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TS5A3357: The OFF isolation is not as expected from datasheet in Sample Hold circuit simulation

user6237525
Prodigy 130 points
Part Number: TS5A3357
Other Parts Discussed in Thread: TMUX1204, TMUX1104, TMUX1119

Dear forum members,

I'd like to build a Sample and Hold (Hold and Track, precisely) circuit with TS5A3357 switch. Since PSpice model is provided, I imported it into my LTSpice and ran simulation.

I followed the example circuit described in Figure 19. in TS5A3357's datasheet. Here's the circuit to simulate:

The SINE(0 5 5Meg 0 0) is the input of Sample Hold circuit. The signal "SW" decides whether to sample or hold. The signal "OUT" is output. The 20p capacitor is for sampling. However, the result of the simulation is different from what I expected. Here's the result:

If the switch is turned on, the V(OUT) follows the input as expected. But when the switch is turned off, much current seems to leak through the switch from input to output. Since the OFF isolation of TS5A3357 is -81 dB when Rl = 50Ohm, this leak seems much larger than that is expected from the datasheet.

Why do I see such a huge leak in this simulation?

  • 0
    TI__Expert 3970 points

    Hi,

    I replicated your schematic and got the same results. 
    I will test it in the lab to see if I can find an error in the set-up or in the model itself. 
    I'll update you during this week.

    Best regards,

    Ambroise

  • 0 in reply to Ambroise Suter
    Prodigy 130 points

    Hi Ambroise,

    Thank you for re-examining my schematic. I'm looking forward to hear the update.

    Best regards,

    Yoshitaka

  • 0 in reply to user6237525
    TI__Expert 3970 points

    Hi Yoshitaka,

    The model itself has no Off isolation built into the simulation model. You can expect an off isolation as per the datasheet, for 5MHz we specify around -70dB for small signals (-10dBm). It can vary depending on your exact layout and measurement set-up. 

    Let us know if you have more questions.

    Best regards,

    Ambroise

  • 0 in reply to Ambroise Suter
    Prodigy 130 points

    Hi Ambroise,

    Thank you for that information. I'm looking for a analog switch that has SPICE model, and that is why I chose TS5A3357 this time.

    Then, would you recoomend me some other analog switch, which has more accurate SPICE model? I would like to run simulation for the proof-of-concept of my schematic. I need to confirm the effect of imparfect off-isolation, rise time, etc to my system, which includes sample and hold part.

    Best regards,

    Yoshitaka

  • 0 in reply to user6237525
    TI__Expert 3970 points

    Hi Yoshitaka,

    Excuse us for the delay. Currently we don' have a replacement with that exact configuration (1ch, 3:1).
    The closest fit would be the TMUX1204 or TMUX1104. There are several ways to use this switch as a 3:1, the simplest would be to pull channel 4 to GND.

    than you and best regards,

    Ambroise

  • 0 in reply to Ambroise Suter
    Prodigy 130 points

    Hi Ambroise,

    The number of ports or channels does not matter at all for my application. Although I will use only one channel to pass or block a signal, I can use one of channel and port when a device has many channels and ports. That sounds redundant, but it is no problem for a proof of concept simulation.

    The most important thing is that the device has SPICE model and the SPICE model is accurate in terms of OFF-isolation. I'm going to simulate an analog circuit with LTSpice. The two devices, TMUX1204 and TMUX1104 has only IBIS model, not SPICE. 

    Would you recommend me an analog switch device, which has SPICE model with accurate OFF-isolation behavior? I don't care the number of channels or ports. I don't care about the value of on-resistance, bandwidth, operation voltage, parasitic capacitance and charge injection, as well.

    Best regards,

    Yoshitaka

  • 0 in reply to user6237525
    TI__Expert 3970 points

    Hi Yoshitaka,

    the TMUX1207 and TMUX1119 both have off-isolation modeled in the spice model.They are both very similar to the TMUX1204 and TMUX1104.

    Let us know if you have more questions.

    Best regards,

    Ambroise

  • 0 in reply to Ambroise Suter
    Prodigy 130 points

    Hi Ambroise,

    Thank you for that suggestion. Now I can proceed to my simulation.

    I really appreciate your help!

    Best regards,

    Yoshitaka