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Low Power RF Transceivers
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Motor Control Hardware Platforms
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Processors and DSP
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Wireless Sensor Networks Reference Library
AD7707:input impedance and currents
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AD4110-1 Datasheet Rev.0
2 RDS 1 Resolver
64-bit driver for EVAL-AD7994EBZ
64-bit driver for old evaluation boards with Cypress controller
64-bit drivers for EVAL-AD7794EBZ
68K resistors in AD2S1205 recommended circuit diagram.
ACE File format for importing filters
AD1139: DC level setting applications
AD1556: Filter coefficients
AD1582 Not Vout seen from the Reference - Family works only with C between Vout and GND
AD1582: Requirements for heat sinking
AD2S1200 LOS, DOS and LOT Detection Errors limits?
AD2S1200 or alternative for use with 2kHz excitation?
AD2S1200 velocity output resolution
AD2S1200: Angular position data format vs, Angular velocity data format
AD2S1200: Can AD2S1200 be used with "SINCOS" sensor?
AD2S1200: Can I use both the serial and the parallel mode for velocity and position information?
AD2S1200: Can I use split supplies of +3.3V for DVDD and +5V for AVDD?
AD2S1200: Connection methods
AD2S1200: Data bus high impedance mode control and max current load
AD2S1200: Does the angular accuracy figure take into account missing codes?
AD2S1200: How are angular accuracy and resolution linked ?
AD2S1200: interpreting the output velocity information
AD2S1200: Maximun phase shift that can be tolerated between the reference excitation and the sin/cos signals?
AD2S1200: MSB to LSB order of Databits is different to other AD2S parts
AD2S1200: SinCos Common Mode Input Voltage
AD2S1200: The full scale value for the velocity output is not given in datasheet
AD2S1200: What is full scale of velocity output and it is programmable or fixed?
AD2S1200: Which device grade is populated on the AD2S1200CB evaluation board?
AD2S1210 Eval Board
AD2S1210 interfaced to an LVDT
AD2S1210 synchronized to external excitation
AD2S1210: EXC/EXC- waveform incorrect issue
AD2S1210: Input connection filtering and shielding
AD2S12xx SIN/COS input CMV query
AD2S75 replacement part?
AD2S80: Maximum power consumption
AD2S80A busy Timing
ad2s80a: Accuracy vs. temperature
AD2S80A: Can 2 AD2S80As be used with the same inductosyn?
AD2S80A: Can I tie both INHIBIT and ENABLE to 0V ?
AD2S80A: Noise on DIR output
AD2S80A: Output is not linear, 2-3 degrees phase difference between sine and cosine input,
AD2S80A_closed loop bandwidth
AD2S80_arc min meaning
AD2S80_RESET signal and sample rate
AD2S80_SC1 SC2 input voltage
AD2S82A: Maximum tracking rate.
AD2S83: Moving bits
AD2S83_Incremental output with low frequency excitation
AD2S90: Angular accuracy and degradation over temperature.
AD2S90: Can I use the unmodulated outputs for perpendicular HALL sensors instead of a resolver?
AD2S99: What is the difference between AD2S99AP and BP versions?
AD598 and AD698: RVDT 5 wire LVDT Application
AD598: Multiple LVDTs
AD598: Temperature range
AD650: Can AD650 Voltage to Frequency converter be used as a Frequency to Voltage converter?
AD650: F/V converter from 0 to 2kHz
AD654 long-term stability data
AD7091: Can Vdrive be greater than Vdd
AD7091R ADC Input Levels
AD7091R Conversion Error After Exiting Power Down Mode
AD7091R Conversion Result Change
AD7091R IBIS Model LFSCP and MSOP
AD7091R IDRIVE current higher than 20uA
AD7091R MSB always 0 after each conversion
AD7091R SPI question
AD7091R VREF to power others parts of a circuit
AD7091R: Second /CONVST pulse occurs before the end of conversion point
AD711_PDIP package pin 8 connection
AD7124-8 FAQ. How do I interface to multiple ADCs over the same serial interface?
AD7124-8 FAQ. Should an anti-alias filter be used with the ADC?
AD7124-8 How do I interface to multiple ADCs over the same serial interface?
AD7124-8 Is the serial interface reset when is taken high?
AD7124-8 Should an anti-alias filter be used with the ADC?
AD7124-8 What are the tradeoffs in performance between the three power modes?
