NSC ADC08031CIN 8-bit high-speed serial i/o a/d converters with multiplexer options, voltage reference, and track/hold function Datasheet

ADC08031/ADC08032/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 approximation A/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 exchange standard for easy interface to the
COPS™ family of controllers, and can easily interface with
standard shift registers or microprocessors.
The ADC08034 and ADC08038 provide a 2.6V band-gap derived reference. For devices offering guaranteed voltage reference performance over temperature see ADC08131,
ADC08134 and ADC08138.
A track/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
n
n
n
Digitizing automotive sensors
Process control monitoring
Remote sensing in noisy environments
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
n
n
n
n
n
Resolution: 8 bits
Conversion time (fC = 1 MHz): 8µs (max)
Power dissipation: 20mW (max)
Single supply: 5VDC ( ± 5%)
Total unadjusted error: ± 1⁄2 LSB and ± 1LSB
No missing codes over temperature
Ordering Information
Industrial (−40˚C ≤ TA ≤ +85˚C)
Package
ADC08031CIN*
N08E
ADC08038CIN*
N20A
ADC08031CIWM,
ADC08032CIWM,
M14B
ADC08034CIWM
ADC08038CIWM
M20B
*Not recomended for new designs.
COPS™ microcontrollers and MICROWIRE™ are trademarks of National Semiconductor Corporation.
© 2000 National Semiconductor Corporation
DS010555
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ADC08031/ADC08032/ADC08034/ADC08038 8-Bit High-Speed Serial I/O A/D Converters with
Multiplexer Options, Voltage Reference, and Track/Hold Function
June 2000
ADC08031/ADC08032/ADC08034/ADC08038
Connection Diagrams
ADC08038
ADC08034
DS010555-3
DS010555-2
ADC08031
Dual-In-Line Package
ADC08032
Small Outline Package
DS010555-5
DS010555-30
ADC08031
Small Outline Package
DS010555-31
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Storage Temperature
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Operating Ratings (Notes 2, 3)
Supply Voltage (VCC)
Voltage at Inputs and Outputs
Input Current at Any Pin (Note 4)
Package Input Current (Note 4)
Power Dissipation at TA = 25˚C
(Note 5)
ESD Susceptibility (Note 6)
Soldering Information
N Package (10 sec.)
SO Package:
Vapor Phase (60 sec.)
Infrared (15 sec.) (Note 7)
−65˚C to +150˚C
Temperature Range
ADC08031BIN, ADC08031CIN,
ADC08032BIN, ADC08032CIN,
ADC08034BIN, ADC08034CIN,
ADC08038BIN, ADC08038CIN,
ADC08031BIWM, ADC08032BIWM,
ADC08034BIWM, ADC08038BIWM
ADC08031CIWM, ADC08032CIWM,
ADC08034CIWM, ADC08038CIWM
Supply Voltage (VCC)
6.5V
−0.3V to VCC + 0.3V
± 5 mA
± 20 mA
800 mW
1500V
235˚C
TMIN ≤ TA ≤ TMAX
−40˚C ≤ TA ≤ +85˚C
4.5 VDC to 6.3 VDC
215˚C
220˚C
Electrical Characteristics
The following specifications apply for VCC = VREF = +5 VDC, and fCLK = 1 MHz unless otherwise specified. Boldface limits
apply for TA = TJ = TMIN to TMAX; all other limits TA = TJ = 25˚C.
