Datasheet ADC08038CIWM, ADC08038CIN Datasheet (NSC)

ADC08031/ADC08034/ADC08038
8-Bit High-Speed Serial I/O A/D Converters with
Multiplexer Options, Voltage Reference, and Track/Hold
Function

General Description

The ADC08031/ADC08032/ADC08034/ADC08038 are 8-bit
successive approximationA/D converters with serial I/O and
configurable input multiplexers with up to 8 channels. The
serial I/O is configured to comply with the NSC MICROW­IRE

™

serial data exchangestandard for easy interface to the

COPS

™

family of controllers, and can easily interface with

standard shift registers or microprocessors.
TheADC08034 and ADC08038 provide a 2.6V band-gap de-

rived reference. For devices offering guaranteed voltage ref­erence performance over temperature see ADC08131,
ADC08134 and ADC08138.

Atrack/hold function allows the analog voltage at the positive
input to vary during the actual A/D conversion.

The analog inputs can be configured to operate in various
combinations of single-ended, differential, or
pseudo-differential modes. In addition, input voltage spans
as small as 1V can be accommodated.

Applications

n Digitizing automotive sensors
n Process control monitoring
n Remote sensing in noisy environments
n Instrumentation

n Test systems
n Embedded diagnostics

Features

n Serial digital data link requires few I/O pins
n Analog input track/hold function
n 2-, 4-, or 8-channel input multiplexer options with

address logic

n 0V to 5V analog input range with single 5V power

supply

n No zero or full scale adjustment required
n TTL/CMOS input/output compatible
n On chip 2.6V band-gap reference
n 0.3" standard width 8-, 14-, or 20-pin DIP package
n 14-, 20-pin small-outline packages

Key Specifications

n Resolution 8 bits
n Conversion time (f

C

=

1 MHz) 8µs (max)

n Power dissipation 20mW (max)
n Single supply 5V

DC

(±5%)

n Total unadjusted error

±

1

⁄2LSB and±1LSB

n No missing codes over temperature

Ordering Information

Industrial (−40˚C ≤ TA≤ +85˚C) Package

ADC08031CIN N08E
ADC08031CIWM, ADC08034CIWM M14B
ADC08038CIWM M20B

TRI-STATE®is a registered trademark of National Semiconductor Corporation.
COPS

™

microcontrollers and MICROWIRE™are trademarks of National Semiconductor Corporation.

June 1999

ADC08031/ADC08034/ADC08038 8-Bit High-Speed Serial I/O A/D Converters with Multiplexer

Options, Voltage Reference, and Track/Hold Function

© 1999 National Semiconductor Corporation DS010555 www.national.com

Connection Diagrams

ADC08038

DS010555-2

ADC08034

DS010555-3

ADC08031

Dual-In-Line Package

DS010555-5

ADC08031

Small Outline Package

DS010555-31

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Absolute Maximum Ratings (Notes 1, 3)

If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.

Supply Voltage (V

CC

) 6.5V

Voltage at Inputs and Outputs −0.3V to V

CC

+ 0.3V

Input Current at Any Pin (Note 4)

±

5mA

Package Input Current (Note 4)

±

20 mA

Power Dissipation at T

A

=

25˚C

(Note 5) 800 mW
ESD Susceptibility (Note 6) 1500V
Soldering Information

N Package (10 sec.)

SO Package:

Vapor Phase (60 sec.)
Infrared (15 sec.) (Note 7)

260˚C
215˚C
220˚C

Storage Temperature −65˚C to +150˚C

Operating Ratings (Notes 2, 3)

Temperature Range T

MIN

≤ TA≤ T

MAX

ADC08031CIN, −40˚C ≤ TA≤ +85˚C
ADC08031CIWM,
ADC08034CIWM, ADC08038CIWM
Supply Voltage (V

CC

) 4.5 VDCto 6.3 V

DC

Electrical Characteristics

The following specifications apply for V

CC

=

V

REF

=

+5 V

DC

, and f

CLK

=

1 MHz unless otherwise specified. Boldface limits

apply for T

A

=

T

J

=

T

MIN

to T

MAX

; all other limits T

A

=

T

J

=

25˚C.

