AD AD7298 8-channel, 1msps, 12-bit sar adc with temperature sensor Datasheet

Preliminary Technical Data
8-Channel, 1MSPS, 12-Bit SAR ADC
with Temperature Sensor
AD7298
FEATURES
FUNCTIONAL BLOCK DIAGRAM
12 bit SAR ADC
8 single-ended inputs
Channel sequencer functionality
Fast throughput of 1Msps
Analog Input Range 0 to 2.5V
12-bit temperature-to-digital converter
Temperature sensor accuracy of ±2°C typical
Temperature range: −40°C to +125°C
Specified for VDD of 2.8 V to 3.6V
Logic Voltage VDRIVE = 1.65V to 3.6V
Power-down current : <10 µA
Internal 2.5V Reference
Internal Power on Reset
High speed serial interface SPI™
20-lead LFCSP
Figure 1.
GENERAL DESCRIPTION
The AD7298 is a 12-bit, high speed, low power, 8-channel,
successive approximation ADC with an internal temperature
sensor. The part operates from a single 3.3V power supply and
features throughput rates up to 1MSPS. The device contains a
low noise, wide bandwidth track-and-hold amplifier that can
handle input frequencies in excess of 70 MHz.
The AD7298 offers a programmable sequencer, which enables
the selection of a pre-programmable sequence of channels for
conversion. The device has an on-chip 2.5 V reference that can be
disabled to allow the use of an external reference.
The AD7298 includes a high accuracy band-gap temperature
sensor, which is monitored and digitized by the12-bit ADC to
give a resolution of 0.25°C. The device offers a 4-wire serial
interface compatible with SPI, and DSP interface standards.
The AD7298 uses advanced design techniques to achieve very
low power dissipation at high throughput rates. The part also
offers flexible power/throughput rate management options. The
part is offered in a 20 lead LFCSP package.
PRODUCT HIGHLIGHTS
1.
Ideally suited to monitoring system variables in a variety of
systems including telecommunications, process and
industrial control.
1.
High Throughput rate of 1Msps per channel with Low
Power Consumption.
2.
Eight Single-Ended Inputs with a Channel Sequencer.
A consecutive sequence of channels can be selected on
which the ADC cycles and converts.
3.
Integrated temperature sensor with 0.25°C resolution.
Table 1. AD7298 and Related Products
Device
AD7298
AD7291
Resolution
12-Bit
12-Bit
Interface
SPI
I2 C
Features
8 Channel ADC & Temp Sensor
8 Channel ADC & Temp Sensor
Rev. PrA
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Preliminary Technical Data
AD7298
SPECIFICATIONS
AD7298 SPECIFICATIONS
VDD = 2.8V to 3.6V; VDRIVE = 1.65 V to 3.6 V; fSAMPLE = 1 MSPS, fSCLK = 20 MHz, fast SCLK mode; VREF = 2.5 V internal/external; TA = −40°C
to +125°C, unless otherwise noted.
Table 2.
Parameter
DYNAMIC PERFORMANCE
Signal-to-Noise Ratio (SNR) 1
Signal-to-Noise (+ Distortion) Ratio (SINAD)1
Total Harmonic Distortion (THD)1
Spurious-Free Dynamic Range (SFDR)1
Intermodulation Distortion (IMD)1
Second-Order Terms
Third-Order Terms
Channel-to-Channel Isolation1
SAMPLE AND HOLD
Aperture Delay2
Aperture Jitter2
Full Power Bandwidth
DC ACCURACY
Resolution
Integral Nonlinearity (INL)1
Differential Nonlinearity (DNL)1
Offset Error
Offset Error Matching
Offset Temperature Drift
Gain Error
Gain Error Matching
Gain Temperature Drift
ANALOG INPUT
Input Voltage Ranges
DC Leakage Current
Input Capacitance
Input Impedance
REFERENCE INPUT/OUTPUT
Reference Output Voltage3
Long-Term Stability
Output Voltage Hysteresis1
Reference Input Voltage Range4
DC Leakage Current
Input Capacitance
VREF Output Impedance
Reference Temperature Coefficient
VREF Noise
Min
Typ
70
70
71
71
−84
−85
Max
Unit
78
80
dB
dB
dB
dB
Test Conditions/Comments
fIN = 50 kHz sine wave
fA = 40.1 kHz, fB = 41.5 kHz
−88
−88
−100
dB
dB
dB
10
40
TBD
TBD
12
±0.5
±0.5
±1
±0.5
4
±1
±0.5
0.5
0
±0.01
32
TBD
2.4875
2.5
150
50
2.0
±0.01
TBD
1
6
60
±1
±0.99
±6
±1
±2
±1
VREF
±1
2.5125
2.5
±1
25
ns
ps
MHz
MHz
Bits
LSB
LSB
LSB
LSB
ppm/°C
LSB
LSB
ppm/°C
V
µA
pF
kΩ
V
ppm
ppm
V
µA
pF
Ω
ppm/°C
µV rms
Rev. PrA | Page 2 of 18
@ 3 dB
@ 0.1 dB
Guaranteed no missed codes to 12 bits
When in track
@ 1 MSPS
±0.5% maximum @ 25°C
For 1000 hours
External reference applied to Pin VREF
Bandwidth = TBD kHz
Preliminary Technical Data
Parameter
LOGIC INPUTS
Input High Voltage, VINH
AD7298
Min
Typ
0.7 ×
VDRIVE
Input Current, IIN
Input Capacitance, CIN2
Output Low Voltage, VOL
Floating State Leakage Current
Floating State Output Capacitance2
TEMPERATURE SENSOR—INTERNAL
Operating Range
Accuracy
±0.01
3
CONVERSION RATE
Conversion Time
±0.01
8
−40
±1
±2
0.25
T2 + 16
× tSCL
100
2.8
1.65
Test Conditions/Comments
V
µA
pF
VIN = 0 V or VDRIVE
V
0.4
±1
+125
±2
±3
V
µA
pF
°C
°C
°C
TA = −40°C to +85°C
TA = >85°C to 125°C
LSB size
μs
For VIN0 to VIN7, with one cycle Latency.