Symbol
Parameter
Conditions
Typical
Limits
Units
(Note 8)
(Note 9)
(Limits)
± 1⁄2
±1
LSB (max)
8
Bits (min)
1.3
kΩ (min)
6.0
kΩ (max)
(VCC + 0.05)
V (max)
CONVERTER AND MULTIPLEXER CHARACTERISTICS
Total Unadjusted Error
(Note 10)
BIN, BIWM
CIN, CIWM
Differential
LSB (max)
Linearity
RREF
VIN
Reference Input Resistance
Analog Input Voltage
(Note 11)
3.5
(Note 12)
DC Common-Mode Error
Power Supply Sensitivity
VCC = 5V ± 5%,
kΩ
(GND − 0.05)
V (min)
± 1⁄4
± 1⁄4
LSB (max)
0.2
µA (max)
LSB (max)
VREF = 4.75V
On Channel Leakage
Current (Note 13)
Off Channel Leakage
Current (Note 13)
On Channel = 5V,
Off Channel = 0V
1
On Channel = 0V,
−0.2
Off Channel = 5V
−1
On Channel = 5V,
−0.2
Off Channel = 0V
−1
On Channel = 0V,
0.2
Off Channel = 5V
1
µA (max)
µA (max)
µA (max)
DIGITAL AND DC CHARACTERISTICS
VIN(1)
Logical “1” Input Voltage
VCC = 5.25V
2.0
V (min)
VIN(0)
Logical “0” Input Voltage
VCC = 4.75V
0.8
V (max)
IIN(1)
Logical “1” Input Current
VIN = 5.0V
1
µA (max)
IIN(0)
Logical “0” Input Current
VIN = 0V
−1
µA (max)
VOUT(1)
Logical “1” Output Voltage
VCC = 4.75V:
2.4
V (min)
IOUT = −360 µA
VOUT(0)
Logical “0” Output Voltage
IOUT = −10 µA
4.5
V (min)
VCC = 4.75V
0.4
V (max)
IOUT = 1.6 mA
3
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ADC08031/ADC08032/ADC08034/ADC08038
Absolute Maximum Ratings (Notes 1, 3)
ADC08031/ADC08032/ADC08034/ADC08038
Electrical Characteristics
(Continued)
The following specifications apply for VCC = VREF = +5 VDC, and fCLK = 1 MHz unless otherwise specified. Boldface limits
apply for TA = TJ = TMIN to TMAX; all other limits TA = TJ = 25˚C.
Symbol
Parameter
Conditions
Typical
Limits
Units
(Note 8)
(Note 9)
(Limits)
DIGITAL AND DC CHARACTERISTICS
IOUT
TRI-STATE ® Output Current
VOUT = 0V
−3.0
µA (max)
VOUT = 5V
3.0
µA (max)
ISOURCE
Output Source Current
VOUT = 0V
−6.5
mA (min)
ISINK
Output Sink Current
VOUT = VCC
8.0
mA (min)
ICC
Supply Current
CS = HIGH
3.0
mA (max)
7.0
mA (max)
ADC08031, ADC08034,
and ADC08038
ADC08032 (Note 16)
REFERENCE CHARACTERISTICS
VREFOUT
Nominal Reference Output
VREFOUT Option
Available Only on
2.6
V
ADC08034 and
ADC08038
Electrical Characteristics
The following specifications apply for VCC = VREF = +5 VDC, and tr = tf = 20 ns unless otherwise specified. Boldface limits
apply for TA = TJ = TMIN to TMAX; all other limits TA = TJ = 25˚C.
Symbol
fCLK
Parameter
Conditions
Clock Frequency
Typical
Limits
(Note 8)
(Note 9)
10
(Limits)
kHz (min)
1
TC
Units
MHz (max)
Clock Duty Cycle
40
% (min)
(Note 14)
60
% (max)
8
1/fCLK (max)
8
µs (max)
12
⁄
1/fCLK(max)
25
ns (min)
20
ns (min)
Conversion Time (Not Including
fCLK = 1 MHz
MUX Addressing Time)
tCA
Acquisition Time
tSELECT
CLK High while CS is High
tSET-UP
CS Falling Edge or Data Input
tHOLD
Data Input Valid after CLK
50
ns
Valid to CLK Rising Edge
Rising Edge
tpd1, tpd0
CLK Falling Edge to Output
Data Valid (Note 15)
t1H, t0H
CL = 100 pF:
Data MSB First
250
ns (max)
Data LSB First
200
ns (max)
TRI-STATE Delay from Rising Edge
CL = 10 pF, RL = 10 kΩ
of CS to Data Output and SARS Hi-Z
(see TRI-STATE Test Circuits)
50
CL = 100 pF, RL = 2 kΩ
ns
180
ns (max)
CIN
Capacitance of Logic Inputs
5
pF
COUT
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 specifications 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 = 0 VDC, unless otherwise specified.
