burned ADS1115 when read voltage (differential)

Other Hardware Sensors
1

Hy guys,

i try to measure 2 different signals, +-10V and +-20mA.

to measure the current of the signal i took a shunt resistor of 10ohm and measured the voltage across the pins A2 and A3 if the ADC (differential mode). this part is working very well.

now i tried to read the voltage signal over an voltage devider (R1 47k, R2 10k).
before i connected the ADC, i checked it with voltmeter --> OK.

Then i connected the ADC with the pins A0 and A1 and activated the differential measuring mode between 0 and 1.
after connecting the ADS1115 burned off immediately!

but why this is happened?
did i something fundamentally wrong?
i read that the ADC has an impedance of 15M ohm, so there is no big current to the ADC.
was it caused of differential mode?! i have absolutely no idea...

i have only one in spare, so for the next test i should know what i do :wink:

i attached a screenshot of the wiring.
thank you!

Anmerkung 2020-06-26 192712.png
Anmerkung 2020-06-26 192712.png1040×766 83.9 KB

2

All input voltages to the ADS1115 MUST be between 0 and the ADS1115 supply voltage, at all times.

3

sorry, i dont understand your reply.

what do i have to connect?

4

Read the sensor data sheet carefully and follow this guide.

You cannot ever connect negative voltages, or voltages above about 5V, directly to the ADS1115.

That is how you destroyed the first one.

5

yes i know, therefore i used the voltage divider (see picture).
isnt it correct?

6

No, it is not correct, which explains exactly why you destroyed your ADC.

7

can you please give me a hint (sketch), how you would wire this?

i know that its caused of my fault, but i dont know why...

8

Post the details of your +/-10V source. What is it, how is it wired, how is it powered, links to data sheet, product page, etc.

9

a simple dc 19V (transformator to charge laptops) output with i guess 4A max.
i designed the resitors that i can measure +-24V too.

so my wiring should be correct, like the img?

10

So far, nothing you have said makes any sense. A 19V laptop power supply is NOT a +/- 10V source.

Please repeat this sentence to yourself until it finally sinks in:

"I cannot measure negative voltages with the ADS1115".

I'm tired of this, so good luck with your project.

11

i think you have less idea then me...

i try to explaine more excately for you:

at my left there is my breakout board with 30 rows and 5 rows, which are marked with the letters f, g, h, i and j.

in this breakout board there are cables with different color. some cables are green, yellow, orange (a bright orange). but i have some fancy colors too, which looks more like neon.

two wires (green and blue) are connected to the laptop charging device. green for + and blue for -.
i was struggeling with myself, cause usually i like the color red and black for the source, but after few seconds i took the blue and the green one. cause it was too chaotic in my wooden wire box to find a cable with 2 females.

when i make the test with the current source i have a signal generator where i can turn a small knob, which is black whith a small white dot on it. this outputs an source between another 2 cables which have the same color - crazy, i know (green and blue). oh, i forgot to mention - the green and blue wires connected to the power supply for the generator.

back to the output. this output deliver 20mA to the green cable and the blue is the reference of the generator (GND). when i rotate the small knob (previously mentioned) counterclockwise, the current decrease to 0mA. whe i still rotate it, it changes the output to the blue cable and now the green cables is the reference GND.

this cables are connected to the shunt 10ohm resistor. the green cable is also connected to the A0, and the blue to the A1. then i can activate the differential mode and the ADS1115 take the reference between A0 and A1 - not to the GND in the ADC.

so... now i test the voltage thing....

i removed all this stuff except the power supply on my breakout board (19V).
the wires still blue and green.

then i have searched a while to find the correct resistors.

after this i took the resistors and plugged it into the breakout board, so that i have connection to the green cable, which is now the positive (19V).
then i put the resitor next to the other one and connected it to the blue cable, which is 0V.

now i connect the green (of the A0 from ADC) to the middle of the voltage devider, and a second blue wire (to A1 of ADC) to the existing blue 0V wire.

now i tested it with my small voltmeter and the voltage was correctly devided.
then i connected the ADC.... now it was making booom... small white smoke was visible... like right after they have choosen the pope in the good old vatican sometimes...

when i want to make a minus V input i can easy change the blue and the green cable... but i didnt reached this step of the test...
.

now i have that much time to explain you the wiring and color of my circuit. previously i thought its easier to make a more professional circuit diagram like this one at my first post. but sometimes people dont understand it correctely.

12

patrick_bit4bit:
previously i thought its easier to make a more professional circuit diagram like this one at my first post. but sometimes people dont understand it correctely.

Yes, we do understand that diagram, much better than the useless story in the last post.

What you don't understand is "differential mode" and "common mode".

You can measure differential voltages, but both inputs of the A/D must stay within the common mode range of the A/D.
Which is in this case between ADS ground and ADS supply.
Leo..

13

thanks for your reply Wawa.

Sorry for this senseless previous post, but i felt a little hopeless cause i still dont know what i have to change in the circuit.

as far as i understand, according the docs of the ADS, it measures the differential voltage across the A0 and A1 without use internal ground (see block diagram of ADS). (of course only when i activate the differential mode in the software)

i know that i have to range the voltage below the ADS supply 5V - this is what i tried with the voltage devider (1,75V to A1/A0 @ 10V input).

it was working properly, when i had done the test with +20mA and also -20mA source.

isnt it the case when i use differential mode that the internal OP is using the input source of A1 as the reference (see img)?
am i wrong in any of these statements?

ads1115-blockdiagram_enh.jpg

Anmerkung 2020-06-27 080337.png
Anmerkung 2020-06-27 080337.png1096×761 87.6 KB

14

We need to see where all those voltages come from, and if/how they are connected to each other.
You can't just connect a 'floating' voltage to ADS inputs.
Leo..

15

i havent more connected, as these wiring you can see in the image...

but why it worked for the +-20mA source...??

16

The two voltages must be connected together somehow, otherwise the ADS (with 10mA input clamping diodes) won't have burned up.
Why won't you show us exactly where they are coming from.
Leo..