TBD
1
μs
ns
MSPS
10
KSPS
3
3
3.6
3.6
V
V
TSENSE Channel
Full-scale step input
fSCLK = 20 MHz, for analog voltage conversions,
one cycle Latency,
For TSENSE channel, one cycle Latency
Digital inputs = 0 V or VDRIVE
See
5
5
3
TBD
60
mA
mA
mA
μA
16.5
9.9
TBD
1.65
mW
mW
mW
μW
1
Track/Hold Acquisition Time2
Throughput Rate
POWER REQUIREMENTS
VDD
VDRIVE
ITOTAL5
Normal Mode (Operational) 6
Normal Mode (Static)
Partial Power-Down Mode
Full Power-Down Mode
Power Dissipation
Normal Mode (Operational)
Normal Mode (Static)
Partial Power-Down Mode
Full Power-Down Mode
+0.3 x
VDRIVE
±1
VDRIVE −
0.2
Resolution
Unit
V
Input Low Voltage, VINL
LOGIC OUTPUTS
Output High Voltage, VOH
Max
VDD = 3.3V
VDD = 3.3V
1
See the Terminology Section.
Sample tested during initial release to ensure compliance.
3
Refers to Pin VREF specified for 25oC.
4
VREF variations from 2.5V will alter the gain error of the temperature sensor, oC per LSB, and a correction factor may be required, See Section X.
5
ITOTAL is the total current flowing in VDD and VDRIVE.
6
Current and power typical specifications are based on results with VDD = 3V and VDRIVE = 1.8V
2
Rev. PrA | Page 3 of 18
Preliminary Technical Data
AD7298
TIMING SPECIFICATIONS
VDD = 2.8V to 3.6V; VDRIVE = 1.65 V to 3.6 V; VREF = 2.5 V internal/external; TA = −40°C to + 125°C, unless otherwise noted. 7
Table 3.
Parameter
tCONVERT
Limit at TMIN, TMAX
16 × tSCLK
1
100
50
20
TBD
Unit
µs max
µs max
µs max
kHz min
MHz max
ns min
TBD
ns min
t2
t3
t4
t5
t6
t7
t8
10
25
TBD
0.4 × tSCLK
0.4 × tSCLK
TBD
15/45
t9
t10
t11
10
5
TBD
ns min
ns max
ns max
ns min
ns min
ns min
ns
min/max
ns min
ns min
ns min
tPOWER-
TBD
μs max
Power-up time from partial power-down
TBD
μs max
Power-up time from full power-down
fSCLK
tQUIET
Test Conditions/Comments
Conversion time
For each ADC channel VIN0 to VIN7, FSCLK = 20MHz
For Temperature Sensor channel
Frequency of external serial clock
Frequency of external serial clock
Minimum quiet time required between the end of serial read and the start of the next
voltage conversion in repeat and non-repeat mode.
Minimum quiet time required between the end of serial read and the start of the next
temperature conversion, for consecutive Temperature conversions.
CS to SCLK setup time
Delay from CS until DOUT three-state disabled
Data access time after SCLK falling edge
SCLK low pulsewidth
SCLK high pulsewidth
SCLK to DOUT valid hold time
SCLK falling edge to DOUT high impedance
DIN setup time prior to SCLK falling edge
DIN hold time after SCLK falling edge
TSENSEBUSY falling edge to CS falling edge
UP_PARTIAL
tPOWER-UP
7
Sample tested during initial release to ensure compliance. All input signals are specified with tR = tF = 5 ns (10% to 90% of VDRIVE) and timed from a voltage level of 1.6 V.
All timing specifications given are with a 25 pF load capacitance. With a load capacitance greater than this value, a digital buffer or latch must be used.
Rev. PrA | Page 4 of 18
Preliminary Technical Data
AD7298
ABSOLUTE MAXIMUM RATINGS
ESD CAUTION
Table 4.
Parameter
VDD to AGND, DGND,
VDRIVE to AGND, DGND,
Analog Input Voltage to AGND
Digital Input Voltage to AGND
Digital Output Voltage to AGND
VREF to AGND
AGND to DGND
Input Current to Any Pin Except
Supplies1
Operating Temperature Range
Storage Temperature Range
Junction Temperature
LFCSP Package
θJA Thermal Impedance
θJC Thermal Impedance
Pb-free Temperature, Soldering
Reflow
ESD
Rating
−0.3 V to +5 V
−0.3 V to + 5 V
−0.3 V to 3V
−0.3 V to VDRIVE + 0.3 V
−0.3 V to VDRIVE + 0.3 V
−0.3 V to +3V
−0.3 V to +0.3V
±10 mA
−40°C to +125°C
−65°C to +150°C
150°C
TBD°C/W
TBD°C/W
260(+0)°C
2 kV
Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
Rev. PrA | Page 5 of 18
AD7298
Preliminary Technical Data
PIN CONFIGURATION AND FUNCTION DESCRIPTION
Figure 2. Pin Configuration
Note: The exposed metal paddle on the bottom of the LFCSP package
should be soldered to PCB ground for proper heat dissipation & performance
Table 5. Pin Function Descriptions
Pin No.