AD7124-8 What is the benefit of the post filters versus the standard sinc filters?
AD7124-8 When single conversion mode is used, can be taken high after the single conversion is initiated?
AD7124-8 Which power mode should I use?
AD7124: Input Offset
AD713: Output considerations
AD713: Output short circuit specifications
AD713: Overvoltage at the inputs
AD7142: Signature capture
AD7147: Can I use it to detect the hand at 10cm away from the button?
AD7147: Can several AD7147 be used in parallel to realize a remote control with 35 buttons?
AD7147: Multiple devices in one application
AD7147: PCB layouts
AD7148: Effect of water
AD714x: maximum glass thickness that can cover a PCB for Captouch using AD7142/3 or AD7147/8
AD7150 AD7746 level sensing demo
AD7150: Capacitive effects as conductive and non-conductive materials approach the sensor
AD7175-2 250kSPS, 24-bit Sigma Delta ADC FAQ
AD7175-2 digital interface check
AD7176 power sqeucing requirements
AD7176-2 Settle in a Single Cycle
AD7190 and oscillation issues (SR#: 1238093)
AD7190 configuration issue
AD7190: How can we combine several ADC on one bus?
AD7190: How to connect MCLK Pins if using the internal clock.
AD7190: Tolerate Overvoltage
AD7190_gain of 2 and gain of 4 setting
AD7192 SPI problems
AD7193 + Arduino Tutorial - Thermocouple measurements
AD7195: Input impedance in buffered mode
AD719x evaluation software Vista and Win 7 compatability
AD7266: Timing for analog input selection to CS falling
AD7280 AD7280A - can the part work with 3 cells?
AD7291 ALERT pin
AD7298, SPI at 20Mhz clk interface and t4 timing issues
AD7305: Settling time
AD7323: output code
AD7328: Overoltage protection
AD7366-5 SPI problems
AD736ARZ Junction temperature
AD737 AGC Bandwidth
AD7398: Usage of LDAC pins
AD7400A: Can this device accept both differential and single ended inputs, datasheet is not clear?
AD7401: SINC3 filtering
AD74111: How to interface
AD7415: Long-term stability data
AD7416: Ambient temperature and PCB temperature
AD7417: SDA line tied to '0' after board initialization
AD7417: unused inputs
AD7418: I2C interface and maximum conversion rate
AD7450: Sample rate estimation (10MHz) and reference voltage
AD7450: Vdrive function
AD7457: Discontinuous clock
AD7457: Filter requirements
AD7484 Minimum Conversion Time
AD7484 Ouput drive capability
AD7484: Initialization process
AD7484: Width of Reset pulse
AD7490: Range Bit
AD7492BRUZ: Thermal, Power and weight information
AD75019: Current limiting on the analog outputs
AD75019: Maximum current
AD7503: J model and K model difference
AD7541A: Gain Tempco
AD7541A: Using a current out MDAC in voltage switching mode
AD7546: Glitch perfomance
AD7568: Input impedance
AD7606 - connection of unused channels
AD7606 - grounding considerations
AD7606 - incorrect output code
AD7606 - operating at half the range setting
AD7606 - powerup sequence
AD7606 - unipolar range configuration
AD7606 - Where can I find schematics for the EVAL-AD7606EDZ board?
AD7606 evaluation board schematic
AD7606 FAQ - What is the variation in the 1Mohm input impedance?
AD7606 internal reference max. drift
AD7606 throughput rate
AD7606-4: No schematic is provided online or out of box for EVAL-AD7606-4EDZ
AD7606: shutdown and standby mode
AD7608 + SPI problem
AD7608, AD7609: Schematic and PCB prints
AD7616-P FAQ - The analog input diagnostic channels are showing an unexpected result, what should I do?
AD7616-P FAQ - What are the differences between the AD7616 and AD7616-P?
AD7616-P FAQ - When using the software sequencer, the first conversion result is not the expected value?
AD7621: Calibrating the zero and full scale error
AD7621: Master read
AD7626 Acquisition Time
AD7631 power supply problem
AD7653: input noise of AD7653
AD7654: Analog Input - CONVST goes low
AD7655: CONVST asstered before conversion
AD7655: Driven significantly below ground
AD7655: REF pins questions
AD7655: Stuck bits
AD7656/ AD7656-1 - grounding considerations
AD7656/AD7656-1 - need for RESET
AD7656/AD7656-1 - problems even after RESET
AD7656/AD7657/AD7658: What order is channel data presented in serial mode, single DOUT line
AD7656: Bipolar inputs
AD7656: Is it possible to read 2 channels as opposed to all 6
AD7656: power and decoupling caps requirements
AD7656: RC between AVcc and DVcc
AD7657-1 DCINA input set-up and hold times & t17 minimum and what is t20?