Symbol Parameter Conditions

ADC08031, ADC08034 and

ADC08038

Units (Limits)

Typical (Note8)Limits (Note 9)

CONVERTER AND MULTIPLEXER CHARACTERISTICS

Total Unadjusted Error (Note 10)

BIN, BIWM

±

1

⁄

2

LSB (max)

CIN, CIWM

±

1 LSB (max)

Differential 8 Bits (min)
Linearity

R

REF

Reference Input Resistance 3.5 kΩ

1.3 kΩ (min)

6.0 kΩ (max)

V

IN

Analog Input Voltage (Note 11) (VCC+ 0.05) V (max)

(GND − 0.05) V (min)

DC Common-Mode Error

±

1

⁄

4

LSB (max)

Power Supply Sensitivity V

CC

=

5V

±

5%,

±

1

⁄

4

LSB (max)

V

REF

=

4.75V
On Channel Leakage On Channel=5V, 0.2 µA (max)
Current (Note 12) Off Channel=0V 1

On Channel=0V, −0.2 µA (max)

Off Channel=5V −1
Off Channel Leakage On Channel=5V, −0.2 µA (max)
Current (Note 12) Off Channel=0V −1

On Channel=0V, 0.2 µA (max)

Off Channel=5V 1

DIGITAL AND DC CHARACTERISTICS

V

IN(1)

Logical “1” Input Voltage V

CC

=

5.25V 2.0 V (min)

V

IN(0)

Logical “0” Input Voltage V

CC

=

4.75V 0.8 V (max)

I

IN(1)

Logical “1” Input Current V

IN

=

5.0V 1 µA (max)

I

IN(0)

Logical “0” Input Current V

IN

=

0V −1 µA (max)

V

OUT(1)

Logical “1” Output Voltage V

CC

=

4.75V:

I

OUT

=

−360 µA 2.4 V (min)

I

OUT

=

−10 µA 4.5 V (min)

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Electrical Characteristics (Continued)

The following specifications apply for V

CC

=

V

REF

=

+5 V

DC

, and f

CLK

=

1 MHz unless otherwise specified. Boldface limits

apply for T

A

=

T

J

=

T

MIN

to T

MAX

; all other limits T

A

=

T

J

=

25˚C.

Symbol Parameter Conditions

ADC08031, ADC08034 and

ADC08038

Units (Limits)

Typical (Note8)Limits (Note 9)

DIGITAL AND DC CHARACTERISTICS

V

OUT(0)

Logical “0” Output Voltage V

CC

=

4.75V 0.4 V (max)

I

OUT

=

1.6 mA

I

OUT

TRI-STATE®Output Current V

OUT

=

0V −3.0 µA (max)

V

OUT

=

5V 3.0 µA (max)

I

SOURCE

Output Source Current V

OUT

=

0V −6.5 mA (min)

I

SINK

Output Sink Current V

OUT

=

V

CC

8.0 mA (min)

I

CC

Supply Current

ADC08031, ADC08034, and
ADC08038

CS=HIGH

3.0 mA (max)

REFERENCE CHARACTERISTICS

V

REF

OUT Nominal Reference Output V

REF

OUT Option
Available Only on 2.6 V
ADC08034 and

ADC08038

Electrical Characteristics

The following specifications apply for V

CC

=

V

REF

=

+5 V

DC

, and t

r

=

t

f

=

20 ns unless otherwise specified. Boldface limits

apply for T

A

=

T

J

=

T

MIN

to T

MAX

; all other limits T

A

=

T

J

=

25˚C.

