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UA9637A: UA9637

Jeff Starkell
Intellectual 505 points
Part Number: UA9637A
Other Parts Discussed in Thread: THVD1400, THVD1500, THVD2410, AM26C32, AM26LV32

Tool/software:

Application is conversion of RS422 type quadrature (A&B) signals to TTL from a magnetic linear scale from a CNC machine in an industrial environment.

Cable length from the encoder is fairly short ~2m, and the signals are twisted pair in shielded cable.

In close proximity (within 0.5m)  is a VFD driving a 2HP motor and the output of the VFD is 240V, 10A square wave so lots of nasty fast high current edges. This causes high frequency spikes on the chassis ground wiring.

The output signals from the encoders will connect directly to the inputs of the UA9637., and we expect a few volts RMS of coupled high frequency common mode noise on those encoder signals. Do we need any additional protection on the inputs to the UA9637?

  • 0
    Guru 318050 points

    You should have termination resistors.

    Differential signals are fairly resistant to common-mode noise, and the shielding helps. But check the waveforms; low-pass filters or a common-mode choke might be necessary.

    2× THVD1500 or THVD1400 would be cheaper and more robust.

  • 0 in reply to Clemens Ladisch
    Intellectual 505 points

    This circuit only receives data, a total of 7 quadrature pairs, so I don't think transceiver circuit is a good fit.

    Is there an equivalent part in terms of the more robust EMI performance, but in a dual or quad receiver only package?

    I'm most concerned with long term damage to the receiver, not so much data integrity.

  • 0 in reply to Jeff Starkell
    Guru 318050 points

    Regarding data integrity, the particular chip used does not affect the EMI resistance; the only question is whether the voltage over the termination resistor changes enough to flip positive or negative.

    To prevent damage, you have to ensure that the absolute voltages at the input pins do not exceed the specified "Fault protection" voltage specifications ("input voltage" in the absolute maximum ratings). This is typically done with TVS diodes, or clamping diodes to the power rails if normal signals do not exceed them. For noise with a higher frequency than the signals, low-pass filters also help.

    There are devices with better fault protection, but only as single-channel transceivers (e.g., THVD2410).

    For fourteen channels, four-channel receivers like the AM26C32 or AM26LV32(E) might be useful.

  • 0 in reply to Clemens Ladisch
    TI__Guru 52752 points

    Hi Jeff,

    Clemens is correct - if you have any other questions though please let me know and I will see what I can do. 

    Best,

    Parker Dodson