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Old 25th May 2007, 10:36 AM   #1
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Default connect power to an op amp (dumb)?

Such a dumb question. So dumb in fact that no-one bothers to mention it. How do you connect power to an op amp? Simple answer might be pos on the pos pin and neg on the gnd, but............. doesn't the signal input need to be referance to the mid voltage?
I.e with a 18V supply. doesn't the signal need to be referanced to 9V? I also assume that gnd needs to connect to the screen?

I see opamps with the max supply voltages at +/-18V ..... so the middle must be zero . Am I thinking too hard? Is that why I'm confused?
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Old 25th May 2007, 11:14 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
most opamps are differential amplifers.
That means they amplify the difference between the inverting input and the non-inverting input.

If the opamp has been designed properly it does not care what voltage is fed to the +ve & -ve supply pins as long as the input signal(s) stay(s) between the supply voltages - ALWAYS!!!

I have seen some builders insisting on absolute accuracy for +-voltage matching and further stating that small errors in supply voltage matching will cause an equal output offset voltage. If the amp is any good this "error" is reduced by the PSRR of the amp.

Have a look at the datasheet and see what it says about PSRR and common mode rejection.
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Old 25th May 2007, 11:20 AM   #3
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Most designs use a bipolar supply which provides a positive voltage and a mirror image negative voltage. These voltages are referenced to a third wire which is called "common."
The Opamp + input is referenced to "common."
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Old 25th May 2007, 02:38 PM   #4
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Dumber please.
So I get from this that I aught to fix the signal to the +18 and -18V rails with (say) 10K resistors, making a voltage divider, and the middle is called "common".

What about gnd? what is that relative to?

One of my realworld projects for the future is to build a DI box running from phantom power (ignore the over-volt for a moment. Pos comes in on the output lines, and neg is on the screen, which is also gnd.
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Old 25th May 2007, 03:08 PM   #5
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Though I don't really like anthropomorphic analogies, I find it helpful to pretend I'm a little guy inside the op-amp, having nothing at my disposal but a DVM and some resistances. I can measure from any pin, to any other, but I have no knowledge of anything outside the op-amp. My task in life is to measure the difference between the two input pins and connect resistances from the power supply pins, to the output, until the difference goes away. I do this quite quickly, so AC signals are no problem. Thinking in these terms, you can float the op-amp at any voltages you like, as long as you observe the CM restrictions. Ground is arbitrary- if you AC couple the input with a cap, you can use a divider to set the input DC voltage to half the supply voltage. You can also create an artificial ground, either with resistor divider, or with that, plus an op-amp buffer, but these approaches have a lot of problems in terms of impedance and stability. Better to AC couple if possible.

As a wild example of floating op-amps, there's a simple high voltage regulator circuit where a pass transistor controls several hundred or more volts. It's driven by an op-amp, with all pins at several hundred volts, the supply voltages being set by zener diodes off the HV line. The whole thing works great until somebody touches the thing with a scope probe, spiking the inputs, which have to charge up the capacitance of the probe, making the whole thing go up in smoke.
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Old 25th May 2007, 04:05 PM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Pos comes in on the output lines, and neg is on the screen, which is also gnd
Hi,
don't call signal and return Pos(+ve) or neg(-ve) that just adds to confusion.

Your signal on the pin and return on the screen are a pair of inputs.
The opamp is just waiting for you to connect these two inputs (signal and return) to it's two input pins. Any difference between those two inputs gets amplified and comes out of the output.
The schematic you use will show how to tie these three pins together.
It is quite common to call the screen, audio ground (but be specific to avoid another confusion). The circuit you build will probably use the screen as the reference for all other voltages going into and out of the opamp.
The opamp does not care where the supply pins voltages are provided you stick to that original rule
Quote:
the input signal(s) stay(s) between the supply voltages - ALWAYS!!!
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Old 25th May 2007, 04:17 PM   #7
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Andrew you misunderstood my post. Phantom power is: 48V DC pos on the 2 signal lines and 0V (return) on the screen ( I shouldn't have called it neg).

Its what to do with 0V DC and ground that confuses me if Common is actually (say) 18V higher than 0V
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Old 25th May 2007, 07:04 PM   #8
Minion is offline Minion  Canada
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If you are useing a Standard Bipolar Supply like a +18v/-18v supply then the 0V is Ground .....(you could also use a Voltage divider between the +18v and -18v to create a Ground)

If you are useing a Single supply like a +36v supply you could Create a dual Supply by putting a Voltive divider between 0v and 36v and referance the input and all grounds to there.......



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Old 25th May 2007, 08:33 PM   #9
Nordic is offline Nordic  South Africa
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OK, now we are moveing into the more relevant replies, with Minion's last post...

Circuits can be powered by dual or single rail supplies.

Dual supplies, will have V+ = xV and V- = -xV and a 0V reference point for inputs and output signal... (gnd)

Single supplies Have a V+ = xV, a common reference of 1/2xV, and a 0V supply (the negative terminal on a battery for example)

Now comes the fun part....

when you use a single supply on its own, lets say with only a loudspeaker connected, the loudspeaker sees only the potential between the output and the reference point.... BUT this reference point is normaly some positive DC voltage... luckily DC is 'silent' allthough it will still melt speakers, so it needs a series capacitor on the output, to act as a gate only letting the AC signals (music) through...

As long as you don't have DC on the input to the power amp, it is irrelivant how a preceeding stage is powered... I.e. dual rail or single... once again, all that is important is the total potential voltage swing, and of course the input current the amp stage can handle...with a good dual rail, the signal could be very low DC offset removing the need for a DC blocking capacitor on the signal...
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Old 25th May 2007, 08:39 PM   #10
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yes I'm down with the voltage divider part, and the decoupling cap, but if the input came down a bit of coax, What do you connect the screen to?
(There's more but that enough for a minute.)
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