Symbol Parameter Conditions Typical Limits Units

(Note 8) (Note 9) (Limits)

f

CLK

Clock Frequency 10 kHz (min)

1 MHz (max)

Clock Duty Cycle 40

%

(min)

(Note 13) 60

%

(max)

T

C

Conversion Time (Not Including f

CLK

=

1 MHz 8 1/f

CLK

(max)

MUX Addressing Time) 8 µs (max)

t

CA

Acquisition Time

1

⁄

2

1/f

CLK

(max)

t

SELECT

CLK High while CS is High 50 ns

t

SET-UP

CS Falling Edge or Data Input 25 ns (min)
Valid to CLK Rising Edge

t

HOLD

Data Input Valid after CLK 20 ns (min)
Rising Edge

t

pd1,tpd0

CLK Falling Edge to Output C

L

=

100 pF:

Data Valid (Note 14) Data MSB First 250 ns (max)

Data LSB First 200 ns (max)

t

1H,t0H

TRI-STATE Delay from Rising Edge C

L

=

10 pF, R

L

=

10 kΩ 50 ns

of CS to Data Output and SARS Hi-Z

(see TRI-STATE Test Circuits)
C

L

=

100 pF, R

L

=

2kΩ 180 ns (max)

C

IN

Capacitance of Logic Inputs 5 pF

C

OUT

Capacitance of Logic Outputs 5 pF

Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur.
Note 2: Operating Ratings indicate conditions for which the device is functional. These ratings do not guarantee specific performance limits. For guaranteed speci-

fications and test conditions, see the Electrical Characteristics. The guaranteed specifications apply only for the test conditions listed. Some performance character­istics may degrade when the device is not operated under the listed test conditions.

Note 3: All voltages are measured with respect to AGND=DGND=0V

DC

, unless otherwise specified.

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Electrical Characteristics (Continued)

Note 4: When the input voltage VINat any pin exceeds the power supplies (V

IN

<

(AGND or DGND) or V

IN

>

VCC) the current at that pin should be limited to 5 mA.

The 20 mA maximum package input current rating limits the number of pins that can safely exceed the power supplies with an input current of 5 mA to four pins.
Note 5: The maximum power dissipation must be derated at elevated temperatures and is dictated by T

JMAX

, θJAand the ambient temperature, TA. The maximum

allowable power dissipation at any temperature is P

D

=

(T

JMAX−TA

)/θJAor the number given in the Absolute Maximum Ratings, whichever is lower. For devices with

suffixes CIN and CIWM T

JMAX

=

125˚C. The typical thermal resistances (θ

JA

) of these parts when board mounted follow: ADC08031CIN 120˚C/W,ADC08031CIWM

140˚C/W, ADC08034 CIWM 140˚C/W, ADC08038CIWM suffixes 91˚C/W.

Note 6: Human body model, 100 pF capacitor discharged through a 1.5 kΩ resistor.
Note 7: See AN450 “Surface Mounting Methods and Their Effect on Product Reliability” or Linear Data Book section “Surface Mount” for other methods of soldering

surface mount devices.
Note 8: Typicals are at T

J

=

25˚C and represent the most likely parametric norm.

Note 9: Guaranteed to National’s AOQL (Average Outgoing Quality Level).
Note 10: Total unadjusted error includes offset, full-scale, linearity, multiplexer.
Note 11: For V

IN(−)

≥ V

IN(+)

the digital code will be 0000 0000. Two on-chip diodes are tied to each analog input (see Block Diagram) which will forward-conduct for

analog input voltages one diode drop below ground or one diode drop greater than V

CC

supply.During testing at low VCClevels (e.g., 4.5V), high level analog inputs
(e.g., 5V) can cause an input diode to conduct, especially at elevated temperatures, which will cause errors for analog inputs near full-scale. The spec allows 50 mV
forward bias of either diode; this means that as long as the analog V

IN

does not exceed the supply voltage by more than 50 mV, the output code will be correct. Ex-

ceeding this range on an unselected channel will corruptthe reading of a selected channel. Achievement of an absolute 0 V

DC

to5VDCinput voltage range will there-

fore require a minimum supply voltage of 4.950 V

DC

over temperature variations, initial tolerance and loading.

Note 12: Channel leakage current is measured after a single-ended channel is selected and the clock is turned off. For off channel leakage current the following two
cases are considered: one, with the selected channel tied high (5 V

DC

) and the remaining seven off channels tied low (0 VDC), total current flow through the off chan­nels is measured; two, with the selected channel tied low and the off channels tied high, total current flow through the off channels is again measured. The two cases
considered for determining on channel leakage current are the same except total current flow through the selected channel is measured.

