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I have a simple system where a motor speed is controlled by a voltage signal via a circuit. Below represents the system where A is the voltage signal device; B is the circuit and M is the motor. The details of the inner circuitry is not important for my question.

enter image description here

All I can say for now is that the voltage input is connected to the circuit via a BNC cable which directly couples to an opAmp of the Circuit B.

When the device A is off or zero volt output, the motor M stops running which is very good. But when I plug off the device or the BNC cable, I think the input to the opAmp starts "floating" and the motor starts jerking.

My question is:

Is there a technique, tool or a way to prevent this floating when the BNC is unplugged?

Edit:

Input part of the complex circuit. DAC represents the input voltage:

enter image description here

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A simple pull-down resistor (connection to GND via resitor) at the input can fix the problem.

It would pull down the floating potential of the open line to 0V.

Note: This works if the output impedance of the "voltage input" part (A) is low enough. It wouldn't be good if the "input voltage" part (A) is just a potentiometer because then the pull down resistor would influence the control voltage.

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  • \$\begingroup\$ Please see my edit. I added the input part of the circuit. So do you mean I should wire a resistor from DAC input to GND? What about the value of it recommended? DAC output (input voltage to B) will be from 0 to around 11V. \$\endgroup\$
    – user16307
    Jul 22, 2016 at 9:06
  • \$\begingroup\$ Forgot to mention DAC cannot output more than 5mA. So can we use a 100K resistor as pulldown? \$\endgroup\$
    – user16307
    Jul 22, 2016 at 9:16
  • \$\begingroup\$ Yes I mean to put a resistor from DAC output to GND. Since there are no other loads (just two OpAmp inputs that have high impedance) the resistor may be even much smaller (the smaller the better as long as the control voltage source can handle the current). So if e.g. max. voltage is 11V it could be 2.2k\$\Omega\$. \$\endgroup\$
    – Curd
    Jul 22, 2016 at 9:23
  • \$\begingroup\$ But why smaller is the better? I was planning to use a 1Meg resistor. I dont want to load the DAC much. Why would you use 2.2k and not 1Meg for instance? \$\endgroup\$
    – user16307
    Jul 22, 2016 at 9:28
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    \$\begingroup\$ The open line acts as an antenna that picks up some noise. The noise volatge will be smaller if the pull-down resistor is smaller (just see the noise source as voltage source with internal series resistance). I guess with resistor in the range of ca. 10k\$\Omega\$ it will be practically 0 (depending on the length of your open input line and the EM fields present) \$\endgroup\$
    – Curd
    Jul 22, 2016 at 9:31

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