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help needed!!!!

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harrythedick

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V IN FREQ OUT
2.5V 1khz
3.0V 1.75khz
3.5V 3.12khz
4.0 6.26khz
5.0V 10khz
6.0V 12.5khz
9.0V 20.8khz

sorry guys but my questions like this. the output from my op amp would be fed into a vco which would convert the voltage into the freq according to the above table. is there anything that i could do which would give me an inverted result? the results i wanna achieve is something like this( the op amp is used to amplify the voltage give from a ultrasonic sensor)
V IN FREQ OUT
2.5V 20.8khz
3.0V 12.5khz
3.5V 10khz
4.0 6.26khz
5.0V 3.12khz
6.0V 1.75khz
9.0V 1khz

thanks!!!!
 
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MikeMl

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Most Helpful Member
Do you already have the VCO? If so, what is it's Freq vs. voltage? Is it the top table?
If so, then all you need is an inverting opamp stage such that 2.5V in makes 9V out and 9V in makes 2.5V out. Do you know how to make that?

hint: Gain of -1, biased to (2.5+9)/2
 
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harrythedick

New Member
Do you already have the VCO? If so, what is it's Freq vs. voltage? Is it the top table?
If so, then all you need is an inverting opamp stage such that 2.5V in makes 9V out and 9V in makes 2.5V out. Do you know how to make that?

hint: Gain of -1, biased to (2.5+9)/2
erm mike im truly sorry but could you explain alil more on how i could achieve that? sorry but im really a noob at electronics. and what about the midde values?? i know how to get the gain i dont see how it makes 2.5v to 9v and 9v to 2.5v
 
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MikeMl

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Write back if you don't understand this:
 

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shokjok

Member
Have you considered using a PLL from a discarded CB radio, such as a PLL02A? With support circuitry, you could use an A-D converter connected to a PROM/ EPROM, connected to the PLL. Experimenting will give you close to the voltages you require.
 

MikeMl

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Most Helpful Member
mike why must vss be 12 volts .
Because the non-inverting input of the opamp has to be at 5.75V to set the middle of the output range. If you use a voltage divider, the voltage at the tap is a function of the resistors in the divider as well as the voltage being divided (Vss, in the example).

If you are starting with a different Vss, then you can recompute the voltage divider resistors, or come up with another way of generating 5.75V. Note that with the voltage divider, the Vss must be clean and well regulated, otherwise any voltage fluctuation on Vss gets divided but is then applied to the non-inverting input of the opamp. It would then appear superimposed on the opamp's output...

The lower limit of Vss is determined by the opamp. If it is not rail-to-rail, then the highest the opamp can pull its output is about 2V below Vss. Since you want 9V out, the Vss must be 11V or higher. That's why I picked 12V. What do you want it to be?
 
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MikeMl

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The one I used in the sim is generic. This opamp is non-critical, so almost anything will work. I have lots of LM358s in my junk box, so I would likely use one of those.
 
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