1-5, 18,
19, 20
14
Mnemonic
VIN1, VN2, VN3,
VN4, VN5, VN6
VN7, VN8
DOUT
16
VDRIVE
10
VDD
7
VREF
6
GND1
9
GND
11
CS
15
SCLK
Description
Analog Inputs. The AD7298 has 8 single-ended analog inputs that are multiplexed into the on-chip track-andhold. Each input channel can accept analog inputs from 0V to 2.5V. Any unused input channels should be
connected to GND1 to avoid noise pickup.
Serial Data Output. The conversion result from the AD7298 is provided on this output as a serial data stream. The
bits are clocked out on the falling edge of the SCLK input. The data stream from the AD7298 consists of four
address bits indicating which channel the conversion result corresponds to, followed by the 12 bits of conversion
data (MSB first). The output coding is straight binary for the voltage channels and two’s complement for the
temperature sensor result.
Logic Power Supply Input. The voltage supplied at this pin determines at the voltage at which the interface
operates. This pin should be decoupled to GND. The voltage range on this pin is 1.65V to 3.6V and may be less
than the voltage at VDD, but should never exceed it by more than 0.3V.
Supply Voltage, 2.8 V to 3.6 V. This supply should be decoupled to GND with 10 µF and 100 nF decoupling
capacitors.
Internal Reference / External Reference supply. The nominal internal reference voltage of 2.5V appears at this pin.
Provided the output is buffered, the on-chip reference can be taken from this pin and applied externally to the
rest of a system. Decoupling capacitors should be connected to this pin to decouple the reference buffer. For best
performance, it is recommended to use a 10 μF decoupling capacitor on this pin to GND1. The internal reference
can be disabled and an external reference supplied to this pin if required. The input voltage range for the external
reference is 2.0 V to 2.5V.
Ground. Ground reference point for the internal reference circuitry on the AD7298. The external reference signals
and all analog input signals should be referred to this GND1 voltage. The GND1 pin should be connected to the
GND plane of a system. All GND1 pins should ideally be at the same potential and must not be more than 0.3 V
apart, even on a transient basis. The VREF should be decoupled to this ground pin via a 10 μF decoupling cap.
Ground. Ground reference point for all analog and digital circuitry on the AD7298. The GND pin should be
connected to the GND plane of the system. All GND pins should ideally be at the same potential and must not be
more than 0.3 V apart, even on a transient basis. Both DCAP and VDD should be decoupled to this GND pin.
Chip Select, Active Low Logic Input. This pin is edge triggered, on the falling edge of this input, the track/hold
goes into hold mode and a conversion is initiated. This input also frames the serial data transfer. When CS is low,
the output bus is enabled, and the conversion result becomes available on the DOUT output.
Serial Clock, Logic Input. A serial clock input provides the SCLK for accessing the data from the AD7298.
Rev. PrA | Page 6 of 18
Preliminary Technical Data
Pin No.
12
Mnemonic
TSENSE_BUSY
13
DIN
17
PD/RST
8
DCAP
AD7298
Description
Busy Output. BUSY transitions high when a temperature sensor conversion starts and remains high until the
conversion completes
Data In. Logic input. Data to be written to the AD7298 control register is provided on this input and is clocked into
the register on the falling edge of SCLK.
Power Down Pin. This pin will place the part into a full power down mode and will enable power conservation
when the parts operation is not required. This pin can be used to RESET the device by toggling the pin LOW for a
minimum of TBD ns and a maximum of 100ns. If the maximum time is exceeded the part will enter power-down
mode.
Decoupling Capacitor Pins. Decoupling capacitors (1 μF recommended) are connected to this pins to decouple
the internal LDO.
Rev. PrA | Page 7 of 18
AD7298
Preliminary Technical Data
CONTROL REGISTER
The control register of the AD7298 is a 16-bit, write-only register. Data is loaded from the DIN pin of the AD7298 on the falling edge of
SCLK. The data is transferred on the DIN line at the same time that the conversion result is read from the part. The data transferred on
the DIN line corresponds to the AD7298 configuration for the next conversion. This requires 16 serial clocks for every data transfer. Only
the information provided on the first 16 falling clock edges (after the falling edge of CS) is loaded to the control register. MSB denotes the
first bit in the data stream. The bit functions are outlined in Table 6 and Table 7. On power up the default content of the control register is
all zero’s.
Table 6. Control Register Bit Functions
LSB
MSB
D15
D14
D13
D12
D11
D10
D9
D8
D7
D6
D5
D4
D3
D2
D1
D0
WRITE
REPEAT
CH1
CH2
CH3
CH4
CH5
CH6
CH7
CH8
TSENSE
DONTC
DONTC
EXT_REF
TSENSE AVG
PD
Table 7. Control Register Bit Function Description
Bit
15
Mnemonic
WRITE
REPEAT
CH1 to CH8
TSENSE
DONTC
EXT_REF
Description
The value written to this bit of the control register determines whether the following 15 bits are loaded to
the control register. If this bit is a 1, the following 15 bits are written to the control register; if it is a 0, then the
remaining 15 bits are not loaded to the control register and it remains unchanged.