Note 13: A40%to 60%duty cycle range insures proper operation at all clock frequencies. In the case that an available clock has a duty cycle outside of these limits
the minimum time the clock is high or low must be at least 450 ns. The maximum time the clock can be high or low is 100 µs.

Note 14: Since data, MSB first, isthe output of the comparator used in the successive approximation loop, an additional delay is built in (see Block Diagram) to allow
for comparator response time.

Typical Performance Characteristics

Linearity Error vs
Reference Voltage

DS010555-32

Linearity Error vs
Temperature

DS010555-33

Linearity Error vs
Clock Frequency

DS010555-34

Power Supply Current vs
Temperature

DS010555-35

Output Current vs
Temperature

DS010555-36

Power Supply Current
vs Clock Frequency

DS010555-37

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Leakage Current Test Circuit

TRI-STATE Test Circuits and Waveforms

Timing Diagrams

DS010555-7

t

1H

DS010555-38

DS010555-39

t

0H

DS010555-40

DS010555-41

Data Input Timing

DS010555-10

*To reset these devices, CLK and CS must be simultaneously high for a period of t

SELECT

or greater. Otherwise these devices are compatible with industry

standards ADC0831/2/4/8.

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Timing Diagrams (Continued)

Data Output Timing

DS010555-11

ADC08031 Start Conversion Timing

DS010555-12

ADC08031 Timing

DS010555-13

*LSB first output not available on ADC08031.
LSB information is maintained for remainder of clock periods until CS goes high.

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Timing Diagrams (Continued)

ADC08034 Timing

DS010555-15

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Timing Diagrams (Continued)

ADC08038 Timing

DS010555-16

*Make sure clock edge

#

18 clocks in the LSB before SE is taken low

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ADC08038 Functional Block Diagram

Functional Description

1.0 MULTIPLEXER ADDRESSING

The design of these converters utilizes a comparator struc­ture with built-in sample-and-hold which provides for a differ­ential analog input to be converted by a successive­approximation routine.

The actual voltage converted is always the difference be­tween an assigned“+” input terminaland a “−” input terminal.
The polarity of each input terminal of the pair indicates which
line the converter expects to be the most positive. If the as­signed “+” input voltage is less than the “−” input voltage the
converter responds with an all zeros output code.

A unique input multiplexing scheme has been utilized to pro­vide multiple analog channels with software-configurable
single-ended, differential, or pseudo-differential (which will
convert the difference between the voltage at any analog in­put and a common terminal) operation. The analog signal
conditioning required in transducer-based data acquisition
systems is significantly simplified with this type of input flex­ibility. One converter package can now handle ground refer­enced inputs and true differential inputs as well as signals
with some arbitrary reference voltage.

A particular input configuration is assigned during the MUX
addressing sequence, prior to the start of a conversion. The
MUX address selects which of the analog inputs are to be

DS010555-17

*Some of these functions/pins are not available with other options.

For the ADC08034, the “SEL 1” Flip-Flop is bypassed.

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Functional Description (Continued)

enabled and whether this input issingle-ended or differential.
Differential inputs are restricted to adjacent channel pairs.
For example, channel 0 and channel 1 may be selected as a
differential pair but channel 0 or 1 cannot act differentially
with any other channel. In addition to selecting differential
mode the polarity may also be selected. Channel 0 may be
selected as the positive input and channel 1 as the negative
input or vice versa. This programmability is best illustrated by
the MUX addressing codes shown in the following tables for
the various product options.

The MUX address is shifted into the converter via the DI line.
Because the ADC08031 contains only one differential input
channel with a fixed polarity assignment, it does not require
addressing.

The common input line (COM) on the ADC08038 can be
used as a pseudo-differential input. In this mode the voltage
on this pin is treatedas the “−” input for any of the other input

channels. This voltage does not have to be analog ground; it
can be any reference potential which is common to all of the
inputs. This feature is most useful in single-supply applica­tions where the analog circuity may be biased up to a poten­tial other than ground and the output signals are all referred
to this potential.

