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AD7818 Schematic ( PDF Datasheet ) - Analog Devices

Teilenummer AD7818
Beschreibung 10-Bit ADC
Hersteller Analog Devices
Logo Analog Devices Logo 




Gesamt 20 Seiten
AD7818 Datasheet, Funktion
Data Sheet
4-Channel/Single-Channel, 9 μs,
10-Bit ADCs with On-Chip Temperature Sensor
AD7817/AD7818
FEATURES
10-bit ADC with 9 μs conversion time
1 AD7818 and 4 AD7817 single-ended analog input channels
On-chip temperature sensor
Resolution of 0.25°C
±2°C error from −40°C to +85°C
−55°C to +125°C operating range
Wide operating supply range: 2.7 V to 5.5 V
Inherent track-and-hold functionality
On-chip reference (2.5 V ± 1%)
Overtemperature indicator
Automatic power-down at the end of a conversion
Low power operation
4 μW at a throughput rate of 10 SPS
40 μW at a throughput rate of 1 kSPS
400 μW at a throughput rate of 10 kSPS
Flexible serial interface
APPLICATIONS
Data acquisition systems with ambient temperature
monitoring
Industrial process control
Automotive
Battery charging applications
GENERAL DESCRIPTION
The AD7817/AD7818 are 10-bit, single- and 4-channel analog-to-
digital converters (ADCs) with an on-chip temperature sensor that
can operate from a single 2.7 V to 5.5 V power supply. Each
device contains a 9 μs successive approximation converter based
around a capacitor digital-to-analog converter (DAC), an on-
chip temperature sensor with an accuracy of ±2°C, an on-chip
clock oscillator, inherent track-and-hold functionality, and an
on-chip reference (2.5 V).
The on-chip temperature sensor of the AD7817/AD7818 can
be accessed via Channel 0. When Channel 0 is selected and a
conversion is initiated, the resulting ADC code at the end of the
conversion gives a measurement of the ambient temperature with a
resolution of ±0.25°C. See the Temperature Measurement section.
The AD7817/AD7818 have a flexible serial interface that allows
easy interfacing to most microcontrollers. The interface is
compatible with the Intel 8051, Motorola SPI and QSPI, and
National Semiconductors MICROWIRE protocols. For more
FUNCTIONAL BLOCK DIAGRAMS
REFIN
VDD
VIN1
VIN2
VIN3
VIN4
AD7817
TEMP
SENSOR
REF
2.5V
MUX
SAMPLING
CAPACITOR
OVERTEMP B
REG
CHARGE
REDISTRIBUTION
DAC
A>B
A
DATA
OUT
CONTROL
LOGIC
REG
CONTROL
VBALANCE
CLOCK
AGND DGND
BUSY
CONVST
Figure 1. AD7817 Functional Block Diagram
VDD
AD7818
TEMP
SENSOR
REF
2.5V
OVERTEMP
REG
B
CHARGE
REDISTRIBUTION
DAC
A>B
A
DATA
OUT
VIN1
MUX
SAMPLING
CAPACITOR
CONTROL
LOGIC
CONTROL
REG
VBALANCE
CLOCK
GENERATOR
AGND
CONVST
Figure 2. AD7818 Functional Block Diagram
OTI
DOUT
DIN
SCLK
RD/WR
CS
OTI
DIN/OUT
SCLK
RD/WR
information, refer to the AD7817 Serial Interface section and
the AD7818 Serial Interface Mode section.
The AD7817 is available in a narrow body, 0.15 inch, 16-lead
SOIC and a 16-lead TSSOP, and the AD7818 comes in an 8-lead
SOIC and an 8-lead MSOP.
PRODUCT HIGHLIGHTS
1. The devices have an on-chip temperature sensor that allows
an accurate measurement of the ambient temperature to be
made. The measurable temperature range is −55°C to +125°C.
