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
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
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
ONLY if the CMOS chip (A) is capable of driving a 74F input, which I doubt.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
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
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
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.
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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
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.
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.
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