[Moved]: reducing stray voltage on pcb

[prateek3790]

I am using one cmos switch in my design (ADG715) , it is a spst switch, now the problem which i am facing is that when the switch is off it should be grounded but we are seeing a stray voltage of 300mv. what things can i try to remove this stray voltage. when the switch is on it is working properly.the datasheets of the switch has been attached. can i provide one more connection to ground to that pin where i am seeing this voltage.

https://www.digikey.be/product-detail/en/analog-devices-inc/ADG715BRUZ/ADG715BRUZ-ND/820112

 

[FvM]

Re: reducing stray voltage on pcb

Switch off means floating signal, unless the node is pulled down by some means. Any reason why not to expect arbitrary signals on the floating node? Jumping into details, we would ask for the type of observed signal (DC, noise, known interfering source)

 

[prateek3790]

yes but floating pin should have given small voltage(i was expecting less than 20mv). i found that this voltage(170mv) is coming only when the board is conncting through an external power supply and works fine when operated through using a battery(12mv). so is it because of noise of the source.

 

[KlausST]

Hi,

"floating" means undefined voltage. Expect any voltage.

To get the "expected" 20mV you should use a resistor to pull down the - before- floating signal. Then it is not "floating" anymore.

A floating signal/wire catches any environmental voltage (noise, signal...), so it is very likely that the (AC) signal with a noisy SMPS is higher than without an SMPS.
An SPMS may introduce several erroneous signals like: AC, DC, mains frequency, switching frequency...
There are several error paths where the signals are introduced. (inductive, capacitive, safety capacitors between prim and sec, SMPS transformer, wiring, flux residuals, ...)


Klaus

 

[prateek3790]

actually to put a pull down resistor for that pin i will need to remove the solder mask, which i do not want to do. any other thing which i can try?

 

[KlausST]

Hi,

use an SPDT switch and properly switch to GND.

Klaus

 

[prateek3790]

actually i am using spst switch (ADG715) and the input signal to one of the pin that is giving high values of voltage that too when an external powr supply i being used. So the volatge i am getting must be because of the supply voltage frequency (50Hz). is there anyway i can suppress the effect of supply frequency on the input pin.

 

[KlausST]

Hi,

We told you that the voltage of an "open" = "floating" signal is not predictable.

Additionally we gave advice how to avoid floating signals.

Unfortunaltey it seems you don´t want to change the circuit.. but without change your problem can´t be solved. Hard to help.


Klaus