This bit enables the repeated conversion of the selected sequence of channels.
Channel selection bits: These eight bits are loaded at the end of the current conversion and select which analog
input channel is to be converted in the next serial transfer, or they may select the sequence of channels for
conversion in the subsequent serial transfers. Each CHX bit corresponds to an analog input channel. A channel or
sequence of channels is selected for conversion by writing a 1 to the appropriate CHX bit/bits. Channel address
bits corresponding to the conversion result are output on DOUT prior to the 12 bits of data. The next channel to be
converted on is selected by the mux on the 14th SCLK falling edge.
Writing a 1 to this bit enables the temperature conversion. When the temperature sensor is selected for conversion
the TSENSE_BUSY pin will go high after the next CS falling edge to indicate that the conversion is in progress, the
previous conversion result can be read while the temperature conversion is in progress. Once TSENSE_Busy goes
low, CS can be brought low Tx ns later to read the TSENSE conversion result.
Don’t care.
Writing a logic 1 to this bit, enables the use of an external reference. The input voltage range for the external
reference is 2V to 2.5V. The external reference should not exceed 2.5V or the device performance will be affected.
Writing a 1 to this bit enables the temperature sensor averaging function. When averaging is enabled, the AD7298
internally computes a running average of the conversion results to determine the final TSENSE result (See page 14
for more details). This mode will reduce the influence of noise on the final TSENSE result. Selecting this feature does
not automatically select the TSENSE for conversion. The TSENSE bit must also be set to start a temperature sensor
conversion.
Partial Power Down. This mode is selected by writing a 1 to this bit in the control register. In this mode, some of
the internal analog circuitry is powered down. The AD7298 retains the information in the control register while in
partial power down mode. The part remains in this mode until a 0 is written to this bit.
TSENSE AVG
PD
Table 8. Channel Address bits
ADD3
0
0
0
0
0
0
0
0
1
1
ADD2
0
0
0
0
1
1
1
1
0
0
ADD1
0
0
1
1
0
0
1
1
0
0
ADD0
0
1
0
1
0
1
0
1
0
1
Analog Input Channel
VIN1
VIN2
VIN3
VIN4
VIN5
VIN6
VIN7
VIN8
TSENSE
TSENSE with averaging
enabled
Rev. PrA | Page 8 of 18
Preliminary Technical Data
AD7298
MODES OF OPERATION
The AD7298 has a number of different modes of operation, which are designed to provide additional flexibility for the user. These options
can be chosen by programming the content of the control register to select the desired mode.
TRADITIONAL MULTI-CHANNEL MODE OF OPERATION
The AD7298 can operate as a traditional multi-channel ADC, where each serial transfer selects the next channel for conversion. One must
write to the control register to configure and select the desired input channel prior to initiating any conversions. In traditional mode of
operation, the CS signal is used to frame the first write to the converter on the DIN pin. In this mode of operation the REPEAT bits in the
control register is set to a low logic level, 0, hence the REPEAT function is not in use. The data, which appears on the DOUT pin during the
initial write to the control register, is invalid. The first CS falling edge will initiate a write to the control register to configure the device, a
conversion is then initiated for the selected analog input channel (VIN1) on the subsequent (2nd) CS falling edge, the third CS falling edge
will have the result (VIN1) available for reading. The AD7298 operates with one cycle latency; hence the conversion result corresponding
to each conversion is available once serial read cycle after the cycle in which the conversion was initiated.
As the device operates with one cycle latency, the control register configuration sets up the configuration for the next conversion, which
is initiated on the next CS falling edge but the first bit of the corresponding result is not clocked out until the subsequent falling CS edge
as shown in Figure 3.
Figure 3. Configuring a conversion and read with the AD7298. One channel selected for conversion.
If more than one channel is selected in the control register, the AD7298 will convert all selected channels sequentially in ascending order
on successive CS falling edges. Once all the selected channels in the control register are converted the AD7298 will cease converting until
the user rewrites to the control register to select the next channel for conversion. This operation is shown in Figure 4
Figure 4. Configuring a conversion and read with the AD7298. Numerous channels selected for conversion
Rev. PrA | Page 9 of 18
AD7298
Preliminary Technical Data
REPEAT OPERATION
The REPEAT bit in the control register allows the user to select a sequence of channels on which the AD7298 will continuously convert.
When the REPEAT bit is set in the control register, the AD7298 will continuously cycle through the selected channels in ascending order,
beginning with the lowest channel and converting all channels selected in the control register. On completion of the sequence, the AD7298
returns to the first selected channel in the control register and recommences the sequence again. The conversion sequence of the selected
channels in the repeat mode of operation continues until such time as the control register of the AD7298 is reprogrammed. If the TSENSE bit is
selected in the Control Register then the temperature conversion will be available for conversion after the last analog input channel in the
sequence has been converted. It is not necessary to write to the control register once a REPEAT operation has been initiated unless a
change in the AD7298 configuration is required. The WRITE bit must be set to zero or the DIN line tied low to ensure that the control
register is not accidentally overwritten, or the automatic conversion sequence interrupted. A write to the control register during REPEAT
mode of operation will reset the cycle even if the selected channels are unchanged. Hence, the next conversion by the AD7298 after a
write operation will be the first selected channel in the sequence.
