Voltage source with adjustable negative and positive output

Nov 14, 2006 3 Replies
M

Hello .... I have an old MAX038 IC. I wanted to build a simple signal generator. One of the pins in the IC datasheet accepts +/- voltages to adjust the frequency at the output. I was wondering if anyone can give me some tips on how I can build a voltage source that has one adjustable output that can range from -2.5V to +2.5V.



I was originally thinking of using two voltage regulators with adjustable positive and negative outputs. I could then switch between the two if I wanted to adjust my frequency up or down from the nominal value, but I figured it'd be more interesting to build just one circuit that has one output that goes from negative to positive.



I appreciate schematics, but what I really want is a resource that explains to me how each part of it works and the concepts behind it. I'm still learning. Thanks!



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V

The MAX038 already requires a symmetric +/- 5V power supply, just use a voltage divider across this power supply to get any voltage between -5 and +5V. You can use a potentiometer and 2 resistors to limit the range to -2.5V - +2.5V.

Something like this : ,--- slider, [-2.5V, +2.5V] v

-5V o---[ R ]---[ 2R potentiometer ]---[ R ]---o +5V

vic

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J

The function of the Fadj pin is explained on page 11 of the data sheet:

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The input is a voltage signal (functioning over a +2.4 to

-2.4 volt range), but the pin also has a -250 uA current load applied internally, so that a variable resistor to the

+2.5 volt reference can produce a variable voltage based on the current passing through it. a 20k variable resistor would drop between zero and 5 volts in that situation, producing +2.5 to -2.5 volts for the input.

This approach is shown on page 9.l.

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C

the data sheet gives a clue if I assume what you are trying todo correctly, a 5vttl pll circuit the output of wich goes to a resistive divider down to -5v, or you could use a 2.5v bandgap device pulled down to -5v that would turn 0 to 5v into 2.5 to -2.5v

Colin =^.^=

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