ADC0808CCV/NOPB - Texas Instruments High-Performance Analog

March 2007

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 successive

approximation register. The 8-channel multiplexer can direct-

ly 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, ex-

cellent long-term accuracy and repeatability, and consumes

minimal power. These features make this device ideally suit-

ed to applications from process and machine control to con-

sumer and automotive applications. For 16-channel multi-

plexer with common output (sample/hold port) see ADC0816

data sheet. (See AN-247 for more information.)

Features

■Easy interface to all microprocessors

■Operates ratiometrically or with 5 VDC or analog span

adjusted voltage reference

■No zero or full-scale adjust required

■8-channel multiplexer with address logic

■0V to VCC input range

■Outputs meet TTL voltage level specifications

■ADC0808 equivalent to MM74C949

■ADC0809 equivalent to MM74C949-1

Key Specifications

■Resolution 8 Bits

■Total Unadjusted Error ±½ LSB and ±1 LSB

■Single Supply 5 VDC

■Low Power 15 mW

■Conversion Time 100 μs

Block Diagram

567201

See Ordering

Information

ADC0808/ADC0809 8-Bit μP Compatible A/D Converters with 8-Channel Multiplexer

Connection Diagrams

Dual-In-Line Package

567211

Order Number ADC0808CCN or ADC0809CCN

See NS Package J28A or N28A

Molded Chip Carrier Package

567212

Order Number ADC0808CCV or ADC0809CCV

See NS Package V28A

Ordering Information

Temperature Range −40°C to +85°C

Package Outline N28A Molded DIP V28A Molded Chip Carrier V28A Molded Chip Carrier

(Tape and Reel)

Error ±½ LSB Unadjusted ADC0808CCN ADC0808CCV ADC0808CCVX

±1 LSB Unadjusted ADC0809CCN ADC0809CCV ADC0809CCVX

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ADC0808/ADC0809

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 (VCC) (Note 3) 6.5V

Voltage at Any Pin −0.3V to (VCC

+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 TA=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 TMIN≤TA≤TMAX

−40°C≤TA≤+85°C

Range of VCC 4.5 VDC to 6.0 VDC

Electrical Characteristics – Converter Specifications

Converter Specifications: VCC=5 VDC=VREF+, VREF(−)=GND, TMIN≤TA≤TMAX and fCLK=640 kHz unless otherwise stated.

Symbol Parameter Conditions Min Typ Max Units

ADC0808

Total Unadjusted Error 25°C ±½ LSB

(Note 5) TMIN to TMAX ±¾ LSB

ADC0809

Total Unadjusted Error 0°C to 70°C ±1 LSB

(Note 5) TMIN to TMAX ±1¼ LSB

Input Resistance From Ref(+) to Ref(−) 1.0 2.5 kΩ

Analog Input Voltage Range (Note 4) V(+) or V(−) GND − 0.1 VCC + 0.1 VDC

VREF(+) Voltage, Top of Ladder Measured at Ref(+) VCC VCC + 0.1 V

Voltage, Center of Ladder (VCC/2) − 0.1 VCC/2 (VCC/2) + 0.1 V

VREF(−) Voltage, Bottom of Ladder Measured at Ref(−) −0.1 0 V

IIN Comparator Input Current fc=640 kHz, (Note 6) −2 ±0.5 2 μA

Electrical Characteristics – Digital Levels and DC Specifications

Digital Levels and DC Specifications: ADC0808CCN, ADC0808CCV, ADC0809CCN and ADC0809CCV, 4.75≤VCC≤5.25V,

−40°C≤TA≤+85°C unless otherwise noted

Symbol Parameter Conditions Min Typ Max Units

ANALOG MULTIPLEXER

IOFF(+) OFF Channel Leakage Current

VCC=5V, VIN=5V,

TA=25°C 10 200 nA

TMIN to TMAX 1.0 μA

IOFF(−) OFF Channel Leakage Current

VCC=5V, VIN=0,

TA=25°C −200 −10 nA

TMIN to TMAX −1.0 μA

CONTROL INPUTS

VIN(1) Logical “1” Input Voltage (VCC − 1.5) V

VIN(0) Logical “0” Input Voltage 1.5 V

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ADC0808/ADC0809

Symbol Parameter Conditions Min Typ Max Units

IIN(1)

Logical “1” Input Current (The Control

Inputs) VIN=15V 1.0 μA

IIN(0)

Logical “0” Input Current (The Control

Inputs) VIN=0 −1.0 μA

ICC Supply Current fCLK=640 kHz 0.3 3.0 mA

DATA OUTPUTS AND EOC (INTERRUPT)

VOUT(1) Logical “1” Output Voltage

VCC = 4.75V

IOUT = −360µA

IOUT = −10µA

2.4

4.5

VOUT(0) Logical “0” Output Voltage IO=1.6 mA 0.45 V

VOUT(0) Logical “0” Output Voltage EOC IO=1.2 mA 0.45 V

IOUT TRI-STATE Output Current VO=5V 3 μA

VO=0 −3 μA

Electrical Characteristics – Timing Specifications

Timing Specifications VCC=VREF(+)=5V, VREF(−)=GND, tr=tf=20 ns and TA=25°C unless otherwise noted.