TABLE 1. Multiplexer/Package Options

Part

Number

Number of Analog

Channels

Number of

Package

Pins

Single-Ended Differential

ADC08031 1 1 8
ADC08032 2 1 8
ADC08034 4 2 14
ADC08038 8 4 20

TABLE 2. MUX Addressing: ADC08038

Single-Ended MUX Mode

MUX Address Analog Single-Ended Channel

#

START SGL/ ODD/ SELECT 01234567COM

DIF

SIGN 1 0

11000+ −
11001 + −
11010 + −
11011 + −
11100 + −
11101 + −
11110 + −
11111 +−

TABLE 3. MUX Addressing: ADC08038

Differential MUX Mode

MUX Address Analog Differential Channel-Pair

#

START SGL/ ODD/ SELECT 0123

DIF

SIGN 1001234567

10000+−
10001 +−
10010 +−
10011 +−
10100−+
10101 −+
10110 −+
10111 −+

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Functional Description (Continued)

TABLE 4. MUX Addressing: ADC08034

Single-Ended MUX Mode

MUX Address Channel

#

START SGL/ ODD/ SELECT 0123

DIF

SIGN 1

110 0+
110 1 +
111 0 +
111 1 +

COM is internally tied to AGND

Since the input configuration is under software control, it can
be modified as required before each conversion. A channel
can be treated as a single-ended, ground referenced input
for one conversion; then it can be reconfigured as part of a
differential channel for another conversion.

Figure 1

illus-

trates the input flexibility which can be achieved.
The analog input voltages for each channel can range from

50mV below ground to 50mV above V

CC

(typically 5V) with-

out degrading conversion accuracy.

2.0 THE DIGITAL INTERFACE

A most important characteristic of these converters is their
serial data link with the controlling processor. Using a serial
communication format offers two very significant system im­provements; it allows many functions to be included in a
small package and it can eliminate the transmission of low
level analog signals by locating the converter right at the
analog sensor; transmitting highly noise immune digital data
back to the host processor.

To understand the operation of these converters it is best to
refer to the Timing Diagrams and Functional Block Diagram
and to follow a complete conversion sequence. For clarity a
separate timing diagram is shown for each device.

1. A conversion is initiated by pulling the CS (chip select)
line low. This line must be held low for the entire conver­sion. The converter is now waiting for a start bit and its
MUX assignment word.

2. On each rising edge of the clock the status of the data in
(DI) line is clocked into the MUX address shift register.
The start bit is the first logic “1” that appears on this line
(all leading zeros are ignored). Following the start bit the
converter expects the next 2 to 4 bits to be the MUX as­signment word.

3. When the start bit has been shifted into the start location
of the MUX register, the input channel has been as­signed and a conversion is about to begin. An interval of

1

⁄2clock period (where nothing happens) is automatically
inserted to allow the selected MUX channel to settle.
The SARS line goes high atthis time tosignal that a con­version is now in progress and the DI line is disabled (it
no longer accepts data).

4. The data out (DO) line now comes out of TRI-STATE
and provides a leading zero for this one clock period of
MUX settling time.

5. During the conversion the output of the SAR comparator
indicates whether the analog input is greater than (high)
or less than (low) a series of successive voltages gener­ated internally from a ratioed capacitor array (first 5 bits)
and a resistor ladder (last 3 bits).After each comparison
the comparator’s output is shipped to the DO line on the
falling edge of CLK. This data is the result of the conver­sion being shifted out (with the MSB first) and can be
read by the processor immediately.

6. After 8 clock periods the conversion is completed. The
SARS line returns low to indicatethis

1

⁄2clock cycle later.

7. The stored data in the successiveapproximation register
is loaded into an internal shift register.If the programmer
prefers the data can be provided in an LSB first format
[this makes use of the shift enable (SE) control line]. On
theADC08038 the SE line is brought out and if held high
the value of the LSB remains valid on the DO line. When
SE is forced low the data is clocked out LSB first. On de­vices which do not include the SE control line, the data,
LSB first, is automatically shifted out the DO line after
the MSB first data stream. The DO line then goes low
and stays low until CS is returned high. The ADC08031
is an exception in that its data is only output in MSB first
format.

8. All internal registers are cleared when the CS line is high
and the t

SELECT

requirement is met. See Data Input Tim­ing under Timing Diagrams. If another conversion is de­sired CS must make a high to low transition followed by
address information.

