Analog.com Analog Dialogue Wiki 简体中文
EngineerZone
EngineerZone
  • Log In
  • Site
  • Search
  • User
  • Support

    Popular Forums

    • RF and Microwave
    • Power Management
    • Video
    • FPGA Reference Designs
    • Precision ADCs
    • Linux Software Drivers
    • SigmaDSP Processors & SigmaStudio Dev. Tool

    Product Forums

    • A2B
    • Amplifiers
    • Analog Microcontrollers
    • Clock and Timing
    • Data Converters
    • Direct Digital Synthesis (DDS)
    • Energy Monitoring and Metering
    • Interface and Isolation
    • MEMS Inertial Sensors
    • Processors and DSP
    • Switches/Multiplexers
    • Temperature Sensors
    • Voltage References
    View All

    Application Forums

    • Audio
    • Automated Test Equipment (ATE)
    • Condition-Based Monitoring
    • Depth, Perception & Ranging Technologies
    • Embedded Vision Sensing
    • Optical Sensing
    • Precision Technology Signal Chains
    • Video
    • Wireless Sensor Networks Reference Library

    Design Center Forums

    • ACE Evaluation Software
    • ADEF System Platforms
    • Design Tools and Calculators
    • FPGA Reference Designs
    • Linux Software Drivers
    • Microcontroller no-OS Drivers
    • Reference Designs
    • Signal Chain Power (SCP)
    • Software Interface Tools
    • System Demonstration Platform (SDP) Support
  • Learn

    Recent Discussions

    • Pluto + GNURADIO frequency change
    • Pluto SDR
    • pluto sdr pyadi-iio continous iq recording
    • attribute error
    • gps-sdr-sim is not working with my ADALM-PLUTO

    Places

    • ADI Education Home
    • ADI Education China
    • ADI Education India
    • ADI Education Philippines
    • StudentZone (Analog Dialogue)
    • Virtual Classroom

    Latest Webinars

    • Optimizing GaN and GaAs RF Amplifier Performance Using Novel Biasing Techniques
    • Webinar Improving Switch-Mode Power Supplies Using Noise Cancelling Technology
    • Webinar Accelerate Your Design With a 2 to 24 GHz Wideband Transceiver Reference Design
    • How to Design an Optimized Motion Control System for Intelligent Edge Based Surveillance Camera
    • How to Enhance High Precision Current Sensing Systems
    View All
  • Community Hub

    Challenge Yourself!

      KCC's Quizzes: Buffers and Fan Out
    View All

    Places

    • Community Help
    • Logic Lounge

    Resources

    • EZ Code of Conduct
    • Getting Started Guide
    • ADI: Words Matter
    • Community Help Videos
    View All
  • Blogs

     

    How To Design Fieldbus Agnostic Smart Factory Sensors

     

    You Passed EMC Testing… But Wait, Is There More?

    Latest Blogs

    • The 5 Things You Need to Know about RF Matching Networks
    • Does Reliability Prediction Uncertainty Justify Mandating Two Channel Safety?
    • An Inside Look at 2022
    • Meet the Person Behind the Quizzes
    • Updates Required: Why Newer is Better
    Read All Blogs

