN
Comparator Input Current f
c
=640 kHz, (Note 6) −2 ±0.5 2 µA
Electrical Characteristics
Digital Levels and DC Specifications: ADC0808CCN, ADC0808CCV, ADC0809CCN and ADC0809CCV, 4.75≤V
CC
≤5.25V,
−40˚C≤T
A
≤+85˚C unless otherwise noted
Symbol Parameter Conditions Min Typ Max Units
ANALOG MULTIPLEXER
I
OFF(+)
OFF Channel Leakage Current V
CC
=5V, V
IN
=5V,
T
A
=25˚C 10 200 nA
T
MIN
to T
MAX
1.0 µA
I
OFF(−)
OFF Channel Leakage Current V
CC
=5V, V
IN
=0,
T
A
=25˚C −200 −10 nA
T
MIN
to T
MAX
−1.0 µA
CONTROL INPUTS
V
IN(1)
Logical “1” Input Voltage V
CC
−1.5 V
V
IN(0)
Logical “0” Input Voltage 1.5 V
I
IN(1)
Logical “1” Input Current V
IN
=15V 1.0 µA
(The Control Inputs)
I
IN(0)
Logical “0” Input Current V
IN
=0 −1.0 µA
(The Control Inputs)
I
CC
Supply Current f
CLK
=640 kHz 0.3 3.0 mA
ADC0808/ADC0809
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Electrical Characteristics (Continued)
Digital Levels and DC Specifications: ADC0808CCN, ADC0808CCV, ADC0809CCN and ADC0809CCV, 4.75≤V
CC
≤5.25V,
−40˚C≤T
A
≤+85˚C unless otherwise noted
Symbol Parameter Conditions Min Typ Max Units
DATA OUTPUTS AND EOC (INTERRUPT)
V
OUT(1)
Logical “1” Output Voltage V
CC
= 4.75V
I
OUT
= −360µA
I
OUT
= −10µA
2.4
4.5
V(min)
V(min)
V
OUT(0)
Logical “0” Output Voltage I
O
=1.6 mA 0.45 V
V
OUT(0)
Logical “0” Output Voltage EOC I
O
=1.2 mA 0.45 V
I
OUT
TRI-STATE Output Current V
O
=5V 3 µA
V
O
=0 −3 µA
Electrical Characteristics
Timing Specifications V
CC
=V
REF(+)
=5V, V
REF(−)
=GND, t
r
=t
f
=20 ns and T
A
=25˚C unless otherwise noted.
Symbol Parameter Conditions MIn Typ Max Units
t
WS
Minimum Start Pulse Width (Figure 5) 100 200 ns
t
WALE
Minimum ALE Pulse Width (Figure 5) 100 200 ns
t
s
Minimum Address Set-Up Time (Figure 5)2550ns
t
H
Minimum Address Hold Time (Figure 5)2550ns
t
D
Analog MUX Delay Time R
S
=0Ω(Figure 5) 1 2.5 µs
From ALE
t
H1
,t
H0
OE Control to Q Logic State C
L
=50 pF, R
L
=10k (Figure 8) 125 250 ns
t
1H
,t
0H
OE Control to Hi-Z C
L
=10 pF, R
L
=10k (Figure 8) 125 250 ns
t
c
Conversion Time f
c
=640 kHz, (Figure 5) (Note 7) 90 100 116 µs
f
c
Clock Frequency 10 640 1280 kHz
t
EOC
EOC Delay Time (Figure 5) 0 8+2 µS Clock
Periods
C
IN
Input Capacitance At Control Inputs 10 15 pF
C
OUT
TRI-STATE Output At TRI-STATE Outputs 10 15 pF
Capacitance
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. DC and AC electrical specifications do not apply when operating
the device beyond its specified operating conditions.
Note 2: All voltages are measured with respect to GND, unless othewise specified.
Note 3: A zener diode exists, internally, from VCC to GND and has a typical breakdown voltage of 7 VDC.
Note 4: Two on-chip diodes are tied to each analog input which will forward conduct for analog input voltages one diode drop below ground or one diode drop
greater than the VCCn supply. The spec allows 100 mV forward bias of either diode. This means that as long as the analog VIN does not exceed the supply voltage
by more than 100 mV, the output code will be correct. To achieve an absolute 0VDC to 5VDC input voltage range will therefore require a minimum supply voltage of
4.900 VDC over temperature variations, initial tolerance and loading.
Note 5: Total unadjusted error includes offset, full-scale, linearity, and multiplexer errors. See Figure 3. None of these A/Ds requires a zero or full-scale adjust.
However, if an all zero code is desired for an analog input other than 0.0V, or if a narrow full-scale span exists (for example: 0.5V to 4.5V full-scale) the reference
voltages can be adjusted to achieve this. See Figure 13.
