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

I read in sci.electronics.design that Pooh Bear wrote (in ) about 'How to select the correct Analog Switch (TTLCMOS)', on Fri, 8 Apr 2005:

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.

--- 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>-------------------------------------------------+

-- John Fields Professional Circuit Designer

newsgroup

other out,

something new is

NC7SZ157 is

function can be

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

threshold to

maximum

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

weren't

Chip B

logic

assuming

The connection is unidirectional.

Very Best Regards and have a nice weekend :)

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.

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.

No, it is not.

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

Good Luck! Rich

Then let's take it to the basics group.

Cheers! Rich

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.

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