2. An overtemperature indicator is implemented by carrying out a
digital comparison of the ADC code for Channel 0 (temperature
sensor) with the contents of the on-chip overtemperature
register. The overtemperature indicator pin goes logic low
when a predetermined temperature is exceeded.
3. The automatic power-down feature enables the AD7817 and
AD7818 to achieve superior power performance at slower
throughput rates, that is, 40 μW at 1 kSPS throughput rate.
Rev. E
Document Feedback
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responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other
rights of third parties that may result from its use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
Trademarksandregisteredtrademarksarethepropertyoftheirrespectiveowners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700 ©2012–2015 Analog Devices, Inc. All rights reserved.
Technical Support
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AD7818 Datasheet, Funktion
AD7817/AD7818
Data Sheet
TIMING CHARACTERISTICS
VDD = 2.7 V to 5.5 V, GND = 0 V, REFIN = 2.5 V. All specifications TMIN to TMAX, unless otherwise noted. Sample tested during initial
release and after any redesign or process changes that may affect the parameters. All input signals are measured with tr = tf = 1 ns (10% to
90% of 5 V) and timed from a voltage level of 1.6 V. See Figure 17, Figure 18, Figure 21, and Figure 22.
Table 2.
Parameter
tPOWER-UP
t1a
t1b
t2
t3
t4
t5
t6
t7
t8
t9
t10
t11
t121
t131
t14a1, 2
t14b1, 2
t15
t16
t17
A Version/B Version
2
9
27
20
50
0
0
10
10
40
40
0
0
20
20
30
30
150
40
400
Unit
μs max
μs max
μs max
ns min
ns max
ns min
ns min
ns min
ns min
ns min
ns min
ns min
ns min
ns max
ns max
ns max
ns max
ns max
ns min
ns min
Test Conditions/Comments
Power-up time from rising edge of CONVST
Conversion time Channel 1 to Channel 4
Conversion time temperature sensor
CONVST pulse width
CONVST falling edge to BUSY rising edge
CS falling edge to RD/WR falling edge setup time
RD/WR falling edge to SCLK falling edge setup
DIN setup time before SCLK rising edge
DIN hold time after SCLK rising edge
SCLK low pulse width
SCLK high pulse width
CS falling edge to RD/WR rising edge setup time
RD/WR rising edge to SCLK falling edge setup time
DOUT access time after RD/WR rising edge
DOUT access time after SCLK falling edge
DOUT bus relinquish time after falling edge of RD/WR
DOUT bus relinquish time after rising edge of CS
BUSY falling edge to OTI falling edge
RD/WR rising edge to OTI rising edge
SCLK rising edge to CONVST falling edge (acquisition time of T/H)
1 These figures are measured with the load circuit of Figure 3. They are defined as the time required for DOUT to cross 0.8 V or 2.4 V for VDD = 5 V ± 10% and 0.4 V or 2 V for
VDD = 3 V ± 10%, as shown in Table 1.
2 These times are derived from the measured time taken by the data outputs to change 0.5 V when loaded with the circuit of Figure 3. The measured number is then
extrapolated back to remove the effects of charging or discharging the 50 pF capacitor. This means that the times quoted in the timing characteristics are the true bus
relinquish times of the part and as such are independent of the external bus loading capacitances.
200µA
IOL
TO OUTPUT
PIN CL
50pF
1.6V
200µA
IOL
Figure 3. Load Circuit for Access Time and Bus Relinquish Time
Rev. E | Page 6 of 20

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AD7818 pdf, datenblatt
AD7817/AD7818
CIRCUIT INFORMATION
The AD7817/AD7818 are single- and four-channel, 9 μs
conversion time, 10-bit ADCs with an on-chip temperature
sensor, reference, and serial interface logic functions on a single
chip. The ADC section consists of a conventional, successive
approximation converter based around a capacitor DAC. The
AD7817/AD7818 are capable of running on a 2.7 V to 5.5 V power
supply, and they accept an analog input range of 0 V to VREF.