Note 4: When the input voltage VIN at any pin exceeds the power supplies (VIN < (AGND or DGND) or VIN > 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 TJMAX, θJA and the ambient temperature, TA. The maximum
allowable power dissipation at any temperature is PD = (TJMAX − TA)/θJA or the number given in the Absolute Maximum Ratings, whichever is lower. For these de-
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(Continued)
vices, TJMAX = 125˚C. The typical thermal resistances (θJA) of these parts when board mounted follow: ADC08031 and ADC08032 with BIN and CIN suffixes
120˚C/W, ADC08038 with CIN suffix 80˚C/W. ADC08031 with CIWM suffix 140˚C/W, ADC08032 140˚C/W, ADC08034 140˚C/W, ADC08038 with CIWM suffix 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 TJ = 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: Cannot be tested for the ADC08032.
Note 12: For VIN(−) ≥ VIN(+) 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 VCC supply. During testing at low VCC levels (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 VIN does not exceed the supply voltage by more than 50 mV, the output code will be correct. Exceeding this range on an unselected channel will corrupt the reading of a selected channel. Achievement of an absolute 0 VDC to 5 VDC input voltage range will therefore require a minimum supply voltage of 4.950 VDC over temperature variations, initial tolerance and loading.
Note 13: 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 VDC) 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 14: A 40% 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 15: Since data, MSB first, is the 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.
Note 16: For the ADC08032 VREFIN is internally tied to VCC, therefore, for the ADC08032 reference current is included in the supply current.
Typical Performance Characteristics
Linearity Error vs
Reference Voltage
Linearity Error vs
Temperature
Linearity Error vs
Clock Frequency
DS010555-32
Power Supply Current vs
Temperature (ADC08038,
ADC08034, ADC08031)
DS010555-34
DS010555-33
Output Current vs
Temperature
Power Supply Current
vs Clock Frequency
DS010555-36
DS010555-37
DS010555-35
Note: For ADC08032 add IREF
5
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ADC08031/ADC08032/ADC08034/ADC08038
Electrical Characteristics
ADC08031/ADC08032/ADC08034/ADC08038
Leakage Current Test Circuit
DS010555-7
TRI-STATE Test Circuits and Waveforms
t1H
DS010555-38
DS010555-39
t0H
DS010555-41
DS010555-40
Timing Diagrams
Data Input Timing
DS010555-10
*To reset these devices, CLK and CS must be simultaneously high for a period of tSELECT or greater. Otherwise these devices are compatible with industry
standards ADC0831/2/4/8.
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ADC08031/ADC08032/ADC08034/ADC08038
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.
ADC08032 Timing
DS010555-14
7
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ADC08031/ADC08032/ADC08034/ADC08038
Timing Diagrams
(Continued)
ADC08034 Timing
DS010555-15
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*Make sure clock edge #18 clocks in the LSB before SE is taken low
DS010555-16
(Continued)
9
ADC08031/ADC08032/ADC08034/ADC08038
ADC08038 Timing
Timing Diagrams
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DS010555-17
*Some of these functions/pins are not available with other options.
For the ADC08034, the “SEL 1” Flip-Flop is bypassed, for the ADC08032, both “SEL 0” and “SEL 1” Flip-Flops are bypassed.
ADC08031/ADC08032/ADC08034/ADC08038
ADC08038 Functional Block Diagram
Functional Description
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
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 terminal and 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.
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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.