    ADI Blogs

    • EZ Spotlight
    • The Engineering Mind
  • Partners

    Electronic Design Services - PartnerZone

    • Boston Engineering
    • Calian, Advanced Technologies
    • Colorado Engineering Inc. (DBA CAES AT&E)
    • Clockworks Signal Processing
    • Epiq Solutions
    • Fidus
    • PalmSens
    • Richardson RFPD
    • Tri-Star Design, Inc.
    • VadaTech
    • Vanteon
    • X-Microwave
    View All
Precision Microcontrollers
  • Analog Microcontrollers
Precision Microcontrollers
Documents Optimizing RCAL,RTIA,CTIA and DAC voltage of ADuCM350 for 4-Wire Measurement.
  • Forums
  • File Uploads
  • Docs/FAQs
  • Members
  • Tags
  • More
  • Cancel
  • +Documents
  • +ADuCM Cortex-M3: FAQ
  • +Aducm320: FAQ
  • -ADuCM350: FAQ
    • 4Wire-Isolated-Bioimpedance ADuCM350 Connections To Sensor
    • ADUCM350 DESIGN SUPPORT COMMUNITY - INTRO
    • ADuCM350 FAQ DFT / Impedance Measurement
    • ADuCM350 FAQ Debug
    • ADuCM350 FAQ Evaluation Kit Software Platform
    • AFE Transmit Stage
    • ADuCM350 FAQ GPIOs and Pinmuxing
    • ADuCM350 FAQ I2C serial interface
    • ADuCM350 FAQ Operational Supply Range / Power Supplies
    • ADuCM350 FAQ Parallel Display Interface(PDI)
    • ADuCM350 FAQ Power Consumption
    • ADuCM350 FAQ Power Management Unit
    • ADuCM350 FAQ Random Number Generator
    • ADuCM350 FAQ SDK Driver Specific FAQs
    • ADuCM350 FAQ SPI
    • ADuCM350 FAQ Switch Matrix
    • ADuCM350 FAQ System Clocks
    • ADuCM350 FAQ System Integrity
    • ADuCM350 SDK v2.3.0.0 now available
    • Bioimpedance Measurement Using ADuCM350
    • Optimizing RCAL,RTIA,CTIA and DAC voltage of ADuCM350 for 4-Wire Measurement.
    • Potentiostat / Amperometric Measurement Using ADuCM350
    • Unipolar Current Measurement Using ADuCM350
    • Errata for ADuCM350 I2C serial interface
    • FAQ ADuCM350 Embedded Software Development
    • How to get SDK + Eval-ADuCM350EBZ Working together.
    • FAQ for Captouch on ADuCM350
    • Known Issues for ADuCM350 Silicon and Support Material
    • ADuCM350 FAQ ADuCM350 Evaluation Kit
    • ADuCM350 FAQ AFE / ADC Measurement Channel
    • ADuCM350 FAQ AFE Measurement Circuit Impedance Range
    • ADuCM350 FAQ AFE TIA
    • ADuCM350 FAQ AFE Sequencer
    • ADuCM350 FAQ Amperometric Measurement Example
    • ADuCM350 FAQ Calibrate
    • ADuCM350 FAQ CRC accelerator
    • ADuCM350 FAQ GP Timers
    • ADuCM350 FAQ LCD controller
    • New Revision of UG-587 Available
    • ADuCM350 FAQ Potentiostat / Amperometric
    • ADuCM350 FAQ Real Time Clock (RTC)
    • First Steps: Quick Measurement with Eval-ADuCM350 & EKSP Labview GUI
  • +ADuCM355: FAQ
  • +ADuCM360: FAQ
  • +ADuCM361: FAQ
  • +ADuCRF101: FAQ
  • +Bio-Impedance & ECG Measurement Solution: FAQ
  • +CN0300: FAQ
  • +Continuously Amperometric Measurement: FAQ

Optimizing RCAL,RTIA,CTIA and DAC voltage of ADuCM350 for 4-Wire Measurement.

Introduction

To achieve higher accuracy and repeatability of measured sensor impedance using ADuCM350, the RCAL, RTIA , CTIA and the DAC output voltage during the measurement needs to be optimized.

The accurate results can be produced when the ADuCM350 SNR is higher. To achieve higher SNR, the DAC output needs to be around 600 mVp and the ADC input should be  around 750 mVp.

600 mVp – Maximum allowable swing from DAC output.

750 mVp – Maximum allowable swing at ADC input.

To calculate the RCAL , RTIA and DAC Output code, the below items needs to be identified.

  1. The minimum sensor impedance.
  2. Access resistance and Capacitance.
  3. Mode of operation such as 2-wire,4-wire and 4-wire Bio-Isolated.
  4. Frequency of interest.

Based on the above parameters, the RCAL , RTIA , CTIA and DAC Output code values can be calculated.

Please refer to the application note “AN-1271-Optimizing the ADuCM350 for Impedance Conversion” for detailed explanation of this calculations.

4-wire Measurement Set-up with RACCESS:

The diagram shown below depicts the 4-wire measurement to be performed using ADuCM350 with RACCESS to identify the unknown Z value connected between the AFE channels 6 and 3.

RCAL,RTIA,CTIA and DAC Output Selection Procedure:

For the 4-wire measurement diagram shown above,consider the below parameters.

  1. Minimum Zsensor = 2000 Ω ;
  2. RACCESS1-4 = 10 KΩ;
  3. Frequency of measurement = 50 KHz.
  4. Mode of measurement = 4-Wire with only access resistances.

With the above mentioned inputs,the RCAL,RTIA,CTIA and DAC Output values can be selected.

RCAL Selection  Procedure:

The RCAL resistor value can be selected as same as the minimum Zsensor value. This allows larger signal during calibration and hence the improved accuracy.

RCAL  = Minimum Zsensor = 2000 Ω.

RTIA Selection  Procedure:

The RTIA value can be calculated by following the below steps:

1.Calculate the total impedance across the channels. In our case, between AFE6-3 :

      = RACCESS1 + RACCESS4+ Zsensor

      =10 KΩ + 2000 Ω + 10 KΩ = 22 KΩ.

2.Calculate the current through the channels(AFE6-3) :

  = (Maximum DAC output voltage * Attenuation Factor) / Total impedance across the channels.

