Society of Robots - Robot Forum

Electronics => Electronics => Topic started by: Sylvestre on August 22, 2013, 06:45:05 PM

Title: Confused About Ground Connection
Post by: Sylvestre on August 22, 2013, 06:45:05 PM

Hello everyone,

I am designing the current sensing circuit for my robot, but I am a little confused about one thing.

I am using this module with an Allegro chip
 http://www.ebay.com/itm/50A-100A-150A-200A-Bi-Uni-AC-DC-Current-Sensor-Module-arduino-compatible-/111040360152 (http://www.ebay.com/itm/50A-100A-150A-200A-Bi-Uni-AC-DC-Current-Sensor-Module-arduino-compatible-/111040360152)

As you can see, it outputs an analog voltage that is directly proportionate with the current passing through the chip.  This analog voltage is too high for the Beaglebone Black (MCU Im using) so I'm using a voltage divider to lower the voltage. I know ground is a reference point from which voltages are measured.

So my question is does it matter where I connect my ground on the voltage divider?

Do I have to connect it to the GND on the board itself or could I connect it say to the 5v- rail on my power distribution board?  Its a bit hard to explain the question so if you want me to clarify, let me know.

Title: Re: Confused About Ground Connection
Post by: jwatte on August 23, 2013, 11:50:54 AM

Does it matter?
If all voltages "seen" by the sensor and BBB are within spec, then you're not doing something "wrong."
If one of the chips will "see" a voltage above VDD or a voltage below VDD, then you'll probably let the magic smoke out.
To figure out which voltages are "seen" by the chips, just do the resistive divider math, checking all the hook-ups.
Remember to account for the source impedance of any output pins involved!

So, for example, if your sensor is connected to +5V and GND, and your BBB is +3V and GND, and you have a -5V rail, and you connect a 2:3 voltage divider to the output of the sensor:

When the sensor is at full output, the voltage across the divider is (+5 - -5) or 10V. The voltage at the division point relative to -5V is (3*10/(2+3)) or 6V. The potential relative ground is 1V.
When the sensor is at ground output, the voltage across the divider is (0 - -5) or 5V. The voltage at the division point relative to -5V is 3V. The voltage potential relative ground is -2V. If this is hooked into a BBB input, it will fry that input.

So, if I understand your question correctly, you probably want to divide between the output and GND if you're forwarding the signal to a single-ended GND-referenced analog input pin.

Title: Re: Confused About Ground Connection
Post by: Sylvestre on August 23, 2013, 09:38:50 PM

Ok thanks for the detailed post. 

Quite a lot of information to swallow!  I think I understand though.

Thanks