Can I increase range on uno by connecting analog inputs in series?

(Sorry if I post this twice, but I can’t seem to find the first post. Maybe the system didn’t register it?).

I have a varying signal up to ~10V, can I connect the signal over 2 resistors i.e. split the signal in two and measure over each resistor with e.g. A1 and A2 (or even more)? Do the analog input always measure in relation to ground? And will the other connected inputs affect the measurement of the others significantly?

Morelight:
Do the analog input always measure in relation to ground?

Yes they do, and even the AVRs that can do differential inputs (like ATtiny85) still require both inputs to be within the supply rails.

This is not possible without an external differential amplifier to read the "top" half of the divider.

That is called a differential analog measurement. The arduino is not designed to do that but the ADS1115 has that feature. Before you load the differential examples sketch you have change the configuration line to select the higher resolution ADC.

#include <Wire.h>

#include <Adafruit_ADS1015.h>

Adafruit_ADS1015 ads1015;

void setup(void)
{
  Serial.begin(9600);
  Serial.println("Hello!");
  
  Serial.println("Getting differential reading from AIN0 (P) and AIN1 (N)");
  Serial.println("ADC Range: +/- 6.144V (1 bit = 3mV)");
  ads1015.begin();
}

void loop(void)
{
  int16_t results;

  results = ads1015.readADC_Differential_0_1();
  Serial.print("Differential: "); Serial.print(results); Serial.print("("); Serial.print(results * 3); Serial.println("mV)");

  delay(1000);
}

What’s missing is this code: (copy and pasted from the Single-ended Example sketch)

// Adafruit_ADS1115 ads;  /* Use this for the 16-bit version */
Adafruit_ADS1015 ads;     /* Use thi for the 12-bit version */

You need to replace the 12-bit config line with this one:

Adafruit_ADS1115 ads ADS1115;  /* Use this for the 16-bit version */

SEE attached screenshot of compiled sketch:
ADS_1115_DIFFERENTIAL SKETCH COMPILED.png

DATASHEET

DIFFERENTIAL INPUTS (ADS1115)
The ADS1115 also features two differential or four single-ended inputs

Morelight:
(Sorry if I post this twice, but I can’t seem to find the first post. Maybe the system didn’t register it?).

I have a varying signal up to ~10V, can I connect the signal over 2 resistors i.e. split the signal in two and measure over each resistor with e.g. A1 and A2 (or even more)? Do the analog input always measure in relation to ground? And will the other connected inputs affect the measurement of the others significantly?

By ~10V do you mean definitely <= 10V? If so a simple 1:1 resistor divider and one analog pin
is all you need. If you force any Arduino pin above the supply or below ground you will break it.

If all the voltage sources connected to pins have an impedance of 10k or so or less, the pins will
be read independently to the resolution of the ADC.

I’d suggest using a 10k+10k resistor divider.

The top 10k in the divider also acts to limit current should the input go above 10V, which is a useful
bit of protection.

You have to verify with a meter that the voltage across each resistor is 5V or less, then you can use two voltage dividers to divide the voltage in half for the measurement that is 10V above ground.
The measurement from ground to 5V does not need a voltage divider. By using two analog inputs and multiplying the higher voltage by 2 to obtain the actual voltage and then measuring the lower voltage and subtracting that from the first, you can obtain the voltage across the resistor that is from 5V to 10V.

Using MarkT's suggestion you would get about 9.766mV per step in your analog reading. Are you trying to achieve better than 0.009766V resolution for 0V to 10V input range?

Splitting the signal in two and using op-amps to interface to 2 analog inputs would achieve 11-bit (0.004883V resolution).

Perhaps the solution would be an external ADC chip in the range and bit depth you need.

I think people (or me) are mis-interpreting his question. I think what he wants to do is measure a 0-10v signal and was thinking of the resistive divider, measuring the two halves and adding them together or something, not realizing that if you have two known (and equal in this case) resistors, you measure the voltage across the lower one and multiply by 2 to get the actual voltage reading (so a reading of 5v across the lower resistor means you have 10 volts applied across the two resistors.

Yes that is right. But I think he wants to increase the resolution of the A/D from 10 bits to 11 bits by doing this.

Of course the answer to the question in the title is a simple NO

I think the title should be :
"measuring 10V with ADC"

raschemmel:
I think the title should be :
"measuring 10V with ADC"

Maybe but it wasn't.

Since he has wandered off apparently, we may never know ... another one of sciences great mysteries :slight_smile:

gpsmikey:
another one of sciences great mysteries :slight_smile:

Here are some more:-
questions2.jpg

Those are all excellent questions....

Excellent questions and definitely points to ponder (don't feel bad about the blood test, I got an O- on mine !! )

I'm glad there's no craters around us...