I need to level shift a clock that toggles between 0 and 3.3v to a clock that toggles between -7v and +7v. I recall that you could use a voltage comparator to do this, however I can't seem to find a schematic in any application notes. I have plenty of Linear Tech LT1011 comparators in my toolbox and was hoping I could do something with them.
How fast of a clock? Could you get away with using a "standard" RS-232 level shifter? They are a dime a dozen.
Jim
What's your maximum frequency? If it's within the usable range of your comparators, you can simply compare your clock signal to ~1.6V from a voltage divider (or a red LED!) & get the result you want. Jim Beck's suggestion of an RS232 output converter should work fine up to a couple of MHz, & is pretty cheap. If the frequency is higher than that, you could use a proper logic level converter, or a push-pull transistor output stage.
-- W "Some people are alive only because it is illegal to kill them." . | ,. w , \\|/ \\|/ Perna condita delenda est ---^----^---------------------------------------------------------------
Lionel,
Thanks for responding. The max frequency is 100khz...well within the bandwidth range of the LT1011. I actually breadboarded a circuit that did as you suggested. I ran my 100khz clock into the + input of the LT1011 and tied the (-) input to a voltage divider (1.65v). The LT1011 has floating transistor output. I have tied the collector to +7v and the emitter is connected to -7v. I would expect the output to toggle
+/-7 volts, however this is not the case. The output swings to the +7v , but not below ground. Relativley simple circuit to breadboard so I doubt I made a mistake.Incidently, the RS232 translator that Jim Beck suggested is a great idea, however most RS232 transceivers invert the outputs. I will have to look a little closer at the LT1011's output stage. Thanks for the input.
-Scott
Hm. Looking at the datasheet, it looks like the chip is designed to interface between split-supply analog & logic-levels. You might get the full output swing by wiring it up for a 14V single supply instead of a +/- 7V split supply. (Looks like you need a pull up resistor on the output as well.)
My pleasure, glad I could help.
-- W "Some people are alive only because it is illegal to kill them." . | ,. w , \\|/ \\|/ Perna condita delenda est ---^----^---------------------------------------------------------------
My MC1488 will do nicely to convert TTL/CMOS levels to +/- whatever, up to +/- 10V.
...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 |
America: Land of the Free, Because of the Brave
Okay, looks like I was on the right track. Check the schematic on the very last page of the datasheet. To get the full voltage swing, you need to connect *both* pins 1 & 4 to your -7V supply, with a pullup resistor from pin 7 to +7V.
-- W "Some people are alive only because it is illegal to kill them." . | ,. w , \\|/ \\|/ Perna condita delenda est ---^----^---------------------------------------------------------------
Yep, plus they're cheap, easy to use, & easy to find. The one thing to watch out for with the OP's 3.3V inputs is that he might need a pullup resistor in the input of the 1488.
-- W "Some people are alive only because it is illegal to kill them." . | ,. w , \\|/ \\|/ Perna condita delenda est ---^----^---------------------------------------------------------------
Nope. My input is classic DTL... threshold is precisely two Vbe's.
...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 |
America: Land of the Free, Because of the Brave
Well then, no problem. (Seemed likely, but I was too lazy to check the datasheet.) ;^)
-- W "Some people are alive only because it is illegal to kill them." . | ,. w , \\|/ \\|/ Perna condita delenda est ---^----^---------------------------------------------------------------
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.