(Continued)
enabled and whether this input is single-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 treated as the “−” input for any of the other input
TABLE 1. Multiplexer/Package Options
Part
Number
Number of Analog
Channels
Single-Ended
Differential
Number of
Package
Pins
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
Analog Single-Ended Channel #
MUX Address
START
SGL/
ODD/
SELECT
DIF
SIGN
1
0
1
1
0
0
0
1
1
0
0
1
1
1
0
1
0
1
1
0
1
1
1
1
1
0
0
1
1
1
0
1
1
1
1
1
0
1
1
1
1
1
0
1
2
3
4
5
6
7
COM
+
−
+
−
+
−
+
−
+
−
+
−
+
−
+
−
TABLE 3. MUX Addressing: ADC08038
Differential MUX Mode
Analog Differential Channel-Pair #
MUX Address
START
SGL/
ODD/
DIF
SIGN
1
SELECT
0
0
0
1
1
0
0
0
0
+
−
1
0
0
0
1
1
0
0
1
0
1
0
0
1
1
1
0
1
0
0
1
0
1
0
1
1
0
1
1
0
1
0
1
1
1
−
1
2
2
3
+
−
3
4
5
+
−
6
7
+
−
−
+
+
−
+
−
11
+
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ADC08031/ADC08032/ADC08034/ADC08038
Functional Description
ADC08031/ADC08032/ADC08034/ADC08038
Functional Description
(Continued)
TABLE 4. MUX Addressing: ADC08034
Single-Ended MUX Mode
Channel #
MUX Address
START
SGL/
ODD/
SELECT
DIF
SIGN
1
1
1
0
0
1
1
0
1
1
1
1
0
1
1
1
1
0
1
2
3
+
+
+
+
COM is internally tied to AGND
MUX Addressing:
ADC08032
Single-Ended MUX Mode
Channel #
MUX Address
START
SGL/
ODD/
DIF
SIGN
1
1
0
1
1
1
0
1
+
+
COM is internally tied to AGND
Differential MUX Mode
Channel #
MUX Address
START
SGL/
ODD/
SELECT
DIF
SIGN
1
1
0
0
0
1
0
0
1
1
0
1
0
1
0
1
1
0
1
+
−
−
+
2
3
+
−
−
+
Differential MUX Mode
Channel #
MUX Address
START
SGL/
ODD/
1
DIF
SIGN
1
0
0
+
−
1
0
1
−
+
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 illustrates the input flexibility which can be achieved.
The analog input voltages for each channel can range from
50mV below ground to 50mV above VCC (typically 5V) without degrading conversion accuracy.
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.
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
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12
7.
The stored data in the successive approximation 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
the ADC08038 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 tSELECT 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.
(Continued)
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
1⁄2 clock period (where nothing happens) is automatically
inserted to allow the selected MUX channel to settle.
The SARS line goes high at this time to signal 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 indicate this 1⁄2 clock cycle later.
8 Single-Ended
8 Pseudo-Differential
DS010555-48
DS010555-49
Mixed Mode
4 Differential
DS010555-50
DS010555-51
FIGURE 1. Analog Input Multiplexer Options for the ADC08038
13
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ADC08031/ADC08032/ADC08034/ADC08038
Functional Description
ADC08031/ADC08032/ADC08034/ADC08038
Functional Description
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 VREFOUT. This
can be tied back to VREFIN. Bypassing VREFOUT 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 VCC
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 VREF/256).
(Continued)
3.0 REFERENCE CONSIDERATIONS
The voltage applied to the reference input on these converters, VREFIN, defines the voltage span of the analog input
(the difference between VIN(MAX) and VIN(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 VREFIN pin can
be tied to VCC (done internally on the ADC08032). 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.
DS010555-52
a) Ratiometric
DS010555-53
b) Absolute with a Reduced Span
FIGURE 2. Reference Examples
4.0 THE ANALOG INPUTS
VPEAK is its peak voltage value
and fCLK is the A/D clock frequency.
