Hi,
I'm using dg506 mux and i noticed something strange:
Ones one of the inputs is enabled and routed to the output the resistence between the input and output is 1k at 5v v+ (v- and Gnd at ov).
At 15v v+ the resistance drop to 300ohm.
My question is: the resistance between the input and output should be short no? In the ic it self there is mosfet transistor that doing the switching if so why i don't see short?
What i'm missing.

 

Because the resistance between a MOSFET's drain and source is determined by the gate-to-source voltage. The higher the power supply voltage, the higher the gate-to-source voltages thus the lower the on resistance. This should be reflected in the datasheet.

For example, with a large signal MOSFET:

 

In muxes like this typically they use small signal MOSFETs not capable of the low
Rdson that bigger power switching MOSFETS have. Also typically they are used
loaded with hi Z buffers/amps so their Rdson is of low concern.

The DG506 is not meant for switching power where low Rdson would be of concern.


Regards, Dana.

 

Thanks every one,
I'm using this mux to transfer uart protocol with cmos voltage levels to multiple rs422 drivers(26ls32) and it's working even the 1kohm resistance in serial.
I was curious why analog mux would have so big resistance and even now i'm a bit confused what is the purpose of such mux.
Any way thank you evey one you have answered my question.

 

I was curious why analog mux would have so big resistance and even now i'm a bit confused what is the purpose of such mux.

The most common usage of analog multiplexers is to select one particular analog signal among several (usually 2, 4, 8 or 16) signals and pass it on to an analog-to-digital converter for measurement. Often the output of the multiplexer is fed to a buffer amplifier with a high input impedance, to prevent the multiplexer's series resistance from causing measurement errors.

 

Thanks every one,
I'm using this mux to transfer uart protocol with cmos voltage levels to multiple rs422 drivers(26ls32) and it's working even the 1kohm resistance in serial.
I was curious why analog mux would have so big resistance and even now i'm a bit confused what is the purpose of such mux.
Any way thank you evey one you have answered my question.

By using a smaller MOSFET geometry in switch, die size goes down, and unit price with it. Because its used,
typically with a hi z buffer following it its a perfectly acceptable compromise with lower cost.


Regards, Dana.

 

Usually a digital multiplexer would be used in a situation like yours. You will notice that you can make one out of a quad gate.



Note that you can make a single 2:1 mux with a single quad nand or nor gage:

The fourth gate is used as an inverter to create complimentary enable signals.