AD7663: PCB layout
AD7665 Start up sequency OVDD DVDD AVDD
AD7665: Operation test
AD766x Current Consumption Calculation
ad7674: Read data when the board is used as a stand-alone board
AD7675 conversion time
AD7682 and AD7688 busy indicator
AD7682 and AD7688 busy indicator usage
AD7683: Stuck bits
AD7684: Stuck bits
AD7685: Not seeing expected resolution
AD7685: Supplied with VDD, but Vref and Vio removed
AD7688: Using with ADuM1301
AD7689 and AD7949 disable internal clock
AD7689 digital interface timing
AD7689 internal temperature sensor
AD7689: Vref voltage
AD7690 SPI Mode SDo High Impedance State
AD7690: Stuck bits
AD7705: General usage
AD7705: grounding on multiple ADC's
AD7705: input impedance
AD7705: Values are jumping
AD7707:input impedance and currents
AD7708: External clock
AD7708: Update rate
AD7709: Choice of decoupling capacitors
AD7709: Clock higher speed
AD7709: Clock source
AD7709: Series resistors into RTDs
AD7709: Using the device with an external current source to increase resolution
AD7710 evaluation baord
AD7710 input impedance different to what is explained in AN-338?
AD7710: Serial interface
AD7711A: Power Up issue
AD7712: Calibration issues
AD7712: Master clock
AD7712: Noise spectral density
AD7712: power supply
AD7712: Variation of input impedance over temperature
AD7713: deduct the actual value of the constant current source
AD7714: ALE pin as a master clock
AD7714: Saturating the I/P channel
AD7714YN: Power up sequence
AD7715: Debugging the output
AD7715: Filter settling time
AD7716: Relationship between SCLK, FCLKIN and resolution
AD7718_ Thermocouple Application
AD7719: Clock driven from the port pin
AD7720: Gain calibration
AD7725: FSK modulator
AD7730 application question re 6-wire loadcell
AD7730 Internal-zero-scale calibration
AD7730 Replacement - AD7195
AD7730: ESD protection
ad7730: Evaluation board
AD7730: Increase in resolution
AD7730: Load cell
AD7730: Matching between different gain steps/stages
AD7730: Maximum ratings
AD7730: Parallel port of the PC
AD7730: RDY signal not working
AD7730: Required capacitance across the XTAL/ Resonator
AD7730: Using the chop mode
AD7731 filter questions?
AD7731: Continuous Read Mode and Conversion Mode
AD7731: PCB join analog and digital ground planes
AD7731: sinking current
AD7738 Figure 10 and Figure 12 (Datasheet Rev. 0)
AD7738: Reading channel data register
AD7738: Timing specs
AD7740 AD7740: maximum input frequency
AD7741 evaluation board user guide
AD7745 : Absolute maximum ratings section in datasheet is confusing regarding lead temperature during soldering ?
AD7745/46/47 addressing for multiple slave devices on I2C bus
AD7745: Capacitance Application
Ad7745: Higher capacitive value
AD7745: Higher input capacitance
ad7745: Sensors with higher capacative range
AD7745: Start a conversion in single conversion mode
AD7745_there is no input protection circuit
AD7746 : External excitation and advice on eliminating interference
AD7746 Software to interface AD7746 Eval Board with my own Application
AD7747_power down mode
AD7760 at 5Msps output datarate
AD7760 EVB & EVAL-CED1Z Contunuous Sampling at 2.5 MSPS
AD7763: input range to the AMP
AD7763: Multiplexing the input using external mux
AD7763: Power dissipation
AD7764 Register Status bits
AD7766 minimum clock MCLK
AD7766 minimum samplerate/MCLK frequency
AD7768 EVB Source Code
AD7768 FAQ - Analog input precharge buffers
AD7768 FAQ - The input seems to be switching faster than the 256 kSPS rate?
AD7768 FAQ - What front end driver amplifier is suitable to drive the AD7768?
AD7768 FAQ - What is a SYNC and when is it required?
AD7768 FAQ - What is the maximum input common-mode voltage?