The DI and DO lines can be tied together and controlled
through a bidirectional processor I/O bit with one wire.
This is possible because the DI input is only “looked-at”
during the MUX addressing interval while the DO line is
still in a high impedance state.

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Functional Description (Continued)

3.0 REFERENCE CONSIDERATIONS

The voltage applied to the reference input on these convert­ers, V

REF

IN, defines the voltage span of the analog input

(the difference between V

IN(MAX)

and V

IN(MIN)

over which the
256 possible output codes apply. The devices can be used
either in ratiometric applications or in systems requiring ab­solute accuracy. The reference pin must be connected to a
voltage source capable of driving the reference input resis­tance which can be as low as 1.3kΩ. This pin is the top of a
resistor divider string and capacitor array used for the suc­cessive approximation conversion.

In a ratiometric system 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

REF

IN pin can

be tied to V

CC

. This technique relaxes the stability require­ments of the system reference as the analog input and A/D
reference move together maintaining the same output code
for a given input condition.

For absolute accuracy, where the analog input varies be­tween very specific voltage limits, the reference pin can be
biased with a time and temperature stable voltage source.
For the ADC08034 and the ADC08038 a band-gap derived
reference voltage of 2.6V (Note 8) is tied to V

REF

OUT. This

can be tied back to V

REF

IN. Bypassing V

REF

OUT with a
100µF capacitor is recommended. 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

CC

supply voltage. The minimum value, however, can be quite
small (see Typical Performance Characteristics) 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

REF/

256).

8 Single-Ended

DS010555-48

8 Pseudo-Differential

DS010555-49

4 Differential

DS010555-50

Mixed Mode

DS010555-51

FIGURE 1. Analog Input Multiplexer Options for the ADC08038

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Functional Description (Continued)

4.0 THE ANALOG INPUTS

The most important feature of these converters is that they
can be located right at the analog signal source and through
just a few wires can communicate with a controlling proces­sor with a highly noise immune serial bit stream. This in itself
greatly minimizes circuitry to maintain analog signal accu­racy which otherwise is most susceptible to noise pickup.
However, a few words are in order with regard to the analog
inputs should the input be noisy to begin with or possibly
riding on a large common-mode voltage.

The differential input of these converters actually reduces
the effects of common-mode input noise, a signal common
to both selected “+” and “−” inputs for a conversion (60 Hz is
most typical). The time interval between sampling the “+” in­put and then the “−” input is

1

⁄2of a clock period. The change
in the common-mode voltage during this short time interval
can cause conversion errors. For a sinusoidal
common-mode signal this error is:

where fCMis the frequency of the common-mode signal,

V

PEAK

is its peak voltage value

and f

CLK

is the A/D clock frequency.

For a 60Hz common-mode signal to generate a

1

⁄4LSB error
(≈5mV) with the converter running at 250kHz, its peak value
would have to be 6.63V which would be larger than allowed
as it exceeds the maximum analog input limits.

Source resistance limitation is important with regard to the
DC leakage currents of the input multiplexer. Bypass capaci­tors should not be used if the source resistance is greater
than 1kΩ. The worst-case leakage current of

±

1µAover tem­perature will create a 1mV input error with a 1kΩ source re­sistance. An op amp RC active low pass filter can provide
both impedance buffering and noise filtering should a high
impedance signal source be required.

5.0 OPTIONAL ADJUSTMENTS

5.1 Zero Error

The zero of the A/D does not require adjustment. If the mini­mum analog input voltage value, V

IN(MIN)

, is not ground a
zero offset can be done. The converter can be made to out­put 0000 0000 digital code for this minimum input voltage by
biasing any V

IN

(−) input at this V

IN(MIN)

value. This utilizes

the differential mode operation of the A/D.
The zero error of the A/D converter relates to the location of

the first riserof the transfer function andcan be measured by
grounding the V

IN

(−) input and applying a small magnitude

positive voltage to the V

IN

(+) input. Zero error is the differ­ence between the actual DC input voltage which is neces­sary to just cause an output digital code transition from 0000
0000 to 0000 0001 and the ideal

1

⁄2LSB value (1⁄2LSB

=

9.8mV for V

REF

=

5.000V

DC

).

