How to select the correct Analog Switch (TTL<->CMOS)

Hi Graham

Thanks for answering I have heard that an Analog Switch would help do that I had a reed relay but it seems the current from the TTLs is not enough to drive it. I even don't know exactly how "analogue" switches function Sorry for the ignorance, if you could recomend a book or information about then would be great

Regards

Why do you think you need an *analogue* switch if you're only switching logic signals ?

Graham

...Jim Thompson

A standard TTL gate is unlikely to be able to drive even a reed relay.

Are your logic levels all 5V ?

You can do what you want simply using logic gates ( much cheaper ).

Graham

Hi Jim,

Thank you very much for the answer

Jim Thomps> >LINE 1: CMOS Chip A -------

The chip B is a 74F Is it possible to use a 74F as selector?

Best Regards

ONLY if the CMOS chip (A) is capable of driving a 74F input, which I doubt.

...Jim Thompson

I think I explained bad and refered wrong to the Analogue Switch modell, I offer an apologize

What I would like to have is following connection:

CMOS Chip A -------(S1) AD7512 (OUT 1) -------- CMOS Chip C

and been able to switch to the following connection

TTL 74F Chip B ---------(S2) AD7512 (OUT 1) -------- CMOS Chip C

Will the AD7512 be the correct one to have those connections?

Is a 74F capable of driving a AD7512 through the (A1) terminal?

Regards

Looks like it would be OK.

The way I read the data sheet, A1...A4 are logic inputs, specified as:

Logic Low maximum level = +0.8V

Logic High minimum level = +2.4V

Which is the TTL specification.

...Jim Thompson

Perfect. Thank you.

Now one more question:

On the following connection:

TTL 74F Chip B ---------(S2) AD7512 (OUT 1) -------- CMOS Chip C

wouldn't be a problem if the 74F terminal has 4.8 Volts and the CMOS Chip can receive only 3.3? Will the AD7512 manage this voltage difference?

Yeah that is a potential problem unless the CMOS Chip C has adequately overvoltage tolerant inputs. In practice the 74F device won't output logic highs of 4.8V, but whatever it does output may still be too high to remain in specifications and avoid activating the CMOS Chip C ESD protection structure. The AD7512 is an analog switch, so it is designed not to interfere with voltage levels. It behaves something more like a 75 ohm resistor would.

Since this is digital logic you really shouldn't be using analog switches anyway. Analog switches are likely more expensive and are less than ideal (IE, slow to switch) than using an all digital approach. By the sounds of it you need the functionality of a multiplexer. By the sounds of your voltage conversion needs, an NC7SZ157 would be perfectly suited.

Use one of these babies and drive it with a Vcc of 3.3V. The device has overvoltage tolerant inputs, so it should handle the 74F logic levels nicely. Additionally with a Vcc of 3.3V and switching threshold requirements of 0.3Vcc (Vin low max) and 0.7Vcc (Vin high min), the required levels are 2.31V. Since the 74F outputs max low voltages of

That said, why are you using 74F logic in the first place. Avoid it and all other TTL logic families (many of which are now officially obsolete anyway) like the plague.

Which is why an HC/HCT device is a good choice - maybe even AC if line 2 has really fast signals ?

Graham

adequately

logic

remain

ohm

Hi Fritz

The 74F outputs 3.3 Volts, apologize again for the mistaken specification.

Thank you very much for the great help

I will wire the NC7SZ157 as follows: CMOS Chip A ----- Terminal 1 Terminal 6 ---- 74F Selector Chip D GND ------------- Terminal 2 Terminal 5 ---- +5 Volts TTL 74F Chip B -- Terminal 3 Terminal 4 ---- CMOS Chip C

Would this function as follows?

Terminal 6 HIGH --> CMOS Chip A would be connected to CMOS Chip C Terminal 6 LOW --> TTL 74F Chip B would be connected to CMOS Chip C

Best Regards and again thank you very much

Greetings Lathe Biosas.

Okay looks good except for Terminal 5. If you expect the input threshold to be within specification, and only want the NC7SZ157 to ouput a maximum voltage of 3.3V, then Terminal 5 should be hooked to a 3.3V supply (presumably the same power supply for CMOS Chip C).

Yeah that is how it would function. Realize however that "connected" doesn't mean they are bidirectionally connected. I assumed you weren't using anything peculiar like two way communications used in some applications such as I2C busses. I assumed CMOS Chip A and TTL 74F Chip B are both producing logical outputs while the NC7SZ157 is feeding the logic input of CMOS Chip C. Assuming these assumptions are correct, and assuming no other relevant details have been left out, then this should work.

I think he is learning some stuff about logic chips. If this newsgroup isn't for sharing information, learning things, and helping each other out, I don't know what it is for.

It is good to have confirmation that your understanding of something new is correct before you build a board around that understanding. The NC7SZ157 is a small SMT part so will presumably need a board before it's function can be personally confirmed.

Heheh- was wondering when anyone was going to notice that- analog switching of digital signals is an oxymoron operation if there ever was one.

Haha! Well said and probably quite accurate.

Try my response - the first reply in this thread.

Uhuh.

Graham

Look !

Why don't you just learn some stuff about logic chips ?

Don't expect easy answers. You need to understand the technology yourself !

Graham

Yeah - agreed - but this is kiddy stuff !

Uh ?

Graham