AD7768 SYNC vs. RESET
AD7768/AD7768-4 FAQ - Clock Selection
AD7778: Input signal out of specified range
AD7780 vs AD7781 - load cell excitation
AD7787: Burnout currents
AD7788: Input ranges
AD7788: Using it with low Vref
AD7790: Serial Interface
AD7790: Shift signals
AD7792 Calibration Routine and Timing
AD7792: AVDD monitor and temperature sensor channel
AD7792: Communication problems
AD7792: On-Chip temperature
AD7792: Using multiple device on the serial bus
AD7793 output rate to input clock ratio
AD7793: Continous conversion mode
AD7793: Digital reading
AD7793: external bias voltage connection to thermocouple
AD7793: ID register
AD7793: Sample rates
AD7793_current output compliance voltage and impedance
AD7794 Evaluation Board firmware
AD7794 evaluation software sample limitation
AD7794 internal short for debug
AD7794 undocumented gain settings
AD7794: Hot to use the Eval Board standalone
AD7794: NOXREF active voltage and temperature sensor calibration
AD7798 v AD7710 for precise measurements in presence of external vibration
AD7798: Electrostatic experiment of AD7798
AD7799 AVDD Monitor 1.17V Internal Reference
AD7799: Changing channels
AD7799: Problems with communications via serial interface
AD7799: SPI issue
AD7799_data reading timing
AD780 long term drift
AD7809: Protecting the analog outputs
AD780S: Do you have any radiation reports, TID or SEE for AD780S ?
AD7811 output stuck at 1023 dec
AD7811: Incorrect O/P code
AD7811: Power up sequence
AD7812 power-up problem
AD7812: Control and addressing
AD7817: clock rate
AD7819: LSB is on DB0
AD7821: Overvoltage protection at the analog inputs
AD7825: Power down pin
AD7825: RD permanently low
AD7829 recommended usage of VDD
AD7829: difference between AD7829-1 and AD7829
AD7835: Influence of the power supply
AD7849: critical item for power supplies
AD7851 evaluation board
AD7854 evaluation board
AD7856: Using a NPO capacitor
AD7856: VREF usage
AD7858: Channel Switching and Software Conversion
AD7859: Functionality after power-up
AD7859: Power up sequence
AD7859: Powering the device
AD7864: Absolute input voltage
AD7864: Chip Select Inputs
AD7864: CLKIN pin
AD7864: Pin Connections
AD7865: Multiple CONVST
AD7874: Power supply sequencing
AD7887: Overvoltage protection
AD7887: Using the interal reference
AD7888: Minimum SCLK
AD7888: Using two 8 bit reads for the SCLK
AD7891-1: First conversion is incorrect
AD7891: IR soldering
AD7891: Thermal impedance
AD7892: Overvoltage protection
AD7895: Leakage current
AD7896: BUSY indicator staying High
AD7899 - CS and RD tied permanently low
AD7921 IBIS model
AD7927: Vdrive voltage
AD7938 Channel ID
AD7938: Overvoltage at analog inputs
AD7949 input configurations
AD7960 LVDS test pattern
AD7960 Test Pattern
AD7961 - power supply sequence
AD797 ESD data HBM
AD797: Maximum junction temperature Tj
AD797: Offset Null configuration
Ad797: PSRR definition
AD7980 input protection
AD7982: Driving the reference input
AD7986 External reference (Vref) typo? and connection queries
AD7988-1 Operating Currents vs. Supply (figure 23 in datasheet Rev A)
AD7988-x in pseudo-differential input applications
AD7988-x: ADC input exceed Vref or GND
AD7993 keeps reading same channel
AD7997 and specific temperature for leakage current info
AD7998: Unused control pin
ADA2200: Replacing RCLK signal with DAC output
ADAS1000 - can it be used in an EEG application?
ADAS1000 - Converting ECG raw data to voltage
ADAS1000 - Example Calculations for each mode
ADAS1000 ACLO Operation
ADAS1000 and using a signal/function generator as the signal source
ADAS1000 Crystal Oscillator Startup
ADAS1000 DRDYb operation
ADAS1000 Drivers and Blackfin Framework
ADAS1000 ECG - What are the advantages/disadvantages of DC coupled ECG system versus AC coupled?
