Shift DC Level of Op Amp Input Signal

I am feeding the non-inverting input of a single supply op amp (LM358) with signals from various sources.

At their output, some are AC coupled and some DC coupled, e.g. wholey positive or negative offset. The latter are LF so I cannot just insert a cap.

What is the best type of circuit to provide a volt or two of adjustment to the DC level of the input signal so that it matches the op amp?

I would prefer passive components if possible.

I have searched for circuits on the web, but most seem to be about shifting the op amp's output not input.

Mike Towner

Reply to
Mike Towner
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So the voltage input range of the opamp is less than the signal? (I would first think about doing the level shift with an opamp.) I'm sure you can do it with a transistro or two also... but I think it has to be active.

What's the frequency?

George H.

Reply to
George Herold

Trying to picture what you have there.. My guess is the (-) input of the Op-amp must be doing something?

Is this being used as a unity gain stage (buffer) ?

Since you have no (-) supply to work with, you can inject a controlable + voltage to the (-) input of the op-amp, that will make the output go lower.

Of course you need to work this in with the existing feedback circuit you now are using.

Jamie

Reply to
M Philbrook

So I cannot simply connect a pot across the +V and GND rails and the wiper to both the signal source and op amp input?

I was hoping for something like that, and that, given there is no negative rail, it would at least allow a postive DC offset to be adjusted.

Mike Towner

Reply to
Mike Towner

Your power-supply choice of 'single supply' is a problem, you cannot feed any large AC signal to the op amp input. Only a fraction of a volt below ground is permissible (and how do you close a feedback loop if the output does NOT go below ground?)

The most flexible situation, is an instrumentation amp configuration; you must then apply both your input signal, and its reference level, to two input pins. The amplifier performs differential input to single-ended output conversion.

The signal input range, and the permissible loading, and the significance of the signal properties, will determine a solution in each case. It's a fine art, to develop an input circuit (probe) to suit an oddball signal.

Reply to
whit3rd

Yes, this is why I need to shift the AC input signal above ground.

Are you are saying this cannot be done with passive components?

Mike Towner

Reply to
Mike Towner

A schematic (LTspice or graphic) would be nice.) ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

Superposition. Tie three resistors to the non-inverting input. R1 goes to your input signa l. R2 goes to your offset. R3 goes to ground. By superposition, you sum the solutions with each source grounded. The voltage at the non-inverting inpu t is Vin *(R2||R3)/(R1+R2||R3) + Voffset * (R1||R3)/(R2 + R1||R3). Isn't th at what you're looking for?

Reply to
Wanderer

No, of course it CAN be done with passive components.

If your signals are microvolts from an antenna, or 110VAC, or 3.2 Amperes DC current driving an electromagnet, or resistance fluctuations in a carbon microphone, the amplification problem gets completely different solutions. For a start, if you can get two power suppllies (one positive, one negative) your op amp no longer needs rewiring for different input polarity signals. If you know source impedance and the input range, you can select an input resistor for an inverting amplifier to suit that range. I'd recommend an inverting voltage-gain amplifier, because that is easy to protect against input overvoltage, and after the FIRST amplifier stage, all the other signal characteristics can be handled without worrying about signal-induced smoke.

An LM358 op amp is flexible enough to do LOTS of different things. We don't know the one you're interested in, yet.

Reply to
whit3rd

A voltage divider between the input signal and some fixed positive voltage. (wikipedia only shows voltage dividers referenced to ground, but you can reference them to other votages too)

This will reduce the peak-to-peak amplitude of the signal and push it towards the fixed positive voltage (by the inverse ratio of the two resistors).

if you can give details on how much load your signals can tolerate what their voltage ranges are and what voltages and how strong the output needs to be someone here will be able to assist with details.

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Reply to
Jasen Betts

Yes, from what I have learned so far that seems to be the simplest solution.

And use a trmpot instead of R2 and R3 to adjust the offset.

Mike Towner

Reply to
Mike Towner

If you use passive, you must necessarily attenuate your signal input. If you go active, the signal does not necessarily get attenuated.

Reply to
bloggs.fredbloggs.fred

You might be able to get a little -ve supply by putting diodes in the supply return line. It all depends.

If using a pot to adjust input dc, put a cap across it & feed the opamp via a resistor. But as pointed out more than once, it's impossible to be specific until you give more info.

NT

Reply to
tabbypurr

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