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ada4254

Category: Hardware
Product Number: ADA4254

Hi,

for RTD measurement I use the current source from output pin 6 "IOUT_HV" and excitate with it RTD and 500 Ohm precise reference resistor in series. With the PGA ADA4254 I measure
the voltage drop through RTD on CH1 (IN1+/IN1-) and the voltage drop through the reference resistor on CH2 of the same PGA (IN2+/IN2-) for ratiometric measurement. I do not measure the
voltage drop at the same time, because I need to switch from CH1 to CH2 measurement. In this two time stamps the value of the excitation current is not the same, because of noise or other influences, so I get
an error in resistor measurement, because of the instability of the excitation current during time. In an older application I sourced the RTD measurement from ADC AD7124 and had more stability
in excitation current as from the PGA excitation current source. I tested with 4kOhm RTD and had an variation of dR=0.53 Ohm with current source from ADC and dR=1.55 Ohm with current source from PGA.
This precision is not enough for my application.

Do you know about the instability of the excitation current source of the PGA ADA4254 and why is the current source from ADC ADA4254 more stable?
Have you recommandations to reduce the instability of the current source or other current source you know that have comparible stability as the ADC?
Could you tell which kind of current source circuit you are using in PGA and ADC which maybe explains the difference of the stability. I could not found values in the datasheet of both chips.

Thank you in advance,

Thomas

Thread Notes

Top Replies

  • Hi Thomas,

    We are looking into this. We will give you an update once we found a relevant information.

    Thanks,

    Mae

  • Hi  ,

    Good day! Thank you for reaching out and sharing your inquiries with the team.

    I would like to ask about several design requirements to understand the problem behind (if any) and give our proper recommendations.

    • Are you using a 3-Wire RTD application?
    • Did you try the recommended circuit and typical procedure as stated on pages 38 to 39 of the ADA4254 datasheet?
    • What supplies are you using for these pins for ADA4254? VDDH, VSSH, AVDD, AVSS, VOCM, DVDD, DVSS?
    • What is the current compliance, clamp current settings of the voltage sources to VDDH, and AVDD? 50mA, 100mA, etc...
    • You mentioned utilizing IOUT_HV as an excitation source to RTD. Did you try IOUT_LV and check if the results are similar?
    • Are you controlling the current excitations settings by an SPI communication?
    • How much excitation are you currently using? What are your settings for EX_CURRENT [3:0] Register?
    • What is your EX_CURRENT_SEL [1:0] Register value?
    • What is your overall value set for EX_CURRENT_CFG Register (0x0F)?
    • Can you please share your application schematic (with PGA ADA4254 and ADC ADA4254) for RTD measurement?

     regards,

  • Good Morning Arsol,

    Thank you for your reply!

    •Are you using a 3-Wire RTD application?
    Yes.

    •Did you try the recommended circuit and typical procedure as stated on pages 38 to 39 of the ADA4254 datasheet?
    Yes we do. The differences between the schematic shown in figure 104.3 and our schematic is, that we use the current source IOUT_HV instead of IOUT_LV and the excitation current goes first through the reference resistor (in our application 499 Ohm) and then through the RTD. But we also measure as figure 104.3 shows with the PGA the voltage drop of the RTD at Channel 1 and the voltage drop of the 499Ohm resistor at Channel 2.

    •What supplies are you using for these pins for ADA4254? VDDH, VSSH, AVDD, AVSS, VOCM, DVDD, DVSS?
    We use VDDH and VSSH supplied by +-15V from DC DC converter TRACO TEC3-0923.
    For AVDD, AVSS, VOCM we use LDO MCP1703T-3602E/CB to get 3.6V output. The LDO is also sourced from our +-15V supply.
    DVDD and DVSS is sourced from Rapsberry Pi Compute Module 4, which has a 3.3V digital voltage output.

    •What is the current compliance, clamp current settings of the voltage sources to VDDH, and AVDD? 50mA, 100mA, etc...
    For VDDH the DC DC converter TRACO TEC3-0923 has a current compliance of 100mA and we dont use clamp current settings.
    For AVDD the LDO MCP1703T-3602E/CB has a current compliance of 250mA and we dont use clamp current settings.

    •You mentioned utilizing IOUT_HV as an excitation source to RTD. Did you try IOUT_LV and check if the results are similar?
    Yes we tried it also with IOUT_LV and got similar results.

    •Are you controlling the current excitations settings by an SPI communication?
    Yes.