ADAS1000 evaluation board and patient cables
ADAS1000 FAQ Index
ADAS1000 Right Leg Drive function
ADAS3022 IBIS Model
ADAS3022 Power Supplies and Power-On Sequence
ADAS3022 recommended ADC driver, external reference and reference buffer options
ADAS3023 - input signals
ADAS3023 - power supply sequencing
ADAS3023: headroom requirement
ADC Channel specs of AD7709
ADC FAQ - In the analog Input specification of a differential ADC, what is the difference between Analog Input Voltage Range and Absolute Input Voltage?
ADC FAQ - In the analog Input specification of a differential ADC, what is the meaning of the common-mode input range?
ADC FAQ - What are the types of ADC input architectures?
ADC for low range pressure transducer
ADI's New 24 bit, Single Channel, Sigma Delta ADC with Power Scaling - The AD7768-1
After powering up the AD7091R why does the conversion result change after approximately 60 samples have completed?
After powering up the device why does the conversion result change after approximately 60 samples have completed?
Amplitude accuracy of AD2S99 vs Input signal amplitude of AD2S90
Analog to digital converter general: worst case resolution / noise free bits
Are there any precautions that should be taken to make the interface for the AD7175-2 more robust?
Are there IBIS models available for the AD7091 LFCSP package?
Are there IBIS models available for the AD7091R LFCSP and MSOP package?
Are there ways to reduce the component count associated with a solution containing the AD7091R-2/4/8?
Bipolar input version of AD7812
burned out issue of AD7656
Bypass Digital FIlters
Can the AD7175-2 be configured to settle in a single cycle?
Can the ADC input exceed the supply voltage or go below ground?
Can the ADC inputs exceed the reference voltage or go below ground?
Can you send me the AD7746 Labview Source Code?
Can't get 24-bit resolution
CDC up to 2nF
Changing R11 and R17 for AD2S1200
Channel Scan Rate
channel selection of AD7655
CMRR and PSRR parameter interpretation
CMRR of AD7683 and AD7684
Common-mode range requirement for the AD7961?
Connection of exposed pad in AD7982 LFCSP package
Continuous read - start and read sequence
Control the Conversion Start
CS tied LOW
Current consumption on AVdd and Vdrive individually - for power supply dimensioning
Current consumption on Vdrive and AVcc
DAC selection: Is there a better recommended choice for new design than AD7805?
Daisy Chain Issues
data sheet error. Hold time of A0 and A1 signal during SPI transfert on AD2S1210
Datasheet error of AD7760 (Figure 55 Decoupling recommendations))
Difference between AD7656 and AD7656-1
Difference between AD7924WYRUZ and AD7924BRUZ
Difference between part AD2S80ATD/B and AD2S80ATD?
Difference between pseudo-differential and single-ended mode of an ADC
Direct interface for resolver with AD2S1210
Do I need an external pull up resistor on the SDO pin if the Busy Indicator feature is not required?
Downloading evaluation software for AD7763 Evaluation Board from CD
EVAL-AD7190EBZ Win7 64-bit issue resolution
EVAL-AD7606 - High resolution schematics
EVAL-AD7625FMCZ: External trigger and storage
EVAL-AD7719-EBZ Evaluation Board
EVAL-AD7746EBZ Evaluation Board Software for Matlab Connection
EVAL-AD7746LDZ - Ground electrode connected to liquid
EVAL-AD7768 Evaluation Board Quick Start Guide
EVAL-AD7768FMCZ FAQ - How can I save sample data to a file?
EVAL-ADAS3022EDZ rev D: PCB dimensions
Evaluation board for the AD7961
evaluation board for the ADAS3023?
Example Parity Check on the AD719X family
Extending the capacitance input range of AD7747
external excitation to AD7746
FAQ: AD7380: The AD7380 has an on chip oversampling, Why is there a need for an oversampling?
FIT and activation energy for AD7942
float the input of AD7606
Gain Drift over time
I am not designing a system for functional safety. Are the diagnostics in AD7124-8 still useful?
I am not designing a system for functional safety. Are the diagnostics still useful?
I don't see a RESET pin on the AD7175-2, How do I reset the ADC?
I2C addressable capacitance-to-digital converter
IBIS model available for the ADAS3023
If a second /CONVST pulse occurs before the end of conversion point, EOC, what will happen? Will the first request be cancelled and a new conversion begun?
In AD7124-8, Is the serial interface reset when /CS is taken high?
In AD7124-8, When single conversion mode is used, can /CS be taken high after the single conversion is initiated?