To select a sequence of channels, the associated channel bit must be set to a logic high state (1) for each analog input whose conversion is
required. For example, if the REPEAT bit = 1 and CH1, CH2 and CH3 =1. The VIN1 analog input will be converted on the first CS falling
edge following the write to the control register, the VIN2 channel will be converted on the subsequent CS falling edge and the VIN1
conversion result will be available for reading, the third CS falling edge following the write operation will initiate a conversion on VIN3
and have the VIN2 result available for reading. The AD7298 operates with one cycle latency; hence the conversion result corresponding to
each conversion is available once serial read cycle after the cycle in which the conversion was initiated.
Figure 5. Configuring a conversion and read in REPEAT mode.
This mode of operation simplifies the operation of the device by allowing consecutive channels to be converted without having to
reprogram the control register or write to the part on each serial transfer. Figure 5 illustrates how to setup the AD7298 to continuously
convert on a particular sequence of channels. To exit REPEAT mode of operation and revert back to the traditional mode of operation of
a multi-channel ADC, ensure that the REPEAT bit = 0 on the next serial write.
Rev. PrA | Page 10 of 18
Preliminary Technical Data
AD7298
POWER-DOWN MODES
The AD7298 has a number of power conservation modes of
operation, which are designed to provide flexible power
management options. These options can be chosen to optimize
the power dissipation/throughput rate ratio for different
application requirements. The power-down modes of operation
of the AD7298 is controlled by the power-down bit, in the
control register and the PD pin on the device. When power
supplies are first applied to the AD7298, care should be taken to
ensure that the part is placed in the required mode of operation
NORMAL MODE
This mode is intended for the fastest throughput rate performance because the user does not have to worry about any powerup times with the AD7298 remaining fully powered at all times.
Figure 6 shows the general diagram of the operation of the
AD7298 in this mode. The conversion is initiated on the falling
edge of CS and the track-and-hold enters hold mode. On the14th
SCLK falling edge the track-and-hold returns to track mode
and starts acquiring the analog input, as described in the serial
interface section. The data presented to the AD7298 on the DIN
line during the first 16 clock cycles of the data transfer are
loaded into the control register (provided the WRITE bit is 1).
The part remains fully powered up in normal mode at the end
of the conversion as long as PD bit is set to 0 in the write transfer
during that conversion. To ensure continued operation in
normal mode, the PD bit should be loaded with 0 on every data
write operation. Sixteen serial clock cycles are required to
complete the conversion and access the conversion result.
For specified performance, the throughput rate should not
exceed 1MSPS. Once a conversion is complete and the CS has
returned high, a minimum of the quiet time, tQUIET, must elapse
before bringing CS low again to initiate another conversion and
access the previous conversion result.
Figure 6. Normal Mode Operation
PARTIAL POWER DOWN MODE
In this mode, part of the internal circuitry on the AD7298 is
powered-down. The AD7298 enters partial power-down on the
CS rising edge once the current serial write operation
containing 16 SCLK clock cycles is completed. To enter partial
power-down the PD bit in the control register should be set to
one on the last required read transfer from the AD7298. Once
in partial power-down mode the AD7298 transmits all ones on
the DOUT pin if CS is toggled low. If the averaging feature for the
temperature sensor is enabled in the control register, the
averaging is reset once the device enters partial power-down
mode.
The AD7298 remains in partial power-down until the powerdown bit, PD, in the control register is changed to a logic level
zero (0). The AD7298 begins powering up on the rising edge of
CS following the write to the control register disabling the
power-down bit. Once TQUITE has elapsed, a full 16-SCLK write
to the control register must be completed to update its content
with the desired channel configuration for the subsequent
conversion. A valid conversion is then initiated on the next CS
falling edge. Since the AD7298 has once cycle latency, the first
conversion result after exiting partial power-down mode is
available in the fourth serial transfer as shown in Figure 7; 1st
cycle to update PD bit, 2nd cycle to update configuration and
Channel ID bits, 3rd to complete conversion, 4th access DOUT
valid result. The use of this mode enables a reduction in the
overall power consumption of the device.
Rev. PrA | Page 11 of 18
AD7298
Preliminary Technical Data
Figure 7. Partial Power Down Mode of Operation
FULL POWER-DOWN MODE
In this mode, all internal circuitry on the AD7298 is powereddown and no information is retained in the control register or any
other internal register. If the averaging feature for the
temperature sensor is enabled in the control register
(TSENSEAVG), the averaging is reset once the device enters
power-down mode. The AD7298 is placed into full power-down
mode by bringing the logic level on the PD pin low for greater
than 100ns. The PD pin is asynchronous to the clock, hence it
can be triggered at any time. The part can be powered up for
normal operation by bringing the PD pin logic level back to a
high logic state. The full power-down feature can be used to
reduce the average power consumed by the AD7298 when
operating at lower throughput rates. The user should ensure
that tPOWER_UP has elapsed prior to programming the control
register and initiating a valid conversion.
POWERING UP THE AD7298
The AD7298 contains a power on reset circuit, which sets the
control register to its default setting of all zero’s, hence the
internal reference is enabled and the device is configured for
normal mode of operation. It takes 100μs to power up the
AD7298 when using the internal reference. When an external
reference is used TBD μs are required to power up the AD7298
with a 1μF decoupling capacitor.
