Hello

Is there any way that I can make this circuit measure negative voltage beside the positive voltage?

 

There might be a better way, but a precision rectifier before it should do it. I am talking about an opamp circuit, not a diode.

 

Thank you, I actually found someone posted a circuit similar to the required. This circuit below supposed to measures voltages with -3V to +3V range I am not sure which amplifier is good but I am thinking of using MCP6021, and I would appreciate any advise.
One thing I don't understand is why the curve is shifted around 0.5V to the right!

 

Hi Sham,
Please post your LTSpice asc file
E

 

I see an amplifier but I see no means of measuring the output except for the meters in the circuit in post #1.

 

Hi again

Yes I will be using an ADS1115 with ATmega2560..
LTSPICE attached..

 

Hi again

Yes I will be using an ADS1115 with ATmega2560..
LTSPICE attached..

The simulator file, which I can not open, will tell me nothing about any meter you will be using.

 

Is there any way that I can make this circuit measure negative voltage beside the positive voltage?

What outputs exactly, do you need for a positive voltage and a negative voltage?
Do you need an indication that it is a positive or negative voltage?

 

why the curve is shifted around 0.5V to the right!

Because your input signal range exceeds the supply voltage. No opamp can generate an output voltage greater than the supply voltage.

Is there any way that I can make this circuit measure negative voltage beside the positive voltage?

Remove R4 and R5. Ground the non-inverting input of the opamp. The output should then go from ~+5v to ~0V as the input goes from -2.5V to +2.5V. You need a rail-to-rail opamp.

 

What outputs exactly, do you need for a positive voltage and a negative voltage?
Do you need an indication that it is a positive or negative voltage?

Yes sir, I want to know the voltage being measured is +ve or -ve..

 

Then you need a negative supply, or a virtual ground. A single supply opamp cannot output a negative voltage.

But... You cannot input a negative voltage to the ATMEGA.

 

Hi Sham
If you bias the OPA to give +2.5V output when the input voltage is zero, you could write your Atmega program to measure an input voltage range of +/-2.5V
E

 

Yes sir, I want to know the voltage being measured is +ve or -ve..

You didn't answer my question about what output you want. so still cannot give you an answer to your post question, other that "yes" you can.
So far, all your statements have been rather vague.

What exactly is the input voltage range you want to measure, and what output voltage do you want for that range?

 

Sorry for any confusion, as ericgibbs said I want to convert the input voltage to analog 0-5V to use it with ADS1115 and Atmega2560.
The mid point between +ve and -ve range should return 2.5V, while the lowest negative of the range is 5V, and highest positive of the range at 0V.

I want to make two circuits: one with +-3V range, and one +-32V. Both circuits should have accuracy better than +-0.3%..

The one I shared before seems working ok for +-3V and for the +-32V the following:

 

With R2 being such a high value, any leakage current through D1 and D2 may cause inaccuracies.

 

So do you think the protection diodes might be a problem?, is it ok to remove them and be safe from overvoltage?

Also if I want to measure voltage with +-220V the resistor R2 will be in the Megaohms range...

 

again, you are not providing negative supply for your opamp.
and the way you wired D2, negative voltages will be clamped so you will not see change below some -55V. also Opamp inputs even when rail to rail, cannot go negative. and LT6020 has outputs that can go rail-to-rail but not inputs so you have less than 50% of desired range.

consider different input protection, creating virtual ground or using summing amplifier.

 

One thing I don't understand is why the curve is shifted around 0.5V to the right!

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You masked out the opamp model that the simulation is using. Why?

Your 0 V input is resulting in an output voltage that is about 300 mV to high, which means that an extra 24 µA of current is flowing through the feedback resistor. This is very possibly due to the bias current requirements of the opamp. But without knowing the model, there's no way for us to determine if that is reasonable or not.

 

Because your input signal range exceeds the supply voltage. No opamp can generate an output voltage greater than the supply voltage.

Where is the input signal exceeding the supply voltage?

The opamp neither knows nor cares what the V1 voltage is, it only knows about the voltages at it's inputs and outputs.

Besides, the shift he is referring to is for an input voltage of 0 V. Ideally, the circuit should output about 2.5 V.

 

Sorry for any confusion, as ericgibbs said I want to convert the input voltage to analog 0-5V to use it with ADS1115 and Atmega2560.
The mid point between +ve and -ve range should return 2.5V, while the lowest negative of the range is 5V, and highest positive of the range at 0V.

I want to make two circuits: one with +-3V range, and one +-32V. Both circuits should have accuracy better than +-0.3%..

But your thread title says that you are wanting to measure millivolt signals.

Have you considered the impact of your component tolerances?

Have you considered the impact of supply voltage variation, since your circuits are relying on a voltage divider using the supply? What happens if your supply drifts by, say, 20 mV?