Interface Not Working
Interfacing the AD7768/AD7768-4 to a microcontroller
Is an IBIS model available for the AD7124-4?
Is an IBIS model available for the AD7124-8?
Is it it is possible to use the AD2S1210EVB without the CED1Z ?
Is it possible to get a PCB footprint for the AD7091?
Is the AD7266 sensitive to SEU (Single Event Upset)?
Latching on power up
LDC - AD7147 Eval-Board
Long Term Drift Bahavior
Long term drift of the AD7792 Reference
Making the Interface more robust
Max sample size in CN0194
Maximum Clock Frequency
Measure current/voltage/resistance using an ADC
Measure up to 150pF
Measuring DC offset in an audio signal
Mosfets for AD7195 AC excitation
Noise when switching from chop to non-chop mode
number of logic gates transistors used
Offset & full-scale registers of AD7190
Only achieving 10-bits of stable resolution, should be 13-bits or more
Output code of AD7321
Part number of the EMC Filter in the AD7656EVB
Power Supply and Capacitance Issues
power up issues
Power-On-Reset problem on the AD7812
Problem getting the digital interface to work reliably. Part sporadically goes into power down mode.
Problem with AD737
Problems initiating calibration on AD7730
Pros and cons of applying SYNC pulse on AD7195
Recommended ADC Driver for the AD7091R
recommended ADC driver, external reference and reference buffer options
recommended ADC driver, external reference and reference buffer options?
Recommended Sequency to change channels over I2C
Reliability data of AD7265
Reserved (RSV) pins in CFG register = always set to 1
Resolver to Digital simulation models
RMS noise spec
Single Micrel FET MIC4427 driver
Single vs. Continuous Conversion Time
Sink current capability of the SDO pin of AD7879
SPI Communication Issues
Synchronising multiple AD7768's to achieve simultaneous sampling of more than 8 channels
Synchronization of multiple AD7175-2 + Set ODR to 20kSPS
Target markets/applications for this product
The AD7175-2 has a maximum output data rate of 250 kSPS but a channel scan rate of
The VDE Viorm = 1250 Vpeak reinforced isolation rating for the AD7403 is much higher than the CSA reinforced rating of 415 Vrms (586 Vpeak). Why is this the case?
There are a number of filter options for the AD7175-2 – which one should I use?
There are three power modes in AD7124-8. Which power mode should I use?
Thermal characteristics (Theta data; Max temp; Max power dissipation)
Time required to settle after a step change in the input
What are the reference options available for the AD7961?
What are the target markets/applications for this product?
What are the tradeoffs in performance between the three power modes of AD7124-8?
What does the MUX_IO bit do on the AD7175-2?
What is a power consumption of AD2S1200?
What is AD7961?
What is ADAS3023?
What is the benefit of the post filters versus the standard sinc filters?
What is the difference between performance range and full scale range of the AD7403?
What is the ESD rating of the AD7124-8?
What package is this part offered in?
What power supplies does the AD7961 require? Does it power down?
When /CS is high, is DOUT is in tri-state mode
Why AD2S1200 pin20 toggling with /RDVEL low and Resolver stationary?
Why after every conversion of AD7091R is the MSB of the conversion result always “0”?
Why do the AD7606 pins in the "Typical Connection Diagram" such as RANGE, /STDBY have resistors and what values should they be?
Why has the power consumption of the AD7403 increased when compared to the AD7401A? 17mA typical at 20MHz and Vdd1 = 5.5V for the AD7401A and 42mA typical for the same conditions for the AD7403
Why the AD7091R ,when I measure the Idrive current during operational mode do I get higher than the 20uA as quoted in the maximum rating in the data sheet?
Why when I measure the Idd current during operational mode do I get higher than the 554uA as quoted in the maximum rating in the data sheet?
Will the AD7793 ADC continue to sample when the CS is disabled?