When supplies are first applied to the AD7298, the user must
wait the specified power up time, tPOWER UP, before programming
the control register to select the desired channels for
conversion.
RESET
The AD7298 includes a reset feature, which can be used to reset
the device and the content of all internal registers including the
control register to their default state. To activate the reset
operation, the PD pin should be brought low for a minimum of
TBD ns and a maximum of 100ns and is asynchronous to the
clock, hence it can be triggered at any time. If the PD pin is held
low for greater than 100ns the part will enter full power-down
mode. It is imperative that the PD pin be held at a stable logic
level at all times to ensure normal operation.
Rev. PrA | Page 12 of 18
Preliminary Technical Data
AD7298
CIRCUIT INFORMATION
The AD7298 is a high speed, 8-channel, 12-bit, ADC with
internal temperature sensor. The part can be operated from a
2.8 V to 3.6 V supply and is capable of throughput rates of
1MSPS per analog input channel.
The AD7298 provides the user with an on-chip, track-and-hold
ADC and a serial interface housed in a 20-lead LFCSP. The
AD7298 has eight single-ended input channels with channel
repeat functionality, which allows the user to select a channel
sequence through which the ADC can cycle with each
consecutive CS falling edge. The serial clock input accesses data
from the part, controls the transfer of data written to the ADC,
and provides the clock source for the successive approximation
ADC. The analog input range for the AD7928 is 0V to VREF. The
AD7298 operates with one cycle latency, which means that
conversion result is available in the serial transfer following the
cycle in which the conversion is performed.
The AD7298 includes a high accuracy band-gap temperature
sensor, which is monitored and digitized by the 12-bit ADC to
give a resolution of 0.25°C. The AD7298 provides flexible power
management options to allow the user to achieve the best power
performance for a given throughput rate. These options are
selected by programming the power-down bit, PD, in the
control register.
CONVERTER OPERATION
The AD7298 is a 12-bit successive approximation ADC based
around a capacitive DAC. Figure 8 and Figure 9 show simplified
schematics of the ADC. The ADC is comprised of control logic,
SAR, and a capacitive DAC that are used to add and subtract
fixed amounts of charge from the sampling capacitor to bring
the comparator back into a balanced condition. Figure 8 shows
the ADC during its acquisition phase. SW2 is closed and SW1 is
in Position A. The comparator is held in a balanced condition
and the sampling capacitor acquires the signal on the selected
VIN channel.
Figure 8. ADC Acquisition Phase
When the ADC starts a conversion (see Figure 9), SW2
opens and SW1 moves to Position B, causing the comparator
to become unbalanced. The control logic and the capacitive
DAC are used to add and subtract fixed amounts of charge to
bring the comparator back into a balanced condition. When the
comparator is rebalanced, the conversion is complete. The
control logic generates the ADC output code. Figure 11 shows
the ADC’s transfer functions.
Figure 9. ADC Conversion Phase
ANALOG INPUT
Figure 11 shows an equivalent circuit of the analog input structure of the AD7298. The two diodes, D1 and D2, provide ESD
protection for the analog inputs. Care must be taken to ensure
that the analog input signal never exceeds the internally
generated LDO voltage of 2.5V (DCAP) by more than 300 mV.
This causes the diodes to become forward biased and start
conducting current into the substrate. 10 mA is the maximum
current these diodes can conduct without causing irreversible
damage to the part. Capacitor C1, in Figure 10 is typically about
TBD pF and can primarily be attributed to pin capacitance. The
Resistor R1 is a lumped component made up of the on
resistance of a switch (track-and-hold switch) and also includes
the on resistance of the input multiplexer. The total resistance is
typically about TBD Ω. The capacitor, C2, is the ADC sampling
capacitor and has a capacitance of TBD pF typically.
Figure 10. Equivalent Analog Input Circuit
For AC applications, removing high frequency components
from the analog input signal is recommended by using
an RC low-pass filter on the relevant analog input pin. In
applications where harmonic distortion and signal-to-noise
ratios are critical, the analog input should be driven from a low
impedance source. Large source impedances significantly
affect the ac performance of the ADC. This may necessitate
the use of an input buffer amplifier. The choice of the op amp
is a function of the particular application performance criteria.
Rev. PrA | Page 13 of 18
AD7298
Preliminary Technical Data
ADC Transfer Function
The output coding of the AD7298 is straight binary for the
analog input channel conversion results and twos complement,
for the temperature conversion result. The designed code
transitions occur at successive LSB values (that is, 1 LSB, 2 LSBs,
and so forth). The LSB size is VREF/4096 for the AD7298. The
ideal transfer characteristic for the AD7298 for straight binary
coding is shown in Figure 11.
111…111
111…110
•
•
111…000
•
011…111
•
•
000…010
000…001
000…000
Theoretically, the temperature measuring circuit can measure
temperatures from –512°C to +511°C with a resolution of
0.25°C. However, temperatures outside TA (the specified
temperature range for the AD7298) are outside the guaranteed
operating temperature range of the device. The temperature
sensor is selected by setting the TSENSE bit in the control register.
