# circuit for mesure positive & negative voltage + Vref resolution

i would like to make a "voltmeter" to read from +2V to -2V and connect it to an Arduino ADC ( 0V to 5V ).
I found different configurations for do this job:

1. From this site:

2. Bipolar to unipolar converter, a summing amp:

3. Instrumental amp, base on this schematic ( ECG ):

The last approach use:
- instrumental amp
- full-wave rectifier
- low pass filter
- inverting opamp + gain

The signal i need to read it's "stable", the ECG circuit i suppose it's too complex for this job. But i'm interested to compare the instrumental amp Vs opamp.

What is the best configuration for my porspose?

For the Vref, I have this question:

In the article he say, for example:
Vref at 5v -> 0-5 V has 4.9 mV resolution
Vref at 3.3v -> 0-3.3 has 3.2mV resolution
But in the video does not say that, he used Vref at 1.1v and apparently does not have any problem.... probably the solution is here in the code.

• "... but i want to discuss about those." This site doesn't do discussions. Ask a question instead. There is none in your post. – Transistor Jun 5 '18 at 21:32
• I have fixed my post, sorry. – MarkCalaway Jun 5 '18 at 21:45

This does not directly answer your question but may give you an idea for a different solution.

simulate this circuit – Schematic created using CircuitLab

Figure 1. The simplest offset adjust.

If you can live with reduced resolution on the ADC the circuit of Figure 1 may be acceptable.

Vin       Vout
-2 V      +1.5 V
0 V      +2.5 V
+2 V      +3.5 V


It reduces the 5 V span to only 2 V with the resistor arrangement shown.

I think the best span you can get with this arrangement will be by reducing R1 to 2k. With this ratio the ADC starts at 0 V.

Vin       Vout
-2 V       0 V
0 V      +1.428 V
+2 V      +2.857 V


You can scale the resistors to suit. The ratio is what is important.

• Nice solution @Transistor, but i had done some changes in the circuit: I must use an external ADC and a single Lipo battery. This is the circuit i draw: ibb.co/cv9Ony You can simulate here: tinyurl.com/y8ct5e87 Is it a good solution? – MarkCalaway Jun 17 '18 at 11:50
• Only to clarify... This humble 2-resistor network is the possibly simplest voltage summer with weighted inputs. It can be easily analysed by the superposition principle: Vadc = Vcc.R1/(R1 + R2) + Vin.R2/(R1 + R2) = (Vcc + Vin)/2. It can be thought as of a voltage divider driven from both ends. It is usually used to convert the bipolar voltage from -5 to +5 V into positive voltage from 0 to +5 V. – Circuit fantasist Dec 6 '19 at 21:41