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

[Fritz Schlunder]

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


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.

Haha! Well said and probably quite accurate.

 

[John Fields]

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.

---
What was wrong with this ? :

+5V>---------------------------+---+
RELAY 6| |1
+-------------O---O------+
| | | |
| +--------+ | |
| |K | +-> | |
| [DIODE] [COIL]- - -| |7
| | | O--O--[330R]--+
| | | | |
+----O--------O----------+ |
74F00 9| 13| |
1+----+ | | |
IN>----+---|A _|3 | | |
| 2| Y|O--+-+--------+ |
+---|B | | |
| +----+ | |
| | |A
| 74F00 | [LED]
| 4+----+ | |
+---|A _|6 | |
| 5| Y|O--+ |
+---|B | |
+----+ |
|
GND>-------------------------------------------------+

 

[Lathe_Biosas]

I think he is learning some stuff about logic chips. If this
newsgroup

Yeah - agreed - but this is kiddy stuff !



Uh ?

Graham

Hi Graham

Thank you for your time, help, interest and information. Indeed I'm
learning and indeed I should learn by myself, but after having $42 USD
shipping costs for parts each time I order them, burning an $8.00 USD
reed relay and waiting 3 weeks of courier delays I thought I should
first try asking S.E.D.

The next shipment would be also $42 USD + $1.15 of 5 NC7SZ157P6X, I
have the board for SOT-363 that I used with an other SC-70 chip.

Unfortunatelly I didn't know how to solve the problem I state on this
thread neither I know of a good advanced book that could help me. The
only information I found was how to convert TTL to CMOS voltages from a
Digital Circuit introductory book.

So this are the reasons I posted at S.E.D. an please don't get me
wrong, I'm not mad an neither is my wish to begin a passionate debate
about something not fully related to this thread.

Best wishes and have a nice weekend

 

[Lathe_Biosas]

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).

All my circuit is connected to 5 Volts and GND
I will try to convert the +5 Volts to add an extra 3.3 Volts line

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.

The connection is unidirectional.

Very Best Regards and have a nice weekend

 

[Fritz Schlunder]

All my circuit is connected to 5 Volts and GND
I will try to convert the +5 Volts to add an extra 3.3 Volts line

Oh. I misunderstood something you said earlier. You said, "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?" I assumed you meant
CMOS Chip C was using a 3.3V supply and was therefore limited to seeing
logic high inputs of no more than 3.3V. Evidently I misunderstood what you
meant. I now assume you really meant that the 74F devices measured 3.3V
output when they produce logical highs, but all logic devices are running
from 5V.

So... That changes things. In that case don't run the NC7SZ157 at 3.3V,
and don't add an extra 3.3V supply rail to your design. Instead run the
NC7SZ157 at 5V and then add two 1k pull up resistors to the inputs that are
fed from the 74F logic devices. In other words put one resistor from
terminal 6 to +5V and one resistor from terminal 3 to +5V assuming you still
use the same connection scheme as you outlined in your previous post.

Alternatively if it is more convenient or cheaper to do so forget the
NC7SZ157 and the two resistors and instead use a 74ACT157 device instead.

http://www.fairchildsemi.com/ds/74/74ACT157.pdf

The 74ACT157 is similar in functionality to the NC7SZ157 except that it is a
quad device, it has a common "enable input", and it has input switching
thresholds suitable for direct interface with TTL type logic families.

 

[Rich Grise]

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?

No, it is not.

The correct chip to use in this situation is 74HCT153 Data Selector.

Good Luck!
Rich

 

[Rich Grise]

[crossposted to sci.electronics.basics,s.e.design, f-u to s.e.basics]

!
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.

Then let's take it to the basics group.

Cheers!
Rich

 

[Rich Grise]

[Crossposted to s.e.design,s.e.basics, followups to s.e.basics]

The 74F outputs a TTL high, which is a very weak pullup to _about_
3.3 volts.

No, that's not what he wants. He wants to drive a CMOS input from
either a FTTL output or another CMOS output. The chip to use is
74HCT157 (or one in that family) - HCMOS with TTL-compatible inputs,
which are also compatible with CMOS, as it happens. Please don't
confuse the newbie with threshold voltages.

All my circuit is connected to 5 Volts and GND I will try to convert the
+5 Volts to add an extra 3.3 Volts line

Forget about 3.3 volts. You have a 5V supply, use 5V chips and let
the thresholds take care of themselves - the chip designers already
did that bit of homework.

But as someone else mentioned, the NC7SZ is in some kind of SMT package -
the 74HCT series at least is in a package that you can _see_! ;-)

Cheers!
Rich