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Preliminary Technical Data
FEATURES
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 2C typical Temperature range: -40C to +125C Specified for VDD of 2.8 V to 3.6V Logic Voltage VDRIVE = 1.65V to 3.6V Power-down current : <10 A Internal 2.5V Reference Internal Power on Reset High speed serial interface SPITM 20-lead LFCSP
8-Channel, 1MSPS, 12-Bit SAR ADC with Temperature Sensor AD7298
FUNCTIONAL BLOCK DIAGRAM
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 0.25C. 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. High Throughput rate of 1Msps per channel with Low Power Consumption. Eight Single-Ended Inputs with a Channel Sequencer. A consecutive sequence of channels can be selected on which the ADC cycles and converts. Integrated temperature sensor with 0.25C resolution.
1. 2.
3.
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
Information furnished by Analog Devices is believed to be accurate and reliable. However, no 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. Trademarks and registered trademarks are the property of their respective owners.
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Preliminary Technical Data
SPECIFICATIONS AD7298 SPECIFICATIONS
AD7298
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 = -40C to +125C, 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 70 70 Typ 71 71 -84 -85 -88 -88 -100 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 0.01 TBD 1 6 60 1 0.99 6 1 2 1 Max Unit dB dB dB dB fA = 40.1 kHz, fB = 41.5 kHz dB dB dB ns ps MHz MHz Bits LSB LSB LSB LSB ppm/C LSB LSB ppm/C V A pF k V ppm ppm V A pF ppm/C V rms Test Conditions/Comments fIN = 50 kHz sine wave
78 80
@ 3 dB @ 0.1 dB
Guaranteed no missed codes to 12 bits
VREF 1
When in track @ 1 MSPS 0.5% maximum @ 25C For 1000 hours
2.5125
2.0
2.5 1
External reference applied to Pin VREF
25
Bandwidth = TBD kHz
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Preliminary Technical Data
Parameter LOGIC INPUTS Input High Voltage, VINH Input Low Voltage, VINL Input Current, IIN Input Capacitance, CIN2 LOGIC OUTPUTS Output High Voltage, VOH Output Low Voltage, VOL Floating State Leakage Current Floating State Output Capacitance2 TEMPERATURE SENSOR--INTERNAL Operating Range Accuracy Resolution CONVERSION RATE Conversion Time 0.01 3 Min 0.7 x VDRIVE +0.3 x VDRIVE 1 Typ Max Unit V V A pF VIN = 0 V or VDRIVE Test Conditions/Comments
AD7298
VDRIVE - 0.2 0.01 8 -40 1 2 0.25 0.4 1
V V A pF
+125 2 3
C C C
TA = -40C to +85C TA = >85C to 125C LSB size
T2 + 16 x tSCL 100
1
s s ns MSPS KSPS V V mA mA mA A
For VIN0 to VIN7, with one cycle Latency. 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 VDD = 3.3V
Track/Hold Acquisition Time2 Throughput Rate
TBD 1 10
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
1 2
2.8 1.65
3 3
3.6 3.6 5 3 TBD 60 16.5 9.9 TBD 1.65
5
VDD = 3.3V mW mW mW W
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
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Preliminary Technical Data
TIMING SPECIFICATIONS
VDD = 2.8V to 3.6V; VDRIVE = 1.65 V to 3.6 V; VREF = 2.5 V internal/external; TA = -40C to + 125C, unless otherwise noted. 7 Table 3.
Parameter tCONVERT Limit at TMIN, TMAX 16 x tSCLK 1 100 50 20 TBD TBD t2 t3 t4 t5 t6 t7 t8 t9 t10 t11 tPOWERUP_PARTIAL
AD7298
fSCLK tQUIET
Unit s max s max s max kHz min MHz max ns min ns min ns min ns max ns max ns min ns min ns min ns min/max ns min ns min ns min s max s max
10 25 TBD 0.4 x tSCLK 0.4 x tSCLK TBD 15/45 10 5 TBD TBD TBD
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 Power-up time from partial power-down Power-up time from full power-down
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.
