• State TI Thinks Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 2 answers
  • Subscribers 16 subscribers
  • Views 194 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TMUX1309-Q1: Unused SxA, SxB input

Shinji Tsuji
Genius 12358 points
Part Number: TMUX1309-Q1
Other Parts Discussed in Thread: TLV9002-Q1

Hi team,

I don't use S1A and S1B pins and plan to connect these pin directly to GND.

Since DA and DB are both connected to the OPAMP positive terminal without any capacitor connected, there should be no current flowing path to S1A, S1B external to the device. 

In this case, Can I connect S1A, S1B pin directly to GND without current limiting resistor?

 I think it should be OK, but would like to double confirm because there is a description below in the pin FMA document on TI.com.

"S1A short to GND failure : Corruption of signal passed onto the DA pin. If there is no limiting resistor in the switch path
device damage possible"

regards,

  • 0
    TI__Mastermind 43375 points

    Hi Shinji,

    The worry is that there could be a voltage on DA/DB when the mux switches to S1A/S1B and while there will be no current from the OP AMP there will be a voltage across the switch where the voltage is V+.  The current through the switch will be V+/R_on - and depending on what V+ is the current can possibly exceed 50mA (the devices current limit).

    As an example at a 5V supply lets say during the transition DX has a 5V signal on it. This is ~100 Ohms typically at this point at room temperature but it could be a little lower - so right off the bat there could be >50mA  through the switch that could damage it.

    So it depends on the exact setup of the system.

    What voltage supply are you planning on using  and what is the max voltage at the V+ terminal w.r.t. to ground of the op amp in this circuit?  Because depending on the exact setup you may be able to avoid adding a current limiting resistor because there isn't a ton of risk.

    Please let me know so I can verify if this is okay for your specific application!

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    TI__Genius 12358 points

    Hi Parker,

    Thank you for your comment. 

    Sorry I missed to add "input". DA and DB terminals are connected to OPAMP positive (non inverted) input terminal.  

    Because it is a high impedance terminal, there should be no worry to directly connect S1A/S1B to GND, correct?

    regards,

  • 0 in reply to Shinji Tsuji
    TI__Mastermind 43375 points

    Hi Shinji,

    The only concern is that there could be a current transient if the voltage on the positive terminal is large enough to induce >50mA of current there could be damage. That could happen if the voltage on the + terminal is near VDD , if VDD is 5V. So in most cases you should be okay - but there is still a chance of failure due to a current transient. 

    The voltage w.r.t. ground on the positive terminal will be shunted to ground. If V+ / R_ON > 50mA there will could be issues. However the largest area of concern is if you power the device with a 5V VDD and the V+ nears VDD. If you are using a 5V VDD but the V+ doesn't get close to 5V its not a concern and if lower power supplies are used than it shouldn't be a problem.

    But you have to verify that V+/R_On <= 50mA at all times of operation. 

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    TI__Genius 12358 points

    Hi Parker,

    VIN+ is high impedance node and if there is no parasitic capacitance, there should not flow that much current, or maybe very limited time spike. 

    I used TLV9002-Q1, its input capacitance is 1.5pF.

    https://www.tij.co.jp/product/jp/TLV9002-Q1

    Even in this case, there is a chance to fail?

    regards,

  • 0 in reply to Shinji Tsuji
    TI__Mastermind 43375 points

    Hi Shinji,

    The risk is very low in most applications as the op amp is high impedance. But there is also capacitance from the multiplexer (the 1309 can have a max of 40pF of capacitance) as well as the bus capacitance. While it is isn't guaranteed to fail those capacitors can source current creating a voltage differential that could cause a transient.

    I am not saying you can't do this as the risk is lower because this part is more resistive which helps in these kinds of cases. That being said there isn't a 100% guarantee that some current transient couldn't happen due the parasitics of the mux, bus, and op amp - it unfortunately isn't just the high Z "source". Honestly even adding like a 10 - 25 Ohm resistor in front could help as if the resistance to ground is 100 Ohms or more at all times than all acceptable voltage ranges for this part will be covered. The issue is there are operational points where the mux can dip below 100 Ohms and the voltage on the + terminal may induce a current transient. If the resistance to ground is 100 Ohms or more the current shouldn't spike >50mA ever.

    Best,

    Parker Dodson