MPC506: Circuit Design for MUX MPC506

The datasheet says:

If the +15V and/or –15V supply voltage is absent or shorted to ground, the MPC507A and MPC506A multiplexers will not be damaged; however, some signal feedthrough to the output will occur. Total package power dissipation must not be exceeded.

The datasheet does not appear to state what the minimum supply voltage is.

V_REF should be connected only if it is larger than 5 V.

Why not use the CD74HC4067?

Hi Basel,

It is not recommended to leave the VSS pin floating. The part of the datasheet the Clemens is referring to is how the device operates when there are no power supplies on either the VDD or VSS pin. Grounding the VSS pin will ensure proper operation of a single sided supply - when left floating the device will not act as expected.

Is it possible for you to GND the VSS pin in your application?

Also is there a specific reason you chose the MPC506 for this application?

Please let me know!

Best,

Parker Dodson

HI 

Thanks for your answer, so no need to connect Vref to +5Volts.

I can't use CD74HC4067 because there is no OVERVOLTAGE PROTECTION feature in it, and the Mux circuit designed to work as a signal sniffer.

Hi Parker

So the negative supply pin is also a VSS pin? no there is no problem connecting it to GND, but it is a problem to connect it to -15Volts.

Yes, there is a specific reason to use this Mux, is that MPC506 has an OVERVOLTAGE PROTECTION feature in it, and I don't want to let the signals interfacing while the MUX turned off, and this is a common case in my application.

Hi Basel,

To Clarify this part is intended to be a bi-polar supplied part - however it has been shown to work when connecting V- to ground. This comes at the cost of some more leakage current that could cause the chip to consume power. 

However what are the expected overvoltage conditions you expect to see at the input and what is nominal signal levels - I want to make sure this part will work for the application. At (V-) = 0V and (V+) = 5V you will have a protection range of [0 - 20V, 5V + 20V] -> [-20V, 25V]. 

One more question on the application, you say you want the mux to act a signal sniffer, but in this application the mux would protect the watchdog MCU from an overvoltage event - it will only pass voltages in the range (V-) to (V+). Maybe I am misunderstanding the application, if you could elaborate a little more that would be helpful. Mainly I want to know what information about the signals that are being passed. Is the goal that if they surpass a predetermined threshold the watchdog timer will react? If the supply is the same for both the watchdog and the Mux an overvoltage event may not be passed through the device - however I could just be misunderstanding the application!

Please let me know!

Best,

Parker Dodson

Hi Parker

those signals are output and input signals to my board and will have a maximum voltage of 4 volts (digital (0V-+3.3V), analog (0V-+4V)).

this application mainly will work under two conditions. in the first one, the mux will be powered on and will pass all the signals sequentially to the Watchdog MCU that will work on logging it as data on a MicroSD card, the second is the critical condition while the mux is turned off  (V-) = 0V and (V+) = 0V, but there are signals on its pins which is still transmitting between the other two MCUs.

however, I have designed an evaluation board that uses this mux ADG406BNZ, but in the off condition, signals were interfacing between each other.

I hope you can now understand it if this didn't help I will make another explanation by a detailed drawing.

Thanks

Hi Basel,

That makes perfect sense, thanks for the explanation! So what you need is powered off protection which this part does provide. 

This part should work in the application you described. However there is one caveat, this part will have a very high Ron at a 5V supply, in the kill-ohm range so depending on the input impedance there could be some insertion loss that could degrade the signal going over the part. That would be my only concern using this part in the application.

There are other options but that are cheaper, use less PCB area, and fit the use case of the application better - the trade off is that it is a multi IC system - so it would increase the needed supply bypass capacitors by 2. One option could be to use 3 SN74CBT3253C devices. It supports the powered-off protection functionality needed. Each IC has 2 4:1 muxes inside that are controlled via the same two control pins. Two of these devices can be used to monitor the 15 signals needed and one device on the backend to select from the incoming signals. It would have the same number of GPIO to control the system as the MPC device. If this is a possible option for you please let me know if you have any questions.

Best,

Parker Dodson

Hi Parker 

Sorry for the late reply but it was the weekend here and couldn't reach the office.

I will try first to stick with the MPC part since it will reduce the complexity and device number.

However, is there any solution to decrease Ron or to avoid any voltage drop on the signal?

Thanks

Basel 

Hi Parker

I used this op-amp as a buffer on the output of the mux, will it do the job and have a high impedance?

If not could you please recommend a buffer?

Thanks 

Basel