1LSB = VREF/4096
0V
1LSB
The temperature conversion consists of two phases, the
integration followed by the conversion. The integration is
initiated on the CS falling edge. It takes a period of
approximately100μs to complete the integration and conversion
of the temperature result. When the integration is completed
the conversion is initiated automatically. Once the temperature
integration is initiated, the TSENSEBUSY signal goes high to
indicate that a temperature conversion is in progress and
remains high until the conversion is completed.
Temperature Sensor Averaging
+VREF – 1LSB
ANALOG INPUT
NOTES
VREF IS EITHER REFIN OR 2 × REFIN.
Figure 11. Straight Binary Transfer Characteristic
TEMPERATURE SENSOR OPERATION
The AD7298 contains one local temperature sensor. The onchip, band gap temperature sensor measures the temperature of
the AD7298 die. The temperature sensor module on the
AD7298 is based on the three current principle (see Figure 12),
where three currents are passed through a diode and the
forward voltage drop is measured, allowing the temperature to
be calculated free of errors caused by series resistance.
The AD7298 incorporates a temperature sensor averaging
feature to enhance the accuracy of the temperature
measurements. To enable the temperature sensor averaging
feature both the TSENSEAVG bit and the TSENSE bit must be
enabled in the control register. In this mode the temperature is
internally averaged to reduce the effect of noise on the
temperature result. The temperature is measured each time a
TSENSE conversion is performed and a moving average method is
used to determine the result in the TSENSE Average Result
Register. The average result is given by the following equation;
TSENSE _ AVG =
7
(Pr evious _ Re sult ) + 1 (Current _ Re sult )
8
8
The TSENSE result read when averaging is enabled is TSENSEAVG
result, a moving average temperature measurement.
Figure 12. Top Level Structure of Internal Temperature Sensor
The first TSENSE conversion result given by the AD7298 after the
temperature sensor and averaging mode has been selected in
the control register (bit D1 & D5) is the actual first TSENSE
conversion result. If the control register is written to and the
content of the TSENSEAVG bit changed the averaging function is
reset and the next TSENSE average conversion result is the current
temperature conversion result. If the status of the TSENSEAVG bit
is not changed on successive writes to the control register, the
averaging function will not be reinitialized and will continue
calculating the cumulative average.
The user has the option of disabling the averaging by setting bit
TSENSEAVG to ‘0’ in the control register. The AD7298 defaults on
power-up with the averaging function disabled. The total time
to measure, a temperature channel is typically 100 μs.
Rev. PrA | Page 14 of 18
Preliminary Technical Data
AD7298
Temperature Value Format
THE REFERENCE
One LSB of the ADC corresponds to 0.25°C. The temperature
reading from the ADC is stored in a 12-bit twos complement
format, to accommodate both positive and negative
temperature measurements. The temperature data format is
provided in Table 9.
The AD7298 can operate with either the internal 2.5V on-chip
reference or an externally applied reference. The EXT_REF bit
in the control register is used to determine whether the internal
reference is used. If the EXT_REF bit is selected in the control
register, an external reference can be supplied through the VREF
pin. On power-up, the internal reference is enabled. Suitable
external reference sources for the AD7298 include AD780,
AD1582, ADR431, REF193, and ADR391.
Table 9. Temperature Data Format
Temperature (°C)
−40
−25
−10
−0.25
0
+0.25
+10
+25
+50
+75
+100
+105
+125
Digital Output
1111 0110 0000
1111 1001 1100
1111 1101 1000
1111 1111 1111
0000 0000 0000
0000 0000 0001
0000 0010 1000
0000 0110 0100
0000 1100 1000
0001 0010 1100
0001 1001 0000
0001 1010 0100
0001 1111 0100
The internal reference circuitry consists of a 2.5V band-gap
reference and a reference buffer. When the AD7298 is operated
in internal reference mode, the 2.5V internal reference is available at the VREF pin, which should be decoupled to AGND using a
10 μF capacitor. It is recommended that the internal reference be
buffered before applying it elsewhere in the system. The internal
reference is capable of sourcing up to TBD μA of current when
the converter is static. The reference buffer requires 10ms to
power up and charge the TBD μF decoupling capacitor during
the power-up time.
Temperature Conversion Formula:
Positive Temperature = ADC Code/4
Negative Temperature = (4096 - ADC Code)/4
VDRIVE
The AD7298 also has the VDRIVE feature. VDRIVE controls the voltage at which the serial interface operates. VDRIVE allows the ADC
to easily interface to both a 1.8V and 3V processors. For
example, if the AD7298 were operated with a VDD of 3.3V, the
VDRIVE pin could be powered from a 1.8V supply. This enables
the AD7298 to operate with a larger dynamic range with an VDD
of 3.3V while still being able to interface to 1.8V processors.
Take care to ensure VDRIVE does not exceed VDD by more than
0.3V (see the Maximum Ratings Section).
Rev. PrA | Page 15 of 18
AD7298
Preliminary Technical Data
SERIAL INTERFACE
Figure 13 shows the detailed timing diagram for the serial
interface to the AD7298. The serial clock provides the
conversion clock and controls the transfer of information to and
from the AD7298 during each conversion.
The CS signal initiates the data transfer and conversion process.