Working with true differential input precision SAR ADCs
AD7707:input impedance and currents
I´ve got some questions about the specifications of the AD7707. I´m working
actually with the low level inputs. I didn´t found clearly values about the
input impedance or input currents in buffered or unbuffered mode in the data
sheet. Could it be that, in unbuffered mode with gain 16, the input current
is in the range of 1uA? This is what I observed. I think you will have a
data sheet in more detail and you will be able to provide me with this
The thing that concerns me about your system is that the 1nF capacitors between
the low level inputs and ground continue to give rise to dynamic currents even
when the part is operating in buffered mode. It is certainly a risky strategy
to operate the part without any anti-aliasing filter at all but if you have
filtering earlier in the signal chain then you can perhaps leave the capacitors
To explain the dynamic current problem, which should only happen in unbuffered
mode, I attach a plot which I did for the AD7712, which also has unbuffered
inputs. I'll try and explain what the plot tells us:
Firstly, the unbuffered input stage of sigma delta converters is a sampling
capacitor of a few pF which is switched between the Ain+ and Ain- channels (in
this case Ain1 and LOCOM). Charge is transferred onto the sampling capacitor
from Ain+ and then charge is transferred off the sampling capacitor to Ain-. To
make the simulation easier, I just model what happens when the sampling
capacitor starts off being charged to 2.5V and an external capaitor starts off
being charged to 0.2V. At t=0, the sampling capacitor is connected to the
external circuit. The sampling capacitor, C2 loses most of it's charge to the
external filter capacitor through R2, as we would expect. Because C1 is much
bigger than C2, the voltage increase in C1 (about 1.8mV) is much smaller than
the voltage decrease in C2 (-2.3V). However, the interesting thing is the
charging time. The sampling capacitor gives up it's charge to the external
capacitor very quickly. However, the external filter capacitor takes a much
longer time to discharge back to 200mV. It is this long time constant which
causes the a gain error in the ADC result. The settling time of the input
voltage is made much longer by the external passive components and has not
settled by the time the conversion takes place, leading to the error. The
numbers in this example aren't correct for the AD7707 but they demonstrate the
To choose a value of capacitor, the table XVI on page 20 is the first guide.
Note that the "time constant" (t = RxC) represented by the R and C combinations
is related to the input sampling frequency T at any particular gain, which is
shown in table XVII on the following page. It turns out that RC = T/7 for the
50pF value of capacitor but only T/2.5 for the 5nF capacitor.
The combination of the two tables is in the attached spreadsheet.
Here is the text of the email which I wrote to another customer, who was
experiencing difficulty with the dynamic input of a sigma delta converter, in
this case on the ADuC824 microconverter.
"The reason for the R and C on Ain1 and Ain2 inputs is as a simple
anti-aliasing filter, to prevent noise at high frequencies being aliased down
into the pass band. The internal buffer means that the R, C circuit doesn't
cause gain errors in the reading. The high oversampling ratio of the ADC means
that a single pole anti-aliasing filter is usually sufficient, even when there
is a lot of out-of -band signal, which needs to be attenuated.
To drive the auxiliary channel from the signal you describe, with such a high
source impedance, you have to buffer the signal so that the bias current drawn
by the ADC (and any dynamic current) doesn't cause a gain error, because of the
voltage drop across the 5k source impedance. You may also have to implement an
anti-aliasing filter, to band-limit the input signal depending on how much
noise is present in the input signal and what your noise requirements are for
the auxiliary channel. For example, if your 2V signal has a lot of out-of-band
noise, this noise will be aliased down to baseband and will appear as low
frequency fluctuations in the sampled result, even though the original noise
may have been at several hundred kHz. The noise may be induced into the analog
inputs from adjacent signals, hf radiation or it may be generated in the buffer
Therefore I recomend that you drive the Auxiliary channel with a unity gain
buffer, followed by a single pole RC filter to prevent aliasing.
One note of caution I have to mention: since you cannot use large values of R
and C at the front end of an unbuffered sigma delta converter without causing a
gain error, the anti-aliasing circuit cannot really have a cutoff frequency
below 15kHz (2.2kOhms and 4.7nF) if gain errors are to be avoided. I have taken
these figures from the AD7714 datasheet, on which the ADuC824's primary ADC is
based. Since the ADuC824 has not yet been released, there is no information on
the datasheet, but I would expect a similar type of limitation to apply to the
ADuC824 as applies to the AD7714 in unbuffered mode.
Since there is a tradeoff between
a) cutoff frequency of the low pass anti-aliasing filter
b) gain error caused by the RC filter,
you cannot optimise both of these by choosing values of R and C.
Some suggestions are
a) use higher values of R and C, accept that there will be a gain error and
calibrate it out in the ADC
b) use low values of R and C to minimise the gain error, and use a low noise
op-amp (e.g. OP113) to minimise the aliased noise
c) use an OP213 (dual op113), with the first stage buffering the 5k source
impedance and the second stage buffering an RC anti aliasing filter and driving
the ADC directly.
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