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LT2862-1 Simulations with LTRA

Category: Hardware
Product Number: LT2862-1

I am using LTSpice's inbuilt macromodel test fixture for LTC2862-1 to simulate its operation. Modified it slightly in the following ways,

  1. Modelled the transmission line using the inbuilt LTRA Model (Lossy Transmission Line Model). Plugged in the RLC values for Belden 3109A into the LTRA Model. Considered a length of 330ft (~100m). Looking into the datasheet, the device should work upto 450ft, at 1MHz. 
  2. Operating the device at 1MHz i.e. sending a finite sequence of 10 pulses at 1MHz, through one of the transceivers (at DI pin), with the ~RE/DE pin driven high (Data In Enabled). The default test fixture uses 10MHz.
  3. The ground references between the 2 isolators are shorted through a 1MΩ resistance (as opposed to a sinusoidal source between the two)
  4. Added parallel termination of 100Ω (nominal) at both transceivers. 
  5. Added capacitance of 375pF from Lines A and B to respective grounds, to model the effect of TVS Diode Capacitance (SZ1SMA33AT3G). Total of 4 x 375pF caps for TVS diode capacitances.

On running the simulation it is observed that the pulses on transmission line experience an unexplained ringing or spiking effect, during the mid point of their "on-times". The ringing does not start from the beginning of the pulse sequence, but starts after 4th pulse. After the DI (data input) pulse sequence at the transmitter has finished, the ringing on the transmission lines is causing the reception of “additional” pulses at the receiver’s RO (Receiver Output).

This is observed only for the 1MHz mode. While operating in 10MHz data transfer mode, the device is able to pass the pulses properly. Note that the “spikes” are at the middle of the pulses’ on-time, and not during rising or falling edges. 

Please advise if there seems to be an issue with the model/simulation or if this is an expected electrical phenomenon. 

  • Hello,

    It looks like the spikes are being caused by reflections due to impedance mismatches of the transmission line bus.  They look to be phase shifted and appear on the receiver after the initial pulses are registered.  The 4 x 375pF capacitance of the TVS diodes can reduce the impedance of the transmission line.  Do you have simulation results without the TVS diodes connected?  

    One more question, for the 10MHz simulations, did you also only run 10 pulses?  If so, the simulation time duration would be 10x less than at 1MHz, so the reflections may not have appeared in the short time interval of the simulation.  I cannot read the horizontal scale but based on the frequency of the 1Mhz pulse width, it looks like a 20us duration.  Would you be able to run the 10MHz simulation with the TVS diodes connected for 10us to see if reflections show up there as well?