Voltage applied to multiplexor pins when multiplexor IC is not powered?

[opampsmoker]

Hi,
Please advise what will happen if 22V is applied to either the S1,S2.....S8 or D pins of the ADG1438 multiplexor when the ADG1438 is not powered?

ADG1438 datasheet:

https://www.analog.com/media/en/technical-documentation/data-sheets/ADG1438_1439.pdf

 

[gophert]

That's the perfect kind of question Analog's tech support team could address. A link to their contact info is on the bottom of the datasheet (Page 1).
I hope they give it a solid review rather that a standard ("our device specs only apply to the conditions listed in the datasheet").
Let us know what they say.

 

[crutschow]

You just need to look at the data sheet's Absolute Maximum Ratings on page 10 (below):
The inputs would blow unless the input current is limited to 30mA if the supply voltage(s) are off.

If the input current is not inherently limited to below that value, a 1k ohm resistor in series with each input should work for 22V maximum input voltage.
That shouldn't have any significant effect on the signal, unless it's very high frequency or the mux output has a low impedance load.

 

[opampsmoker]

Thanks but the analog switchs may well be off if the adg1438 is not powered......but still we fear 22v may blow the chip if its not powered and 22v is applied to pins S1, S2, etc?

 

[crutschow]

Thanks but the analog switchs may well be off if the adg1438 is not powered......but still we fear 22v may blow the chip if its not powered and 22v is applied to pins S1, S2, etc?

I understand they are not powered.
But the added series resistor will protect them.
Can you add them, as I suggested?

 

[schmitt trigger]

Elaborating on Cruts idea:

You can also add, after the resistor, a dual series diode, with the cathode connected to V+, anode to V- and the center connection to the input being protected.
Schottkys are usually too leaky for analog circuits, but you could use a high conductance Si dual like the MMBD1503.

 

[opampsmoker]

I understand they are not powered.
But the added series resistor will protect them.

Thanks, but no we cant add these, as we need the path through the analog switch to be low resistance.
I'm wondering if we shoudl put series diodes in there though.....anode to S1, etc

 

[ronsimpson]

I think there are diodes from the switches to V+ and V- inside the IC. These are only 30mA diodes. When applying power to the pins you will power up all the circuits on the power supply. If the circuits pull less than 30mA the diodes will hold.

Adding dual diodes on the outside does not help much because the internal 30mA diodes turn on first (lower turn on voltage) and the external 200mA diodes will not turn on until you are way past 30mA.

 

[crutschow]

What is the source of the 22V, and what is its impedance?

How is it that you are applying 22V to the mux input when it's rated for only 15V maximum (with 15v supplies)?

 

[crutschow]

You really need to give us more details about the application so we don't give unworkable solutions.

 

[tomizett]

I think there are diodes from the switches to V+ and V- inside the IC. These are only 30mA diodes. When applying power to the pins you will power up all the circuits on the power supply. If the circuits pull less than 30mA the diodes will hold.

Ron makes a pertinent point: If you can't limit the current with resistance in the signal source, perhaps you can stop the signal source "back feeding" the supply rails to the rest of the circuit via the clamping diodes in the multiplexer. Simply supplying the multiplexer supplies through diodes might work.

 

[crutschow]

opampsmoker, you've given me thumbs-up but haven't answered my questions.
What's with that?