For a 60Hz common-mode signal to generate a 1⁄4 LSB 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 ± 1µA over 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.
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 1⁄2 of 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:
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, VIN(MIN), is not ground a
zero offset can be done. The converter can be made to out-
where fCM is the frequency of the common-mode signal,
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14
should be properly adjusted first. A VIN (+) voltage which
equals this desired zero reference plus 1⁄2 LSB (where the
LSB is calculated for the desired analog span, using 1 LSB =
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 00HEX to 01HEX code transition.
(Continued)
put 0000 0000 digital code for this minimum input voltage by
biasing any VIN (−) input at this VIN(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 riser of the transfer function and can be measured by
grounding the VIN (−) input and applying a small magnitude
positive voltage to the VIN (+) 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 1⁄2 LSB value (1⁄2 LSB =
9.8mV for VREF = 5.000VDC).
The full-scale adjustment should be made [with the proper
VIN (−) voltage applied] by forcing a voltage to the VIN (+) input which is given by:
5.2 Full Scale
The full-scale adjustment can be made by applying a differential input voltage which is 11⁄2 LSB down from the desired
analog full-scale voltage range and then adjusting the magnitude of the VREFIN input (or VCC for the ADC08032) for a
digital output code which is just changing from 1111 1110 to
1111 1111.
where:
VMAX = the high end of the analog input range
and
VMIN = the low end (the offset zero) of the analog range.
(Both are ground referenced.)
The VREFIN (or VCC) voltage is then adjusted to provide a
code change from FEHEX to FFHEX. This completes the adjustment procedure.
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
15
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ADC08031/ADC08032/ADC08034/ADC08038
Functional Description
ADC08031/ADC08032/ADC08034/ADC08038
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.
Low-Cost Remote Temperature Sensor
DS010555-45
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16
ADC08031/ADC08032/ADC08034/ADC08038
Applications
(Continued)
Digitizing a Current Flow
DS010555-22
Operating with Ratiometric Transducers
DS010555-23
*VIN(−) = 0.15 VCC
15% of VCC ≤ VXDR ≤ 85% of VCC
17
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ADC08031/ADC08032/ADC08034/ADC08038
Applications
(Continued)
Span Adjust; 0V ≤ VIN ≤ 3V
DS010555-46
Zero-Shift and Span Adjust: 2V ≤ VIN ≤ 5V
DS010555-47
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18
ADC08031/ADC08032/ADC08034/ADC08038
Applications
(Continued)
Protecting the Input
High Accuracy Comparators
DS010555-26
DS010555-25
DO = all 1s if +VIN
DO = all 0s if +VIN
> −VIN
< −VIN
Diodes are 1N914
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
19
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ADC08031/ADC08032/ADC08034/ADC08038
Applications
(Continued)
4 mA-20 mA Current Loop Converter
DS010555-28
•
•
All power supplied by loop
1500V isolation at output
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20
ADC08031/ADC08032/ADC08034/ADC08038
Applications
(Continued)
Isolated Data Converter
DS010555-29
•
•
No power required remotely
1500V isolation
21
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ADC08031/ADC08032/ADC08034/ADC08038
Physical Dimensions
inches (millimeters) unless otherwise noted
Order Number ADC08031CIWM, ADC08032CIWM, or ADC08034CIWM
NS Package Number M14B
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ADC08031/ADC08032/ADC08034/ADC08038
Physical Dimensions
inches (millimeters) unless otherwise noted (Continued)
Order Number ADC08038CIWM
NS Package Number M20B
Order Number ADC08031CIN
NS Package Number N08E
23
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ADC08031/ADC08032/ADC08034/ADC08038 8-Bit High-Speed Serial I/O A/D Converters with
Multiplexer Options, Voltage Reference, and Track/Hold Function
Physical Dimensions
inches (millimeters) unless otherwise noted (Continued)
Order Number ADC08038CIN
NS Package Number N20A
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