Symbol Parameter Conditions MIn Typ Max Units

tWS Minimum Start Pulse Width (Figure 5) 100 200 ns

tWALE Minimum ALE Pulse Width (Figure 5) 100 200 ns

tsMinimum Address Set-Up Time (Figure 5) 25 50 ns

tHMinimum Address Hold Time (Figure 5) 25 50 ns

tDAnalog MUX Delay Time From ALE RS=0Ω (Figure 5) 1 2.5 μs

tH1, tH0 OE Control to Q Logic State CL=50 pF, RL=10k (Figure 8) 125 250 ns

t1H, t0H OE Control to Hi-Z CL=10 pF, RL=10k (Figure 8) 125 250 ns

tcConversion Time fc=640 kHz, (Figure 5) (Note 7) 90 100 116 μs

fcClock Frequency 10 640 1280 kHz

tEOC EOC Delay Time (Figure 5) 0 8 + 2 μSClock

Periods

CIN Input Capacitance At Control Inputs 10 15 pF

COUT TRI-STATE Output Capacitance At TRI-STATE Outputs 10 15 pF

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 otherwise 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 kΩ resistor.

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ADC0808/ADC0809

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.

TABLE 1. Analog Channel Selection

SELECTED ANALOG

CHANNEL

ADDRESS LINE

CBA

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 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.

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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ADC0808/ADC0809

567202

FIGURE 1. Resistor Ladder and Switch Tree

567213

FIGURE 2. 3-Bit A/D Transfer Curve

567214

FIGURE 3. 3-Bit A/D Absolute Accuracy Curve

567215

FIGURE 4. Typical Error Curve

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ADC0808/ADC0809

Timing Diagram

567204

FIGURE 5.

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ADC0808/ADC0809

Typical Performance Characteristics

567216

FIGURE 6. Comparator IIN vs. VIN

(VCC=VREF=5V)

567217

FIGURE 7. Multiplexer RON vs. VIN

(VCC=VREF=5V)

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ADC0808/ADC0809

TRI-STATE Test Circuits and Timing Diagrams

t1H, tH1

567218

t1H, CL = 10 pF

567219

tH1, CL = 50 pF

567220

t0H, tH0

567221

t0H, CL = 10 pF

567222

tH0, CL = 50 pF

567223

FIGURE 8.

Applications Information

OPERATION

1.0 RATIOMETRIC CONVERSION

The ADC0808, ADC0809 is designed as a complete Data

Acquisition System (DAS) for ratiometric conversion systems.

In ratiometric systems, the physical variable being measured

is expressed as a percentage of full-scale which is not nec-

essarily related to an absolute standard. The voltage input to

the ADC0808 is expressed by the equation

(1)

VIN= Input voltage into the ADC0808

Vfs= Full-scale voltage

VZ= Zero voltage

DX= Data point being measured

DMAX= Maximum data limit

DMIN= Minimum data limit

A good example of a ratiometric transducer is a potentiometer

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 in-

puts, (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 absolute

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 VCC=VREF=5.12V, then the

full-scale range is divided into 256 standard steps. The small-

est standard step is 1 LSB which is then 20 mV.

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 volt-

age 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 satis-

fied in ratiometric systems and can be easily met in ground

referenced systems.

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ADC0808/ADC0809

Figure 10 shows a ground referenced system with a separate

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.

567207

FIGURE 9. Ratiometric Conversion System

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 VCC and

ground, respectively, but they can be symmetrically less than

VCC 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 Fig-

ure 13, a 2.5V reference is symmetrically centered about

VCC/2 since the same current flows in identical resistors. This

system with a 2.5V reference allows the LSB bit to be half the

size of a 5V reference system.

567224

FIGURE 10. Ground Referenced

Conversion System Using Trimmed Supply

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ADC0808/ADC0809

567225

FIGURE 11. Ground Referenced Conversion System with

Reference Generating VCC Supply

567226

FIGURE 12. Typical Reference and Supply Circuit

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ADC0808/ADC0809

567227

RA=RB

*Ratiometric transducers

FIGURE 13. Symmetrically Centered Reference

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: VIN=Voltage at comparator input

VREF(+)=Voltage at Ref(+)

VREF(−)=Voltage at Ref(−)

VTUE=Total unadjusted error voltage (typically

VREF(+)÷512)

4.0 ANALOG COMPARATOR INPUTS

The dynamic comparator input current is caused by the peri-

odic switching of on-chip stray capacitances. These are con-

nected alternately to the output of the resistor ladder/switch

tree network and to the comparator input as part of the oper-

ation of the chopper stabilized comparator.

The average value of the comparator input current varies di-

rectly with clock frequency and with VIN 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 cur-

rent should not introduce converter errors, as the transient

created by the capacitance discharge will die out before 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 characteris-

tics of a DC bias current whose effect can be predicted

conventionally.

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ADC0808/ADC0809

Typical Application

567210

*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)

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ADC0808/ADC0809

Physical Dimensions inches (millimeters) unless otherwise noted

Molded Dual-In-Line Package (N)

Order Number ADC0808CCN or ADC0809CCN

NS Package Number N28B

Molded Chip Carrier (V)

Order Number ADC0808CCV or ADC0809CCV

NS Package Number V28A

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ADC0808/ADC0809

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ADC0808/ADC0809

Notes

ADC0808/ADC0809 8-Bit μP Compatible A/D Converters with 8-Channel Multiplexer

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