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I am working with two sensors with 4 pin as output.
My question is simple: I want to make circuit that allows me to short circuit the output of either one of the sensors individually when needed.

I am considering using quad 1:2 demux (AGD774a) and shorting either the pins on channel B together while measuring with channel A and vice versa.
So when I want the channel for normal measurement I will use channel A and when I want output of sensor shorted I change the channel of the switch.

So, my main question is whether using a demux to solve the requirement is a wise choice or there is some other way it can be achieved.


Notes:

  • The sensor is an ultrasonic transducer which produces only millivolt level outputs.

    In use the sensor is placed in a heating chamber to increase its temperature.
    When the temperature of the transducer is varied, it produces spurious temperature-change related voltages.
    After heating we need to short the output pins to release all the residual voltage.
    The charge itself is small.

    May be I can do a worst case calculations and attach a resistance as well with it.

  • The voltage drop is small - in the mV range.
    I do not think that an electromechanical relay is required.

  • The sensor will not be connected to Micro-Controller.
    Shorting is needed to discharge the piezo in the sensor which occurs when the temperature is changed.

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    \$\begingroup\$ Details of the sensor are really needed. \$\endgroup\$ – Andy aka Jul 16 '14 at 12:19
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    \$\begingroup\$ And explain what you want to achieve shorting outputs. Maybe you want to average? Or do you just want to connect your (uC?) input to a selected sensor? \$\endgroup\$ – Wouter van Ooijen Jul 16 '14 at 12:22
  • \$\begingroup\$ Specifically, how much current can the sensor drive through the short, and how much voltage drop can you tolerate? You might need to use an electromechanical relay. \$\endgroup\$ – Dave Tweed Jul 16 '14 at 12:35
  • \$\begingroup\$ Actually the sensor(ultrasonic) I am using is not producing any voltage that is the shorting of signal is only required because if we put it in a temprature chamber to increase the temp we need to short the output pins to release all the residue charge. The charge it self is small. May be I can do a worst case calculations and attach a resistance as well with it . But my main question is using a demux to solve a problem like I explained before a wise choice or there is someother way it can be acheived. thanks \$\endgroup\$ – user49470 Jul 16 '14 at 12:36
  • \$\begingroup\$ the voltage drop is small of mV range. No need of electromechanical relays for sure. \$\endgroup\$ – user49470 Jul 16 '14 at 12:39
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A reference to a data sheet would be very helpful.

Using a multiplexer for shorting is a potentially good solution. You need to determine what the impedance of the sensor is under short circuit conditions and what residual voltage be present when the chosen multiplexer is used.

If you want a precision measurement, a well defined and very low resistance short may be required. If the polarity of the pins to be shorted is always the same then using a MOSFET across the pins with gate driven on as required will short the sensor. Choosing a very low Rdson MOSFET will give a lower shorting resistance than is available from most multiplexers.

If the sensor output can be of either polarity you could use two MOSFETS in series opposed polarity. eg 2 x NChannels, join sources , join gates, connect two drains to sensor pins. [Possibly optional: Connect high value resistor gates to sources (say 1 megohm +) to hold FETS off usually.] Drive gates with input resistor - saya 1 megohm+. Drive low = off. drive high = shorted. Drive voltage and MOSFET ratings to suit.

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  • \$\begingroup\$ Downvoter - "More soon" comment was there to stop question being closed and preventing me placing an answer - as happens. \$\endgroup\$ – Russell McMahon Jul 16 '14 at 13:38
  • \$\begingroup\$ You're also going to have to provide some sort of return path for the Vgs control voltage; perhaps a medium value resistor from the two sources to ground -- assuming, of course, that it's OK to short the sensor to ground while you're dissipating the charge. If not, you'll have to find some other way of referencing the control (gate) voltage to the sources. \$\endgroup\$ – Dave Tweed Jul 16 '14 at 15:39
  • \$\begingroup\$ @DaveTweed - a return path would indeed be needed in an ideal case. My experience of this configuration is that it is "self starting. Once it is on it has a reference point. If it did not start in real life you'd have no grounds for complaint. \$\endgroup\$ – Russell McMahon Jul 17 '14 at 8:05

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