5.2 Full Scale

The full-scale adjustment can be made by applying a differ­ential input voltage which is 1

1

⁄2LSB down from the desired
analog full-scale voltage range and then adjusting the mag­nitude of the V

REF

IN input for a digital output code which is

just changing from 1111 1110 to 1111 1111.

5.3 Adjusting for an Arbitrary Analog Input
Voltage Range

If the analog zero voltage of the A/D is shifted away from
ground (for example,to accommodate an analog input signal
which does not go to ground), this new zero reference
should be properly adjusted first. A V

IN

(+) voltage which

equals this desired zero reference plus

1

⁄2LSB (where the

LSB is calculated for the desired analog span, using 1LSB

=
analog span/256) is applied to selected “+” input and the
zero reference voltage at the corresponding “−” input should
then be adjusted to just obtain the 00

HEX

to 01

HEX

code tran-

sition.
The full-scale adjustment should be made [with the proper

V

IN

(−) voltage applied] by forcing a voltage to the VIN(+) in-

put which is given by:

DS010555-52

a) Ratiometric

DS010555-53

b) Absolute with a Reduced Span

FIGURE 2. Reference Examples

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Functional Description (Continued)

where:

V

MAX

=

the high end of the analog input range

and

V

MIN

=

the low end (the offset zero) of the analog range.
(Both are ground referenced.)
The V

REF

IN (or VCC) voltage is then adjusted to provide a
code change from FE

HEX

to FF

HEX

. This completes the ad-

justment procedure.

Applications

A “Stand-Alone” Hook-Up for ADC08038 Evaluation

DS010555-44

*Pinouts shown for ADC08038.
For all other products tie to pin functions as shown.

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Applications (Continued)

Low-Cost Remote Temperature Sensor

DS010555-45

Digitizing a Current Flow

DS010555-22

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Applications (Continued)

Operating with Ratiometric Transducers

DS010555-23

*VIN(−)=0.15 V

CC

15%of VCC≤ V

XDR

≤ 85%of V

CC

Span Adjust; 0V ≤ VIN≤ 3V

DS010555-46

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Applications (Continued)

Zero-Shift and Span Adjust: 2V ≤ V

IN

≤ 5V

DS010555-47

Protecting the Input

DS010555-25

Diodes are 1N914

High Accuracy Comparators

DS010555-26

DO=all 1s if +V

IN

>

−V

IN

DO=all 0s if +V

IN

<

−V

IN

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Applications (Continued)

Digital Load Cell

DS010555-27

•

Uses one more wire than load cell itself

•

Two mini-DIPs could be mounted inside load cell for digital output transducer

•

Electronic offset and gain trims relax mechanical specs for gauge factor and offset

•

Low level cell output is converted immediately for high noise immunity

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Applications (Continued)

4 mA-20 mA Current Loop Converter

DS010555-28

•

All power supplied by loop

•

1500V isolation at output

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Applications (Continued)

Isolated Data Converter

DS010555-29

•

No power required remotely

•

1500V isolation

www.national.com21

Physical Dimensions inches (millimeters) unless otherwise noted

Order Number ADC08031CIWM or ADC08034CIWM

NS Package Number M14B

www.national.com 22

Physical Dimensions inches (millimeters) unless otherwise noted (Continued)

Order Number ADC08038CIWM

NS Package Number M20B

Order Number ADC08031CIN

NS Package Number N08E

www.national.com23

Notes

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COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:

1. Life support devices or systems are devices or
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whose failure to perform when properly used in
accordance with instructions for use provided in the
labeling, can be reasonably expected to result in a
significant injury to the user.

2. A critical component is any component of a life
support device or system whose failure to perform
can be reasonably expected to cause the failure of
the life support device or system, or to affect its
safety or effectiveness.

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Corporation

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www.national.com

ADC08031/ADC08034/ADC08038 8-Bit High-Speed Serial I/O A/D Converters with Multiplexer

Options, Voltage Reference, and Track/Hold Function

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.