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ISO7140CC Carrier Frequency/Noise Performance

Steve Hobbs39
Prodigy 50 points
Other Parts Discussed in Thread: ISO7140CC

The data sheet (and the design guide) for the ISO71xxCC 4242-VPK Small-Footprint Low-Power Triple and Quad Channels Digital
Isolators references a carrier frequency used for PWM of low frequency (<100kHz) inputs.  They also reference a LPF at the output to remove the carrier.


From data sheet:

Because low-frequency input signals require the internal capacitors to assume prohibitively large values, these
signals are pulse-width modulated (PWM) with the carrier frequency of an internal oscillator, thus creating a
sufficiently high frequency signal, capable of passing the capacitive barrier. As the input is modulated, a low-pass
filter (LPF) is needed to remove the high-frequency carrier from the actual data before passing it on to the output
multiplexer.


Can you provide any additional information regarding this?  What is the carrier frequency?  Do you have any plots of the frequency of the LPF (referenced in Figure 14 of the data sheet)?  Short of a plot of the filter performance, do you have a spec on the leakage of the carrier at the output?

  • 0
    TI__Expert 6335 points
    Steve,

    I am moving this to the Industrial Interface forum for proper support.

    Best Regards,
    Nirav
  • 0
    TI__Intellectual 2200 points

    Hi Steve,

    Sorry for the delay in this response.

    The PWM modulated communication is packetized. The packets repeat every 8us. The packet structure has a custom header and tail, and the body of the packet has the sampled low frequency information of all the channels. The smallest pulse width inside the PWM packet is ~20ns.

    It is difficult to define a value for the carrier frequency for the PWM path.

    This is a digital channel, and the filter performance or carrier leakage cannot be defined. We can however say the following:

    1. Any transitions on the input pins (which are high frequency by definition) are conveyed to the outputs with a fixed propagation delay.

    2. If the inputs are in a DC state (low frequency by definition), and the outputs happen to be in error, for example due to to excess noise or common mode transients beyond spec  (not expected normally) the outputs will be corrected roughly wihtin 16us (this is the response time of the low frequency path).

    Hope this clarifies.

    Rgds,

    Anant

  • 0 in reply to Anant Kamath
    Prodigy 50 points
    Anant,

    Thank you for your response. However, I am looking more for information regarding any possible noise that may be generated in the component that will then propagate into sensitive analog circuits on the isolated side. In my particular application, the signals I am transmitting through the device will not be changing very rapidly (expected to be < 100kHz).

    In the past, I have used the Analog Devices ADUM series components in a similar manner. I realize that they use a different technology than this component, but the problem is the same (i.e., how to "refresh" a "steady-state" (or slow changing) input across the isolation barrier). The ADUM part actually specifies a refresh rate (see the refresh rate specified to 1.2Mbps on page 4 of the datasheet for the ADUM1200ARZ). In the design where I used the ADUM part, I saw 1.2MHz noise in my analog circuits.

    I am trying to determine what, if any, noise will be sourced by the ISO7140 in a similar manner in my circuit.
  • 0 in reply to Steve Hobbs39
    TI__Intellectual 2200 points

    Hi Steve,

    The refresh tha the competition implements is on every channel and is periodic, whereas we have a separate watchdog/low-frequency channel that packetizes data for all channels in a given direction. There's no fixed tone for us, with some frequency bands dominating depending on board layout and physical dimensions.

    In general, we have found that our capacitive isolators are much lower noise than magnetic isolators - difference could be 10dB to 50dB depending on frequency. For example magnetic isolators require stiching capacitors between GND1 and GND2 to meet radiated emissions specifications, whereas TI isolators do not. The difference is due to lower capacitive coupling between side1 and side2 through the isolator, and differential operation.

    I believe you'll see much better performance with ISO7140CC.

    Rgds,

    Anant