The on-chip temperature sensor allows an accurate measurement
of the ambient device temperature to be made. The working
measurement range of the temperature sensor is −55°C to +125°C.
The AD7817/AD7818 require a 2.5 V reference, which can be
provided from their internal reference or from an external
reference source. The on-chip reference is selected by connecting
the REFIN pin to analog ground.
CONVERTER DETAILS
Conversion is initiated by pulsing the CONVST input. The
conversion clock for the device is internally generated; therefore,
an external clock is not required, except when reading from and
writing to the serial port. The on-chip, track-and-hold goes from
track mode to hold mode, and the conversion sequence is started
on the falling edge of the CONVST signal. At this point, the BUSY
signal goes high and low again 9 μs or 27 μs later (depending on
whether an analog input or the temperature sensor is selected)
to indicate the end of the conversion process. A microcontroller
can use this signal to determine when the result of the conversion is
to be read. The track-and-hold acquisition time of the AD7817/
AD7818 is 400 ns.
A temperature measurement is made by selecting the Channel 0
of the on-chip mux and carrying out a conversion on this channel.
A conversion on Channel 0 takes 27 μs to complete. Temperature
measurement is explained in the Temperature Measurement
section.
The on-chip reference is not available, however, REFIN can be
overdriven by an external reference source (2.5 V only). The effect
of reference tolerances on temperature measurements is discussed
in the Temperature Measurement Error Due to Reference Error
section.
Tie all unused analog inputs to a voltage within the nominal
analog input range to avoid noise pickup. For minimum power
consumption, tie the unused analog inputs to AGND.
Data Sheet
TYPICAL CONNECTION DIAGRAM
Figure 8 shows a typical connection diagram for the AD7817.
The AGND and DGND are connected together at the device for
good noise suppression. The BUSY line is used to interrupt the
microcontroller at the end of the conversion process, and the
serial interface is implemented using three wires (see the AD7817
Serial Interface section for more details). An external 2.5 V
reference can be connected at the REFIN pin. If an external reference
is used, connect a 10 μF capacitor between REFIN and AGND.
For applications where power consumption is a concern, use the
automatic power-down at the end of a conversion to improve
power performance. See the Power vs. Throughput section.
SUPPLY
2.7V TO 5.5V
10µF
0.1µF
3-WIRE
SERIAL
INTERFACE
0V TO 2.5V
INPUT
VDD SCLK
AIN1
AIN2
AIN3
AIN4
RD/WR
DOUT
DIN
AD7817
OPTIONAL
EXTERNAL
REFERENCE
AD780/
REF-192
AGND
DGND
CONVST
BUSY
OTI
REFIN
CS
10µF
EXTERNAL
REFERENCE
Figure 8. Typical Connection Diagram
ANALOG INPUTS
Analog Input
Figure 9 shows an equivalent circuit of the analog input structure of
the AD7817/AD7818. The two diodes, D1 and D2, provide ESD
protection for the analog inputs. Take care to ensure that the
analog input signal never exceeds the supply rails by more than
200 mV. This causes these diodes to become forward-biased
and start conducting current into the substrate. The maximum
current these diodes can conduct without causing irreversible
damage to the device is 20 mA. The C2 capacitor in Figure 9 is
typically about 4 pF and can mostly be attributed to pin
capacitance. The R1 resistor is a lumped component made up
of the on resistance of a multiplexer and a switch. This resistor
is typically about 1 kΩ. The C1 capacitor is the ADC sampling
capacitor and has a capacitance of 3 pF.
VDD
AIN
C2
4pF
D1
R1
1k
C1
3pF
VBALANCE
D2
CONVERT PHASE—SWITCH OPEN
TRACK PHASE—SWITCH CLOSED
Figure 9. Equivalent Analog Input Circuit
Rev. E | Page 12 of 20

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