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ISO7220A: ISO7220ADR LF AND HF PULSE COUNTER

Michael Soldierram
Prodigy 30 points
Part Number: ISO7220A

Tool/software:

We want to read High frequency (hf) and low frequency (lf) signals via ISO7220ADR. We made a drawing like this, is it correct? (MCU = STM32F401RET6)
Thank you in advance for your help.
Michael

  • 0
    TI__Expert 4995 points

    Hi Michael,

    Thanks for reaching out. Couple of Questions and inputs, since the whole picture is not just with above snapshot

    1. What is the need of Q7 and Q8 and how are you biasing it? I'm not sure how you are forcing HIGH to Gate with respect to the Source.

    2. What kind of signal is HF and LF?

    Regards
    Varun
  • 0 in reply to Varun Kumar
    Prodigy 30 points

    First of all, thank you for your feedback.
    Let me explain, but we may have mistakes in usage.

    1.

    Q7 and Q8 (SI2356DS-T1-GE3) act as signal conditioning and protection layers for each input.
    We put the MOSFET for the following cases.

    It prevents the input signal from reaching a dangerously high level and ensures that the voltage to the isolation integrated circuit remains within a certain range.
    It forms a protection layer together with MOSFETs and diodes against high voltage surges such as ESD (electrostatic discharge).

    Diodes D27 and D28 (4.7V Zener) connected between the drain end of the MOSFET and GND limit the signal line at approximately 4.7V. When the input signal starts to exceed this level, the Zener diode becomes conductive and limits the voltage.

    A small resistor marked as 2.2Ω seen on the same line or the internal diode of the MOSFET also helps to protect the Zener diode and MOSFET by limiting it in case of overcurrent. In general, in this way the MOSFET and Zener diode work like a "clamping" circuit.

    2.  
    We use a Pulse Output Gas Meter.
    HF and LF pulse inputs of Pulse Output Gas Meter.

    LF INPUT

    Digital output, TTL level (5V / 3.3V)
    • Usually potential-free contact (dry contact)

    HF INPUT

    Signal character:
    • Digital output, high frequency square wave
    • More sensitivity and resolution
    • Usually open collector or NPN output type


    Regards
    Michael




  • 0 in reply to Michael Soldierram
    TI__Expert 4995 points

    Hi Michael,

    Thanks for the details. Please find my comments below:

    1. The GND on both sides of the isolator looks same. If it is two ground system, then make sure you correct this
    2. In PCB Layout, make sure to place decaps close to the device and have solid ground connections on both sides

    I don't see any other concern on ISO7220 connections.

    Although, please take care of clamping circuit you mentioned above, if the MOSFET is Enhancement type, I don't see a way of signal passing through. If you wish to use depletion type, then maybe there is a way,

    Michael Soldierram said:
    Diodes D27 and D28 (4.7V Zener) connected between the drain end of the MOSFET and GND limit the signal line at approximately 4.7V.

    The above statement doesn't seem right as the Zener is connected between Gate and Source.

    Regards
    Varun
  • 0 in reply to Varun Kumar
    Prodigy 30 points

    1. The grounding to which the two points are connected is the same.

    2. The gas meter that we will connect must support the measurement of all pulse values ​​between 0-30V. Therefore, we have established a MOSFET circuit as shown in the figure to dampen this voltage value. But if this is wrong, how should we proceed when giving pulses between 0-30V to these inputs?

    3. We updated this MOSFET connection as follows. Could you please check again?

  • 0 in reply to Michael Soldierram
    TI__Expert 4995 points

    Hi Michael,

    Thanks for the input pulse range information (0-30V).

    Coming to the MOSFET circuit, I recommend you to run PSPICE simulations with exact FET models and connections to see the actual waveforms.

    FYI, I still don't see how the signal is gonna pass through the FET, GND is connected to drain and biasing to gate is done through the 30V signal (Make sure to see VGS spec in MOSFET datasheet).My recommendation is to rather use a High Side Switch with a PMOS whose source is connected to 3.3V supply, and gate to 30V input from gas meter and drain is connected to INA/INB.

    Again, Please Simulate the final MOSFET circuit for accurate waveforms.

    ISO7220 should work fine as long as INA/INB pins don't see a voltage above Recommended Operating Condition as per datasheet.

    Regards
    Varun