# ADC Common Ground

**URL:** <https://forums.pimoroni.com/t/adc-common-ground/8914>\
**Category:** Support\
**Created:** [October 15, 2018, 9:38pm UTC](https://forums.pimoroni.com/t/adc-common-ground/8914 "2018-10-15T21:38:22Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![notkevinjohn](https://avatars.discourse-cdn.com/v4/letter/n/f05b48/32.png) [@notkevinjohn](https://forums.pimoroni.com/u/notkevinjohn)\
**Post date:** [October 15, 2018, 9:38pm UTC](https://forums.pimoroni.com/t/adc-common-ground/8914/1 "2018-10-15T21:38:22Z")

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I am working with a sensor that requires a 10-30VDC source. I am not seeing the proper way to tie together the ground from my voltage source with the ground for the ADC on the board. I tried to connecting the ground pin on the voltage source to the ground pin on the board, but that did not give me the effect I was looking for. Is there any documentation on the proper way to read voltage from a device NOT being powered by the 5V source on the board?

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**Author:** ![gadgetoid](https://sea2.discourse-cdn.com/flex016/user_avatar/forums.pimoroni.com/gadgetoid/32/13_2.png) [@gadgetoid](https://forums.pimoroni.com/u/gadgetoid)\
**Post date:** [October 16, 2018, 10:51am UTC](https://forums.pimoroni.com/t/adc-common-ground/8914/2 "2018-10-16T10:51:07Z")

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That is- insofar as I’m aware- the only way to tie the grounds between your external circuit and an ADC. However:

1. What board are you using?
2. What results did you expect when tying grounds?
3. What results did you get?

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<div class="post-metadata">

**Author:** ![notkevinjohn](https://avatars.discourse-cdn.com/v4/letter/n/f05b48/32.png) [@notkevinjohn](https://forums.pimoroni.com/u/notkevinjohn)\
**Post date:** [October 16, 2018, 4:16pm UTC](https://forums.pimoroni.com/t/adc-common-ground/8914/3 "2018-10-16T16:16:42Z")

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1. I am using the automation hat
2. I expect to get a potential difference between the data pin on the sensor and the ground of about 4.2V, this is the potential I measure from the ground to the data pin using my multimeter.
3. I get a value of around 0.02V, which is why I think the ground is not being properly tied to the ADC ground even when I connect the ground from my 24V source (to power the sensor) to one of the three ground pins on the automation hat.

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**Author:** ![notkevinjohn](https://avatars.discourse-cdn.com/v4/letter/n/f05b48/32.png) [@notkevinjohn](https://forums.pimoroni.com/u/notkevinjohn)\
**Post date:** [October 16, 2018, 5:11pm UTC](https://forums.pimoroni.com/t/adc-common-ground/8914/4 "2018-10-16T17:11:37Z")

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Measuring (successfully) with DMM:

 ![Diagram2](https://us1.discourse-cdn.com/flex016/uploads/pimoroni/original/2X/e/e96c97f30c70cd339218f0dc83e5bae76b670659.png)

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<div class="post-metadata">

**Author:** ![notkevinjohn](https://avatars.discourse-cdn.com/v4/letter/n/f05b48/32.png) [@notkevinjohn](https://forums.pimoroni.com/u/notkevinjohn)\
**Post date:** [October 16, 2018, 5:12pm UTC](https://forums.pimoroni.com/t/adc-common-ground/8914/5 "2018-10-16T17:12:23Z")

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Measuring (unsuccessfully) with ADC/Automation Hat:

 ![Diagram1](https://us1.discourse-cdn.com/flex016/uploads/pimoroni/original/2X/0/03d2df79f5e3c6a52178f600a48612ee4ad623ec.png)

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**Author:** ![gadgetoid](https://sea2.discourse-cdn.com/flex016/user_avatar/forums.pimoroni.com/gadgetoid/32/13_2.png) [@gadgetoid](https://forums.pimoroni.com/u/gadgetoid)\
**Post date:** [October 17, 2018, 11:04am UTC](https://forums.pimoroni.com/t/adc-common-ground/8914/6 "2018-10-17T11:04:54Z")

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The ADC inputs are protected by a voltage divider which is effectively a 940k Ohm resistor to ground from the perspective of the sensor. This small load on the sensor might be what’s affecting the voltage- if the sensor is only capable of supplying a tiny current then it would encounter voltage drop.