    •How much excitation are you currently using? What are your settings for EX_CURRENT [3:0] Register?
    We tried different excitation currents in the range of 100uA to 1000uA. For my last tests I used 4k RTD with an excitation of 500uA.
    EX_CURRENT [3:0] = 0x5

    •What is your EX_CURRENT_SEL [1:0] Register value?
    EX_CURRENT_SEL [1:0] = 0x2


    •What is your overall value set for EX_CURRENT_CFG Register (0x0F)?
    EX_CURRENT_CFG = 0x85

    •Can you please share your application schematic (with PGA ADA4254 and ADC ADA4254) for RTD measurement?
    We used appended schematic for RTD measurement:

    Thank you!

    Thomas

    /cfs-file/__key/communityserver-discussions-components-files/382/3wire-RTD.bmp

  • Hi  

    My Additional questions:

    1. What is your connection to VOCM? Is there any voltage being sourced to this pin?
    2. What is the Gain set to the PGIA?

    I checked the design application circuit, and you use a high-side 3-wire RTD configuration (reference is placed right after the excitation source), Which is typical when utilizing the excitation source from the ADCs.

    As for the PGIA (ADA4254), it is recommended to use the low-side 3-wire RTD configuration, and I suggest following the typical circuit in Fig104.

    Then these modifications for your existing design circuit, are below:

    • Removing R279, R282 and R283
    • Replace R279 with value of R287
    • Remove C71 and C74
    • Connecting the Reference of the RTD to CH2
    • Remove EXC1 connection to R281, replacing it to GNDA connection
    • Remove R285 and R286
    • Remove the RTD connection to GNDA
    • Connect the IOUT_HV1 between R287 and C73

    Follow on these calibration and measurement procedures (using a handheld DMM or a 7.5 DMM):

    1. Run a calibration to null any offset voltage error in the system by shorting the input of the instrumentational amplifier to ground.
    2. Set the ADA4254 input multiplexer to Channel 1, +IN1 and –IN1, by writing 0x60 to the INPUT_MUX register.
    3. Switch IOUT_LV on by writing 0x40 to the EX_CURRENT_CFG register. Also, set the current by writing to the EX_CURRENT[3:0] bits.
    4. Set the gain of the ADA4254 to the appropriate gain via the GAIN_MUX register.
    5. Take a reading on the ADC. This reading shows the voltage drop across RL1, the RTD element. Measure Pin 9, 12, 24, and 1 of ADA4254
    6. Switch the ADA4254 input mux to Channel 2, +IN2 and −IN2, by writing 0x18 to the INPUT_MUX register.
    7. Take a reading on the ADC. This reading is the voltage drop across RREF. IOUT_LV can be calculated from this reading. Measure Pin 9, 12, 24, and 1 of ADA4254
    8. Measure between –IN1 and +IN2. First the input multiplexer protection is disabled by setting the MUX_PROT_DIS bit to allow a –IN1 and +IN2 configuration
    9. Set the INPUT_MUX register to 0x30.
    10. Repeat Step 4.
    11. Take a reading on the ADC. This reading is the voltage drop across RL3 and REF. Measure Pin 9, 12, 24, and 1 of ADA4254

    Regards

  • What is the tolerance of the resistor being used inR281 (499 Ohms)?

    +sharing the modified design circuit for the 3-wire RTD:

  • Hi Arsol,

    1.) VOCM is half of the AVDD voltage (3,6V/2=1,8V) generated by voltage divider 2,7kOhm --- 2,7kOhm.

    2.) We use different gain settings according to our measurement range. In case of 4kOhm 3V/500uA. But the voltage measurements are okay, so that is not the problem.

    We tested our design circuit with your modifications and could not improve the accuracy. Now we have a variation of dR=8,8Ohm (relative to 4kOhm+7,5kOhm: 765.2 ppm) and in our design circuit as mentioned dR=1.55 Ohm (relative to 4kOhm: 387.5 ppm).

    Conclusion: The current soure of the PGA ADA4254 has not the stability as our application needs, so we are looking for an alternative current source for RTD measurement.

    Regards

    Thomas

  • Hi  

    ADA4254 may not be able to support your intended applications; though I would suggest an option for the RTD solution, you may refer to this ADI Circuit Note: CN-0383 (Rev.D) (analog.com)

    Regards,

  • Hi Thomas,

    Please let us know how we can assist you further with this query.

    L

  • Hi Arsol,

    Thank you for your help and the Circuit Note Link. We will try it to source it with the ADC.

    Regards,