Level shifting a clock

[Spatch]

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.

 

[James Beck]

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

 

[Lionel]

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.

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.

 

[Scott]

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 logiclevelconverter, or a push-pull
transistor output stage.

--
W "Some people are alive only because it is illegal to kill them."
. | ,. w ,
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---^----^---------------------------------------------------------------

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

 

[Lionel]

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.

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.)

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.

My pleasure, glad I could help.

 

[Jim Thompson]

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.

My MC1488 will do nicely to convert TTL/CMOS levels to +/- whatever,
up to +/- 10V.

...Jim Thompson

 

[Lionel]

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.)

<reads futher> 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.

 

[Lionel]

My MC1488 will do nicely to convert TTL/CMOS levels to +/- whatever,
up to +/- 10V.

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.

 

[Jim Thompson]

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.

Nope. My input is classic DTL... threshold is precisely two Vbe's.

...Jim Thompson

 

[Lionel]

Nope. My input is classic DTL... threshold is precisely two Vbe's.

Well then, no problem. (Seemed likely, but I was too lazy to check the
datasheet.) ;^)