The falling edge of CS puts the track-and-hold into hold mode
at which point the analog input is sampled and the bus is taken
out of three-state. The conversion is also initiated at this point
and requires 16 SCLK cycles to complete. The track-and-hold
goes back into track on the 14th SCLK falling edge as shown in
Figure 13 at Point B. On the 16th SCLK falling edge or on the
rising edge of CS , the DOUT line goes back into three-state. If the
rising edge of CS occurs before 16 SCLKs have elapsed, the
conversion is terminated, the DOUT line goes back into tri-state,
and the control register is not updated; otherwise DOUT returns
to three-state on the 16th SCLK falling edge. Sixteen serial clock
cycles are required to perform the conversion process and to
access data from the AD7298.
For the AD7298, four-channel address bits (ADD3 to ADD0)
identify which channel the conversion result corresponds, to
precede the 12 bits of data. The CS going low provides the first
remaining data is then clocked out by subsequent SCLK falling
edges, beginning with a second address bit. Thus, the first
falling clock edge on the serial clock has the first address bit
provided for reading and also clocks out the second address bit.
The 3 remaining address bits and 12 data bits are clocked out by
subsequent SCLK falling edges. The final bit in the data transfer
is valid for reading on the 16th falling edge having been clocked
out on the previous (15th) falling edge.
In applications with a slower SCLK, it may be possible to read
in data on each SCLK rising edge depending on the SCLK
frequency. The first rising edge of SCLK after the CS falling
edge would have the first address bit provided, and the 15th
rising SCLK edge would have last data bit provided.
Writing information to the control register takes place on the
first 16 falling edges of SCLK in a data transfer, assuming the MSB
(that is, the WRITE bit) has been set to 1. The 16-bit word read
from the AD7298 always contains four channel address bits that
the conversion result corresponds to, followed by the 12-bit
conversion result.
address bit to be read in by the microcontroller or DSP. The
Figure 13. Serial Interface Timing Diagram
TEMPERATURE SENSOR READ.
The temperature sensor conversion involves two phases, the
integration phase and the conversion phase as detailed in the
Temperature Sensor Operation Section. The integration phase
is initiated on the falling edge of CS and once completed the
conversion is automatically initiated internally by the AD7298.
When a temperature conversion integration is initiated, the
TSENSEBUSY signal goes high to indicate that a temperature
conversion is in progress and remains high until the conversion
is completed.
The total time to measure and convert a temperature channel
with the AD7298 is 100 μs max. Once the TSENSEBUSY signal
goes low to indicate that the temperature conversion is
completed, t11ns must elapse prior to the next falling edge of CS.
If a minimum of t11 ns is not adhered to between the falling
edge of TSENSEBUSY and the subsequent falling edge of CS, the
next conversion will be corrupted but the temperature result
that is framed by the CS will not be affected. This restriction is
in place to ensure that sufficient acquisition time is allowed for
the next conversion.
Rev. PrA | Page 16 of 18
Preliminary Technical Data
AD7298
Once the TSENSEBUSY signal goes high, the user may provide a
CS falling edge to frame the read of the previous conversion and
program the control register if required, see Figure 14. Once the
previous conversion result has been read, any subsequent CS
falling edges which occur while the TSENSEBUSY signal is high
are internally ignored by the AD7298. If additional CS falling
edges are provided while TSENSEBUSY is high, the AD7298 will
provide an invalid digital output of all 1’s. Alternatively, if CS
remains high while, TSENSEBUSY is high then the DOUT bus will
remain in three-state.
If the user writes to the control register during the first 16 SCLK
cycles following TSENSEBUSY going high, the configuration of the
device for the next conversion, which will be initiated on the
subsequent CS falling edge after TSENSEBUSY goes low, is altered.
TSENSEBUSY going high, the temperature sensor conversion will
be aborted and the part will enter partial power down on the
16th SCLK falling edge.
It is thus recommended not to write to the control register if the
CS signal will be toggling while TSENSEBUSY is high. Hence, care
should be taken to ensure that the write bit is set to zero during
the temperature conversion phase when CS is toggling.
If an SCLK frequency of more than 10kHz is used, the
temperature conversion will require more than one standard
read cycle to complete. In this case, the user can monitor the
TSENSEBUSY signal to determine when the conversion is
completed and the result is available for reading.
If the user configures the part for partial power down in a write
to the control register during the first 16 SCLK cycles following
Figure 14. Serial Interface Timing Diagram for the Temperature Sensor Conversion.
Rev. PrA | Page 17 of 18
AD7298
Preliminary Technical Data
OUTLINE DIMENSIONS
0.30
0.25
0.18
0.50
BSC
PIN 1
INDICATOR
20
16
15
1
EXPOSED
PAD
2.75
2.60 SQ
2.35
11
TOP VIEW
0.80
0.75
0.70
0.50
0.40
0.30
5
10
6
0.25 MIN
BOTTOM VIEW
0.05 MAX
0.02 NOM
COPLANARITY
0.08
0.20 REF
SEATING
PLANE
COMPLIANT TO JEDEC STANDARDS MO-220-WGGD.
020509-B
PIN 1
INDICATOR
4.10
4.00 SQ
3.90
Figure 15. 20 Lead- Lead Frame Chip Scale Package
ORDERING GUIDE
Model
Temperature Range
Package Description
Package Option
AD7298BCPZ
−40°C to +125°C
20 Lead - Lead Frame Chip Scale package
CP-20-8
−40°C to +125°C
20 Lead - Lead Frame Chip Scale package
CP-20-8
AD7298BCPZ-RL7
©2009 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
PR08754-0-12/09(PrA)
Rev. PrA | Page 18 of 18
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