Note 6: Comparator input current is a bias current into or out of the chopper stabilized comparator. The bias current varies directly with clock frequency and has
little temperature dependence (Figure 6). See paragraph 4.0.
Note 7: The outputs of the data register are updated one clock cycle before the rising edge of EOC.
Note 8: Human body model, 100 pF discharged through a 1.5 kΩresistor.
ADC0808/ADC0809
www.national.com 4
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
address is latched into the decoder on the low-to-high tran-
sition of the address latch enable signal.
TABLE 1.
SELECTED ADDRESS LINE
ANALOG CHANNEL C B A
IN0 L L L
IN1 L L H
IN2 L H L
IN3 L H H
IN4 H L L
IN5 H L H
IN6 H H L
IN7 H H H
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
approximation 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
oscillations that will be catastrophic for the system. Addition-
ally, the 256R network does not cause load variations on the
reference voltage.
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-
tion occurs when the analog signal has reached +
1
⁄
2
LSB
and succeeding output transitions occur every 1 LSB later up
to full-scale.
The successive approximation register (SAR) performs 8
iterations to approximate the input voltage. For any SAR
type converter, n-iterations are required for an n-bit con-
verter. Figure 2 shows a typical example of a 3-bit converter.
In the ADC0808, ADC0809, the approximation technique is
extended 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 conversion pulse. A conversion in process will be
interrupted 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
applied after power up. End-of-conversion will go low be-
tween 0 and 8 clock pulses after the rising edge of start
conversion.
The most important section of the A/D converter is the
comparator. It is this section which is responsible for the
ultimate accuracy of the entire converter. It is also the com-
parator drift which has the greatest influence on the repeat-
ability of the device. A chopper-stabilized comparator pro-
vides the most effective method of satisfying all the
converter requirements.
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
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.
ADC0808/ADC0809
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Functional Description (Continued)
00567202
FIGURE 1. Resistor Ladder and Switch Tree
00567213
FIGURE 2. 3-Bit A/D Transfer Curve
00567214
FIGURE 3. 3-Bit A/D Absolute Accuracy Curve
00567215
FIGURE 4. Typical Error Curve
ADC0808/ADC0809
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Timing Diagram
00567204
FIGURE 5.
ADC0808/ADC0809
www.national.com7
Typical Performance Characteristics
00567216
FIGURE 6. Comparator I
IN
vs V
IN
(V
CC
=V
REF
=5V)
00567217
FIGURE 7. Multiplexer R
ON
vs V
IN
(V
CC
=V
REF
=5V)
ADC0808/ADC0809
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TRI-STATE Test Circuits and
Timing Diagrams
Applications Information
OPERATION
1.0 RATIOMETRIC CONVERSION
The ADC0808, ADC0809 is designed as a complete Data
Acquisition System (DAS) for ratiometric conversion sys-
tems. In ratiometric systems, the physical variable being
measured is expressed as a percentage of full-scale which is
not necessarily related to an absolute standard. The voltage
input to the ADC0808 is expressed by the equation
(1)
V
IN
=Input voltage into the ADC0808
V
fs
=Full-scale voltage
V
Z
=Zero voltage
D
X
=Data point being measured
D
MAX
=Maximum data limit
D
MIN
=Minimum data limit
A good example of a ratiometric transducer is a potentiom-
eter used as a position sensor. The position of the wiper is
directly proportional to the output voltage which is a ratio of
the full-scale voltage across it. Since the data is represented
as a proportion of full-scale, reference requirements are
greatly reduced, eliminating a large source of error and cost
for many applications. A major advantage of the ADC0808,
ADC0809 is that the input voltage range is equal to the
supply range so the transducers can be connected directly
across the supply and their outputs connected directly into
the multiplexer inputs, (Figure 9).
Ratiometric transducers such as potentiometers, strain
gauges, thermistor bridges, pressure transducers, etc., are
suitable for measuring proportional relationships; however,
many types of measurements must be referred to an abso-
lute standard such as voltage or current. This means a
system reference must be used which relates the full-scale
voltage to the standard volt. For example, if
V
CC
=V
REF
=5.12V, then the full-scale range is divided into
256 standard steps. The smallest standard step is 1 LSB
which is then 20 mV.
t
1H
,t
H1
00567218
t
1H
,C
L
=10pF
00567219
t
H1
,C
L
=50pF
00567220
t
0H
,t
H0
00567221
t
0H
,C
L
=10pF
00567222
t
H0
,C
L
=50pF
00567223
FIGURE 8.
ADC0808/ADC0809
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Applications Information (Continued)
2.0 RESISTOR LADDER LIMITATIONS
The voltages from the resistor ladder are compared to the
selected into 8 times in a conversion. These voltages are
coupled to the comparator via an analog switch tree which is
referenced to the supply. The voltages at the top, center and
bottom of the ladder must be controlled to maintain proper
operation.
