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ISOTMP35: Questions about connecting the output to a buffer and then ADC

Eric Norton
Prodigy 70 points
Part Number: ISOTMP35

Tool/software:

Hi All,

 I will incorporate two ISOTMP35 devices into my design to measure two SMT triac temperatures. I want to amplify the output from the ISOTMP35 so that the voltage output seen by the ADC is 0-5V. The opamp is inside a PSoC 4 (CY8C4246AZI-L423) device. Since I am going to be using an OPAMP buffer/amplifier before going into the ADC do I still need to add a capacitor on the output of the ISOTMP35? Here is the input buffer/amplifier circuit inside the PSoC 4 device:

The amplifier has a gain of 2.5 so that an input of 0-2V is amplified to 0-5V. TT_TRIAC input will have a 10K series resistor. Since the ADC sample rate will be 1ms per channel (1000 SPS) will a 680pF capacitor at the output of the ISOTMP35 be sufficient? What would you recommend I do in this scenario? Since there is an opamp/amplifier in between the output of the ISOTMP35 and the ADC, is the charging capacitor required? I want to ensure that I design this correctly so any advice is appreciated.

Thanks,

Eric Norton 

  • 0
    TI__Genius 9535 points

    Hi Eric,

    I do not think you will need a capacitor to act as a charger for the ADC since the inputs to the amplifier will be high Z. However, there are two other reasons why you may want to add a capacitor: noise filtering, and CMTI.

    CMTI stands for Common Mode Transient Immunity, and describes how well the ISOTMP output can withstand large changes on the input. If you do not anticipate that the SMT Triac temperature sources will be doing high voltage switching (switching on and off or doing large voltage jumps), then this isn't a concern. If you do anticipate some high voltage switching though, a load cap 1nF should be added to the ISOTMP35 output to prevent errors appearing on the output from the switching. You may also want to add a capacitor for noise filtering, which will depend on the noise floor of the application. The value you choose will depend on what frequency of noise you are trying to filter out. A 10KOHm series resistor and 680pF would be appropriate to provide filtering but not slow the ISOTMP35 output to the point where it interferes with your 1kHz sampling rate.

    Here is what the ISOTMP35 output would look like with the filter and load cap, if desired:

    In this case, instead of going straight to the ADC input, it goes to the amplifier input.

    Regards

    -Alex Thompson

  • 0 in reply to Alex Thompson
    Prodigy 70 points

    Hi Alex,

     Excellent. Do you think my general approach is sound and I can proceed with prototyping my circuit? Is amplifying the 0-2V output from the ISOTMP35 to 0-5V a good idea? I want the ISOTMP35 scaled to the 0-5V ADC range. I will have a mapping function in my code so that the voltage will scale to the temperature scale specified in the ISOTMP35 datasheet.

    The application will measure the temperature of two triacs switching 120V AC. One triac will switch a low current heater load to a max of 1.2 amps and the other will switch roughly 10 amp heater load. Since I am employing a pulse-skipping modulation scheme the temperatures of the two triacs will never go beyond 120 degrees Fahrenheit. I know this because I have a similar system working now but trying to downsize it for manufacturability and the triac temp never goes beyond 120 degrees even at full power. Most of the time the triacs are in a PID cycle and I see them running quite cool at about 60-85 degrees Fahrenheit. The SMT triacs will have rather hefty SMT heatsinks (7W thermal resistance, compared to the 10W thermal resistance of the current heatsinks) and a fan that is PID-controlled. As the temperature rises the fan speed also rises thus keeping the triacs relatively cool.

    Now that you know the general application do you think the 10K series resistor and 680pF capacitor on the input of the opamp amplifier circuit is sufficient? I know you mentioned the 1nF capacitor. Should I add this in parallel with the 680pF capacitor since I will be switching high voltage (120V)? I do worry about CMTI and want to mitigate this as best as possible. Let me know what you think.

    Thanks,

    Eric Norton

  • 0 in reply to Eric Norton
    TI__Genius 9535 points

    Eric,

    Thanks for the quick response! Yes I think you should use both the CMTI loading cap of 1nF and a filter cap (680pF should be fine), just make sure the 10Kohm resistor is between the two caps like in the diagram above to avoid putting to much capacitive load directly on the ISOTMP output. I don't think these should cause the output to respond too slowly, and I would hate to see some noise on the ISOTMP output get amplified into the ADC.

    Regards

    -Alex Thompson

  • 0 in reply to Alex Thompson
    Prodigy 70 points

    Thank you Alex for your quick response and answers! Very much appreciated!

    Thanks,

    Eric Norton

  • +1 in reply to Eric Norton
    TI__Genius 9535 points

    No problem, please feel free to post here in the future so we can assist you further!

    -Alex

  • 0 in reply to Alex Thompson
    Prodigy 70 points

    Hi Alex,

     I hate to open this again but hope you can help me out with this one Slight smile. I have selected the T2535-800G triac (similar to the BTA24-800CWRRG that is currently in use in my system) and I want to use this heatsink V-1102-SMD/A-L (Digikey part #: A10751-ND). Would the heatsink work for my application if the highest current is 10 amps at 110V? I use PID-controlled pulse skipping modulation to control two TRIACs that control AC heating elements.I use the BTA24 devices currently and want to switch everything to surface mount. Would the heatsink I specified above work with the SMT TRIACs at 10 amps at 110V? They will be actively cooled with PID-controlled fans and I will use the ISOTMP35 devices to sense the temperature of the two SMT TRIACs to make sure they do not overheat or get anywhere near the breakdown temperature. Unfortunately, I have not had much exposure to thermal design and therefore do not trust my math in something this critical. If you could help me out it would be greatly appreciated Slight smile.

    Thanks,

    Eric Norton

  • 0 in reply to Eric Norton
    TI__Genius 9535 points

    Hi Eric,

    I will be honest with you, this question is outside my expertise since it is not a question about using the ISOTMP35.

    Regards

    -Alex Thompson

  • 0 in reply to Alex Thompson
    Prodigy 70 points

    Hi Alex,

     Okay, thank you anyway I do appreciate your response.

    Thanks,

    Eric Norton