Re: How to select the correct Analog Switch (TTL<->CMOS)
- From: John Fields <jfields@xxxxxxxxxxxxxxxxxxxxx>
- Date: Sat, 09 Apr 2005 12:27:51 -0500
On Sat, 09 Apr 2005 16:13:59 GMT, Rich Grise <richgrise@xxxxxxxxxxx>
wrote:
>[Crossposted to s.e.design,s.e.basics, followups to s.e.basics]
>On Fri, 08 Apr 2005 14:39:16 -0700, Lathe_Biosas wrote:
>> Fritz Schlunder wrote:
>>> "Lathe_Biosas" <lathe_biosas@xxxxxxxxxxx> wrote in message
>>>
>>> Greetings Lathe Biosas.
>>>
>>> > The 74F outputs 3.3 Volts, apologize again for the mistaken
>>> > specification.
>
>The 74F outputs a TTL high, which is a very weak pullup to _about_
>3.3 volts.
>
>>>
>>> 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).
>
>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.
>
>>> > 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"
>...
>>> are both producing logical outputs while the NC7SZ157 is feeding the
>
>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_! ;-)
---
According to what the OP asked for:
>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
all he needs is this, where the gates are the four NANDs in any flavor
of bipolar '00:
CMOS IN>---------A _
Y--+ +5V
TTL SW>--+---+---B | |
| | | [10k]
A B +--A _ |
_ Y--+-->CMOS OUT
Y +--B
| |
+-----A _ |
Y--+
TTL IN>----------B
this,where the gates are the four NANDs in an HCT00,
CMOS IN>---------A _
Y--+
TTL SW>--+---+---B |
| | |
A B +--A _
_ Y---->CMOS OUT
Y +--B
| |
+-----A _ |
Y--+
TTL IN>----------B
or this, where the gates are the four NANDs in an HC00.
CMOS IN>--------------A _
Y--+
TTL SW>--+----+---+---B |
| | | |
[10k] A B +--A _
| _ Y---->CMOS OUT
+5V>-----+ Y +--B
| | |
[10k] +-----A _ |
| Y--+
TTL IN>--+------------B
--
John Fields
Professional Circuit Designer
.
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