Modulating DC Level Relative to Fixed Voltage

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Bill Sloman, sydney 
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> This would be in relation to a fixed voltage positive and negative 
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> rail. 
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> Klaus Jensen

That's not "modulation, that's _additive_. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

OK. I'll call it "additive" if you can answer the question. Sounds like a good deal to me.

Klaus Jensen

** You are asking the impossible.

As the DC level increases, the peak signal level MUST go down.

If the DC level approaches peak signal level, then the signal disappears.

Surely that is not what you want.

FYI: an AC signal may have a large DC component, but that implies the wave form is highly assymetrical.

.... Phil

What if there is enough headroom on the raisl to allow a full swing of the varying DC offset?

For example, if the positive rail was 12VDC, and the DC level varied sinusouidally by 6V at 10Hz. If the rail was clamped at 12V, wouldn't it and the 10Hz sinewave be unaffected?

Please correct me if I am wrong.

Perhaps I did not explain well, but the DC level variation would never exceed the rail voltage.

Klaus Jensen

I can't say I understand what you want to do. If you vary a DC voltage at 10 Hz, it is no longer DC, it is a 10 Hz sine wave.

How do you want to create this signal? Is there some other signal on the same wire? If you are ask if you can create a 10 Hz sine wave, the answer is yes. Are you trying to put this signal *on* the positive rail?

If you want to put a 6 VAC signal on the positive rail it would either be in addition to the 12 VDC rail which means the rail would swing between 9 and 15 volts. Or if you don't want the rail to ever go above

12 volts it would have to be reduced to 9 volts with a 10Hz 6 Vpp sine wave added to it making the peak voltage 12 volts and the minimum voltage 6 volts. Is that what you mean?

Maybe you can explain what this will accomplish for you and we can understand better what you need to do.

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Rick

you mean like a clamp?

It sounds like you want to sum _your_ ac signal and the bias signal and then _clamp_ the result.

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umop apisdn 


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It's quite unclear what you seek. Is this what you want...

V = VDCnom + Vwander*sin(OmegaWander*t) + Vpeak*sin(OmegaSignal*t)

(OmegaWander is a very low frequency) ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

Is this what you had in mind...

...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

Ok, I'll bite..

A rail to rail op-amp on a single rail supply, configured as a

1:1 gain inverted input with a variable DC set point sitting on the (+) input. Output has no decoupling cap.

Moving the DC set point on the (+) will allow you to set your DC constant voltage on the output.

The inverted input as a amplifier of course passes your sine wave information, from another source elsewhere.

If sine wave information has no negative properties and you use no coupling capacitor on the input, it'll force the output to move towards the common rail and not allow it to move above the +DC set point.

Basically, from what you said, I would say a non-cap-coupled single rail Op-Amp circuit, with the (+) as the base line DC set point etc..

How close was I ? :)

I feel this is a game of charades.

Jamie

That is similar, Jim, but involves a modulation frequency.

If I can explain in another way.

There is a split rail supply +/-12VDC. I want to reference a 10Hz sinewave to each rail instead of ground. In other words the 10Hz appears as a varying DC offset centered between the two rails but never exceeding them.

Klaus Jensen

Clear as mud... but doesn't cover the ground. Explain the outcome. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

You need to brush up on Op-Amp basics.

Home work?

Jamie

So you are saying you want a 10 Hz AC signal of 24 volts peak to peak amplitude centered at 0 volts?

I don't know why you are referencing the power supply rails or talking about a "varying DC offset". Your varying DC offset is an AC signal.

What does "referencing" the rails have to do with it?

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Rick

** OK now I get it.

Your "DC level", "DC offset" and "10Hz sinewave" are all the same damn signal.

FFS learn to use tech language properly !!

One can "modulate" a signal in various ways (amplitude, frequency, phase) but a voltage simply varies.

A voltage that varies cannot be called DC.

... Phil

Normally, a DC coupled signal would be ground (earth) referenced. IOW the voltage varies in relation to the 0V of the supply, and is accessed with one wire to the active and the other to ground.

What I am meaning instead is to center the signal between the positive and negative rails and then access TWO outputs, one between active and positive and the other between active and negative.

In other words, at one output, one wire would lead to the active of the signal, but the other would connect to the positive rail. The other output would be between the active and negative rail.

This would provide two outputs, one positive going and one negative going, but inverted from each other.

Klaus Jensen

Yes, but it is referenced to two different voltages, e.g. the positive and negative rails instead of ground.

The idea is, by doing this, to derive two signals of differing polarity with a single generator.

Not normaly done I realise.

Klaus Jensen

This *won't* give you opposite polarity signals from one output. The signal will have the same polarity, it will just appear to be offset from the two rails by a different DC voltage. When the output goes positive, it doesn't matter what your reference is, it is still going positive.

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Rick

Oh. Do you mean you want a waveform (sine?) whose zero starts at the positive rail and goes negative to zero volts AND, at the same time, you have a similar waveform whose zero starts at the negative rail and goes positive to zero volts? Such that your negative half of the sine goes from the + rail to zero and the positive half goes from the - rail to zero?