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Preliminary Technical Data 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 JA Thermal Impedance JC 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
AD7298
-0.3 V to +3V -0.3 V to +0.3V 10 mA -40C to +125C -65C to +150C 150C TBDC/W TBDC/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.
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AD7298 PIN CONFIGURATION AND FUNCTION DESCRIPTION
Preliminary Technical Data
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 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 F 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 F 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 F 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.
16
VDRIVE VDD VREF
10 7
6
GND1
9
GND
11
CS
15
SCLK
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Preliminary Technical Data
Pin No. 12 13 17 Mnemonic TSENSE_BUSY DIN PD/RST
AD7298
8
DCAP
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 F recommended) are connected to this pins to decouple the internal LDO.
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AD7298 CONTROL REGISTER
Preliminary Technical Data
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
MSB
D15 WRITE D14 REPEAT D13 CH1 D12 CH2 D11 CH3 D10 CH4 D9 CH5 D8 CH6 D7 CH7 D6 CH8 D5 TSENSE D4 DONTC D3 DONTC D2 EXT_REF D1 TSENSE AVG
LSB
D0 PD
Table 7. Control Register Bit Function Description
Bit 15 Mnemonic WRITE 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.
REPEAT CH1 to CH8
TSENSE
DONTC EXT_REF 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
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Preliminary Technical Data MODES OF OPERATION
AD7298
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
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AD7298
REPEAT OPERATION
Preliminary Technical Data
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.
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Preliminary Technical Data
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
AD7298
Figure 6. Normal Mode 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.
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.
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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.
AD7298 when using the internal reference. When an external reference is used TBD s are required to power up the AD7298 with a 1F 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.
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 100s to power up the
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Preliminary Technical Data 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 0.25C. 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.
AD7298
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 . The capacitor, C2, is the ADC sampling capacitor and has a capacitance of TBD pF typically.
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 10. Equivalent Analog Input Circuit
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
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.
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AD7298
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 0V 1LSB
Preliminary Technical Data
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 approximately100s 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. Theoretically, the temperature measuring circuit can measure temperatures from -512C to +511C with a resolution of 0.25C. 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
+VREF - 1LSB ANALOG INPUT
Temperature Sensor Averaging
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;
NOTES VREF IS EITHER REFIN OR 2 x 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.
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. 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 s.
Figure 12. Top Level Structure of Internal Temperature Sensor
Rev. PrA | Page 14 of 18
Preliminary Technical Data
Temperature Value Format
One LSB of the ADC corresponds to 0.25C. 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. 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
AD7298
THE REFERENCE
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. 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 F 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 A of current when the converter is static. The reference buffer requires 10ms to power up and charge the TBD F 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 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 address bit to be read in by the microcontroller or DSP. The
Preliminary Technical Data
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.
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 s 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
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. If the user configures the part for partial power down in a write to the control register during the first 16 SCLK cycles following
AD7298
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.
Figure 14. Serial Interface Timing Diagram for the Temperature Sensor Conversion.
Rev. PrA | Page 17 of 18
AD7298 OUTLINE DIMENSIONS
PIN 1 INDICATOR 4.10 4.00 SQ 3.90 0.50 BSC 0.30 0.25 0.18
16 15 EXPOSED PAD 20 1
Preliminary Technical Data
PIN 1 INDICATOR
2.75 2.60 SQ 2.35
5 6
11
TOP VIEW 0.80 0.75 0.70 SEATING PLANE
0.50 0.40 0.30
10
0.25 MIN
BOTTOM VIEW
0.05 MAX 0.02 NOM COPLANARITY 0.08 0.20 REF COMPLIANT TO JEDEC STANDARDS MO-220-WGGD.
020509-B
Figure 15. 20 Lead- Lead Frame Chip Scale Package
ORDERING GUIDE
Model Temperature Range Package Description Package Option
AD7298BCPZ AD7298BCPZ-RL7
-40C to +125C -40C to +125C
20 Lead - Lead Frame Chip Scale package 20 Lead - Lead Frame Chip Scale package
CP-20-8 CP-20-8
(c)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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