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

[Lathe_Biosas]

Hi

Would the following Analog Switch (AD7510)

http://www.analog.com/en/prod/0,,768_833_AD7510,00.html

help to switch between two signal lines that are connected as follows?


LINE 1: CMOS Chip A -------
\
SWITCHING AD7510 ---- CMOS Chip C
/
LINE 2: TTL Chip B -------


An aditional TTL Chip D would control the switching of AD7510

Unfortunatelly I don't know that much about currents and I don't want
to burn my circuit. If the AD7510 is the correct one, what voltage
should I apply to V_ss and V_dd?

While selecting the Analag Switch I get options with different ranges
of impedances, i.e. 50 to 100 Ohm, what is the meaning of that?

Any help or information would be kindly appreciated
Best Regards

 

[Pooh Bear]

Hi

Would the following Analog Switch (AD7510)

http://www.analog.com/en/prod/0,,768_833_AD7510,00.html

help to switch between two signal lines that are connected as follows?

LINE 1: CMOS Chip A -------
\
SWITCHING AD7510 ---- CMOS Chip C
/
LINE 2: TTL Chip B -------

An aditional TTL Chip D would control the switching of AD7510

Unfortunatelly I don't know that much about currents and I don't want
to burn my circuit. If the AD7510 is the correct one, what voltage
should I apply to V_ss and V_dd?

While selecting the Analag Switch I get options with different ranges
of impedances, i.e. 50 to 100 Ohm, what is the meaning of that?

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


Graham

 

[Lathe_Biosas]

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

 

[Pooh Bear]

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.

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

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

Are your logic levels all 5V ?

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


Graham

 

[Lathe_Biosas]

Hi Jim,

Thank you very much for the answer

LINE 1: CMOS Chip A -------
\
SWITCHING AD7510 ---- CMOS Chip C
/
LINE 2: TTL Chip B -------

[snip]
^
|
|
|
Just use an HCT selector here --

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

Best Regards

 

[Jim Thompson]

Hi Jim,

Thank you very much for the answer

LINE 1: CMOS Chip A -------
\
SWITCHING AD7510 ---- CMOS Chip C
/
LINE 2: TTL Chip B -------

[snip]
^
|
|
|
Just use an HCT selector here --

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

 

[Lathe_Biosas]

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

 

[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

 

[Pooh Bear]

Hi Jim,

Thank you very much for the answer

LINE 1: CMOS Chip A -------
\
SWITCHING AD7510 ---- CMOS Chip C
/
LINE 2: TTL Chip B -------


[snip]
^
|
|
|
Just use an HCT selector here --

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.

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


Graham

 

[Lathe_Biosas]

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?

 

[Fritz Schlunder]

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.

http://www.fairchildsemi.com/ds/NC/NC7SZ157.pdf

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 <0.99V and >2.31V. Since the 74F outputs max low voltages of
0.5V and min high voltage of 2.5V, this should work very nicely with
everything within specification.

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.

 

[Fred Bloggs]

Since this is digital logic you really shouldn't be using analog switches
anyway.

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

 

[Pooh Bear]

Heheh- was wondering when anyone was going to notice that-

Try my response - the first reply in this thread.

analog switching of digital signals is an oxymoron operation if there ever was
one.

Uhuh.


Graham

 

[Lathe_Biosas]

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.

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

 

[Pooh Bear]

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

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

 

[Fritz Schlunder]

Hi Fritz

Greetings Lathe Biosas.

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

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).

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

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.

 

[Fritz Schlunder]

Look !

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

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

!


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.

 

[Pooh Bear]

!

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.

Yeah - agreed - but this is kiddy stuff !

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.

Uh ?

Graham

 

[Pooh Bear]

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.

And that's meant to be understood by a beginner ?


Graham

 

[John Woodgate]

I read in sci.electronics.design that Pooh Bear
<[email protected]>) about 'How to select the correct Analog

And that's meant to be understood by a beginner ?

Of course not. On s.e.d., most of the explanations (except mine) are
solely intended to impress the other contributors. Someone asks a
question about DC Ohm's Law and gets a lecture on complex numbers.