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### Topic: Wiring advice of the TI ADS1115 (Read 697 times)previous topic - next topic

#### MLUND92

##### Dec 09, 2017, 07:06 pmLast Edit: Dec 09, 2017, 07:16 pm by MLUND92
Hi there,

I need to read a 4 - 20 ma signal from a pressure transducer using an Arduino Mega. I've considered using a 250 Ohm resistor, and measure the voltage drop with the Mega's analog pins, but the 10 bit resolution of the Mega's ADC is not high enough for this particular application.

For this reason I've found interrest in the ADC 4-20 module from Circuitar, which has an 16 bit ADC (TI ADS1115DGS) : https://www.circuitar.com/nanoshields/modules/adc-4-20/ .

Unfortunately, Circuitar ships outside the EU, so I've decided to make one myself by using the provided circuit from Circuitar: https://www.circuitar.com/media/product/142/files/ADC_4-20_v1.2_EN.pdf .

My questions are:
- What is the function of the 3k3 resistors (R3, R6, R12, R9) on the input of the analog pins of the ADS1115DGS ADC?
- Is this circuit more advantageous compared to just measuring the voltage drop of a 250/150 Ohm resistor with the ADS1115DGS ADC?

Thanks in advance

#### TonyWilk

#1
##### Dec 09, 2017, 07:49 pm
My questions are:
- What is the function of the 3k3 resistors (R3, R6, R12, R9) on the input of the analog pins of the ADS1115DGS ADC?
- Is this circuit more advantageous compared to just measuring the voltage drop of a 250/150 Ohm resistor with the ADS1115DGS ADC?
Referring to channel A0:

R3 and C3 are input filtering for the ADS1115's AIN0 input

R1 and R2 form a 150ohm load to ground for the input 0..30mA

At 30mA, there will be 4.5V across R1+R2, and 3v across R2 which is what the ADC will read.

It looks like they divided the input to provide a 0v - 3v input to the ADC because there is a 'Power supply selection' jumper giving the option of running this from a 3.3v supply.

If you are powering the ADC at 5v and you want to read 4 - 20mA, you can replace R2 with 250ohm and remove R1. Keep R3,C3 for filtering.

Yours,
TonyWilk

#### Wawa

#2
##### Dec 09, 2017, 08:38 pm
I need to read a 4 - 20 ma signal from a pressure transducer using an Arduino Mega. I've considered using a 250 Ohm resistor, and measure the voltage drop with the Mega's analog pins, but the 10 bit resolution of the Mega's ADC is not high enough for this particular application.
Did you read the datasheet of the pressure sensor.
Most of them have a total possible error of 1-2.5%. Way more than a 10-bit A/D (0.1%).
You will get more numbers with a 16-bit A/D, but do they mean anything.

You could use a 120ohm current sense resistor and the Mega's buildin 2.56volt reference.
Add an RC filter between that sense resistor and the analogue pin for protection (like in the module).
Leo..

#### Mikeb1970

#3
##### Dec 09, 2017, 09:24 pm
I am sorry, but did you mean they don't ship to Europe or they do ship to Europe. Because I have tried, and they are willing to ship what you need to where I live (Belgium) I don't have a clue where you live though.

#### MLUND92

#4
##### Dec 09, 2017, 09:34 pm
Referring to channel A0:

R3 and C3 are input filtering for the ADS1115's AIN0 input

R1 and R2 form a 150ohm load to ground for the input 0..30mA

At 30mA, there will be 4.5V across R1+R2, and 3v across R2 which is what the ADC will read.

It looks like they divided the input to provide a 0v - 3v input to the ADC because there is a 'Power supply selection' jumper giving the option of running this from a 3.3v supply.

If you are powering the ADC at 5v and you want to read 4 - 20mA, you can replace R2 with 250ohm and remove R1. Keep R3,C3 for filtering.

Yours,
TonyWilk

Thank you very much for the thorough explanation TonyWilk. That makes sense.

Did you read the datasheet of the pressure sensor.
Most of them have a total possible error of 1-2.5%. Way more than a 10-bit A/D (0.1%).
You will get more numbers with a 16-bit A/D, but do they mean anything.

You could use a 120ohm current sense resistor and the Mega's buildin 2.56volt reference.
Add an RC filter between that sense resistor and the analogue pin for protection (like in the module).
Leo..
You certainly have a point there. Thank you for the suggestion.

I am sorry, but did you mean they don't ship to Europe or they do ship to Europe. Because I have tried, and they are willing to ship what you need to where I live (Belgium) I don't have a clue where you live though.

Thank you for the info. What I meant was that since they don't ship from within the EU (I believe they ship from Brazil), you have to pay customs, which is a tedious process here in Denmark.

#### Mikeb1970

#5
##### Dec 09, 2017, 09:45 pm
MLUND92,

Should you get stuck, and really need the original shield I am willing to order it for you, we only have to pay 21% taxes here, and this can be done by just replying to a text message, but this is normally only for amounts above 30 Euros. If I should send it then to you maybe you wouldn't have to pay custom duties, since we are both in the EU.

I'm just offering my help. Anyway, good luck with your project, I hope you can get it working.

#### MLUND92

#6
##### Dec 10, 2017, 03:12 pm
Mikeb1970,

Thank you very much for offering your help. It's much appreciated. I will let you know, if I don't succeed with making the circuit myself.

All the best

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