  • The Maximum DAC output voltage allowable is 600 mVp.
  • The attenuation factor is the attenuation of RCF incorporated at the DAC. The RCF has a cutoff frequency of 50 KHz and based of the frequency of operation , the signal attenuation varies.

The attenuation factor can be calculated by using the below equation :

1/(SQRT((1+(F/Fc)^2))).

Where as

  • Fc – cutoff frequency of RCF which is 50 KHz.
  • F – DAC output frequency.

Considering our frequency of interest which is 50 KHz,The attenuation factor is 

= 1/(SQRT((1+(50000/50000)^2))) = 0.707

3.The current through channels can be calculated by substituting the above values.

= 600 mVp * 0.707  / 22 KΩ

= 19.28  µA

4.Now, Calculate the optimal RTIA by dividing the maximum allowable ADC input(750 mVp) with the calculated current through the channel.

  = 750 mVp / 19.28 µA = 38.9 KΩ.

5. Atlast,select the closest available standard resistor value

i.e. RTIA  = 39.2 KΩ 1%.

CTIA Selection  Procedure:

The CTIA value can be selected by identifying the cutoff frequency of the measurement signal, RTIA and attenuation factor.

CTIA = 1/(2*∏*Fc* RTIA).

Fc – Cutoff frequency of the RC filter.

The Fc needs to be carefully selected here. If Fc is too close to desired frequency of interest , the ADC input will be attenuated due to the RC filter rolloff.

Considering ,the cutoff frequency ten times the measurement frequency gives a attenuation factor of 0.995.The cutoff frequency selected is 500 KHz.

Now, the CTIA can be calculated by

= 1/(2*∏*500E3*39.2E3)

= ~ 8 pF.

Select the closest available standard capacitor value

i.e. CTIA  = 10 pF 5%.

DAC Output Voltage Selection Procedure:

To calculate the DAC output voltage that needs to be programmed in code, multiply the current through the channel with the sensor impedance.

  = 19.28 µA * 2000 Ω.

  =~39 mV

RCAL,RTIA,CTIA and DAC Output Calculator:

The excel sheet"RTIA_RCAL_CTIA_CALCULATOR.xlsx" attached with this FAQ incorporates the above mentioned procedure and calculates the RCAL,RTIA,CTIA and DAC Output as per measurement inputs.

Signal Swings Across AFE Channels

By using the calculated the RCAL , RTIA , CTIA and DAC Output code values, the DAC output swing captured across the AFE channels 3-6 are shown below :

Please note that the maximum swing across the AFE channels are only 848 mVp-p instead of 1.2 Vp-p. This is due to the 0.707 attenuation factor introduced by the RCF in TX path of ADuCM350.

Also  the DAC output code, we have programmed is ~39 mV only. This larger swing is introduced by the Raccess resistance of 10 K. The 39 mV swing will be present across the sensor impedance of 2K.

Programming a wrong DAC output in code could cause DAC output saturation or reduced SNR thereby affecting the measurement results.

Signal Swings Across ADC (RTIA) Input:

By using the calculated the RCAL , RTIA , CTIA and DAC Output code values, the signal swing captured across the ADC input is shown below :

Please note that the maximum swing across the ADC input is ~1.5 Vp-p. This helps in achieving maximum SNR and thereby producing accurate measurements.

Conclusion :

Thus optimum values for RCAL, RTIA,CTIA and DAC output needs to be selected for obtaining accurate results using ADuCM350.Use the excel sheet calculator attached with this FAQ for selecting them.

Attachments:
RTIA_RCAL_CTIA_CALCULATOR.xlsx
  • ctia
  • aducm350
  • rcal
  • rtia
  • impedance measurement
  • aducm35x
  • Share
  • History
  • More
  • Cancel
Related
Recommended
 
Social
Quick Links
  • About ADI
  • Partners
  • Analog Dialogue
  • Careers
  • Contact us
  • Investor Relations
  • News Room
  • Quality & Reliability
  • Sales & Distribution
  • Analog Garage
Languages
  • English
  • 简体中文
  • 日本語
  • Руccкий
Newsletters

Interested in the latest news and articles about ADI products, design tools, training and events? Choose from one of our 12 newsletters that match your product area of interest, delivered monthly or quarterly to your inbox.

Sign Up
Switch to mobile view
Analog Logo
© 1995 - 2023 Analog Devices, Inc. All Rights Reserved 沪ICP备09046653号-1
  • ©
  • 1995 - 2023 Analog Devices, Inc. All Rights Reserved
  • 沪ICP备09046653号-1
  • Sitemap
  • Privacy & Security
  • Privacy Settings
  • Terms of use
EngineerZone Uses cookies to ensure you get the best experience in our community. For more information on cookies, please read our Privacy & Security Statement.