The top of the ladder, Ref(+), should not be more positive
than the supply, and the bottom of the ladder, Ref(−), should
not be more negative than ground. The center of the ladder
voltage must also be near the center of the supply because
the analog switch tree changes from N-channel switches to
P-channel switches. These limitations are automatically sat-
isfied in ratiometric systems and can be easily met in ground
referenced systems.
Figure 10 shows a ground referenced system with a sepa-
rate supply and reference. In this system, the supply must be
trimmed to match the reference voltage. For instance, if a
5.12V is used, the supply should be adjusted to the same
voltage within 0.1V.
The ADC0808 needs less than a milliamp of supply current
so developing the supply from the reference is readily ac-
complished. In Figure 11 a ground referenced system is
shown which generates the supply from the reference. The
buffer shown can be an op amp of sufficient drive to supply
the milliamp of supply current and the desired bus drive, or if
a capacitive bus is driven by the outputs a large capacitor will
supply the transient supply current as seen in Figure 12. The
LM301 is overcompensated to insure stability when loaded
by the 10 µF output capacitor.
The top and bottom ladder voltages cannot exceed V
CC
and
ground, respectively, but they can be symmetrically less than
V
CC
and greater than ground. The center of the ladder
voltage should always be near the center of the supply. The
sensitivity of the converter can be increased, (i.e., size of the
LSB steps decreased) by using a symmetrical reference
system. In Figure 13, a 2.5V reference is symmetrically
centered about V
CC
/2 since the same current flows in iden-
tical resistors. This system with a 2.5V reference allows the
LSB bit to be half the size of a 5V reference system.
00567207
FIGURE 9. Ratiometric Conversion System
ADC0808/ADC0809
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Applications Information (Continued)
00567224
FIGURE 10. Ground Referenced
Conversion System Using Trimmed Supply
00567225
FIGURE 11. Ground Referenced Conversion System with
Reference Generating V
CC
Supply
ADC0808/ADC0809
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Applications Information (Continued)
3.0 CONVERTER EQUATIONS
The transition between adjacent codes N and N+1 is given
by:
(2)
The center of an output code N is given by:
(3)
The output code N for an arbitrary input are the integers
within the range:
(4)
Where: V
IN
=Voltage at comparator input
V
REF(+)
=Voltage at Ref(+)
V
REF(−)
=Voltage at Ref(−)
V
TUE
=Total unadjusted error voltage (typically
V
REF(+)
÷512)
00567226
FIGURE 12. Typical Reference and Supply Circuit
00567227
R
A
=R
B
*Ratiometric transducers
FIGURE 13. Symmetrically Centered Reference
ADC0808/ADC0809
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Applications Information (Continued)
4.0 ANALOG COMPARATOR INPUTS
The dynamic comparator input current is caused by the
periodic switching of on-chip stray capacitances. These are
connected alternately to the output of the resistor ladder/
switch tree network and to the comparator input as part of
the operation of the chopper stabilized comparator.
The average value of the comparator input current varies
directly with clock frequency and with V
IN
as shown in
Figure 6.
If no filter capacitors are used at the analog inputs and the
signal source impedances are low, the comparator input
current should not introduce converter errors, as the tran-
sient created by the capacitance discharge will die out be-
fore the comparator output is strobed.
If input filter capacitors are desired for noise reduction and
signal conditioning they will tend to average out the dynamic
comparator input current. It will then take on the character-
istics of a DC bias current whose effect can be predicted
conventionally.
Typical Application
00567210
*Address latches needed for 8085 and SC/MP interfacing the ADC0808 to a microprocessor
TABLE 2. Microprocessor Interface Table
PROCESSOR READ WRITE INTERRUPT (COMMENT)
8080 MEMR MEMW INTR (Thru RST Circuit)
8085 RD WR INTR (Thru RST Circuit)
Z-80 RD WR INT (Thru RST Circuit, Mode 0)
SC/MP NRDS NWDS SA (Thru Sense A)
6800 VMA•φ2•R/W VMA•φ•R/W IRQA or IRQB (Thru PIA)
ADC0808/ADC0809
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Physical Dimensions inches (millimeters)
unless otherwise noted
Molded Dual-In-Line Package (N)
Order Number ADC0808CCN or ADC0809CCN
NS Package Number N28B
ADC0808/ADC0809
www.national.com 14
Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
Molded Chip Carrier (V)
Order Number ADC0808CCV or ADC0809CCV
NS Package Number V28A
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support device or system whose failure to perform
can be reasonably expected to cause the failure of
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ADC0808/ADC0809 8-Bit µP Compatible A/D Converters with 8-Channel Multiplexer
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
