What do you connect to IN+ and IN- for ADC differential input configuration

adcdifferentialraspberry pi

I was trying to understand how to read analog inputs on a raspberry pi. I found this video post and I think I understood how it works.
https://www.youtube.com/watch?v=oW0mTwFck_c
This video uses a potentiometer and read its analog outputs through MCP3004 microchip.

In this video, he says to use "1" for the single / diff control bit, and he didn't explain much about the differential input. I was curious, and after some searches, I found that it was used to reduce noises when to read analog outputs.

Now, when I look at this diagram,
http://i.gyazo.com/f86370f39c199b4b8a4fe345b1486b09.png
I understand how to use CH1, 2, 3, 4 for single input type (you just connect analog sensor output to one of them), but for the differential, it says IN+ or IN-, and this gets me really confused.

What do I need to connect for the + and -?

Can anybody explain what to do when you have a potentiometer to read just like the video example?

Best Answer

As the name suggests, a differential ADC is used to measure the difference between two signals.

For a simple potentiometer, there isn't really two different signals to measure, so you're measuring the difference between the wiper and ground.

schematic

simulate this circuit – Schematic created using CircuitLab

When working in single-ended mode the - input of the ADC is connected to ground internally for you, so you basically end up with the same circuit anyway.

Now, the fun comes when you have a more complex circuit. If your potentiometer forms just part of a chain of resistances, and you need to know exactly the voltage across just the potentiometer, then the differential ADC comes in very handy:

schematic

simulate this circuit

Now with the ADC you are measuring the difference just between the wiper and the lower point of the potentiometer. The other resistors are then ignored by the ADC.

Any strange fluctuations in the voltages in that resistor network are also ignored, since the same fluctuations will be proportionally the same throughout - at both the wiper and at the lower side of the ADC, so they will not be sensed by the ADC.

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