ADS1015 Convert bytes to voltage

Felix Pregitzer

Prodigy 170 points

Other Parts Discussed in Thread: ADS1015

Hey there,

I want to meassure the voltage of a battery (from 3.2 to 4.2 Volts).

So I have written a Programm in Python and use a Raspberry Pi. Im using the SMBus library.

So first I write to the register:

bus.write_word_data(0x48, 0x1, 0x03c0)

I am in the +- 6.144V gain Mode single ended and in continious mode (right??).

and the I read the conversion register (2 Bytes)

bus.read_i2c_block_data(0x48, 0, 2)

So when my Battery is plugged to channel A0 with exactly 4.2V and to GND I am getting back an array with [87, 112].

us.write_word_data(add, 0x1, 0x03c0)
How can I convert this 2 Bytes to my Voltage ( 4,2Volts)??

I tried the following:

the 2 readed bytes together 696 + 896 (in Bits) Im getting are1592 right?

the ADS1015 is connected to %v (from raspberry Pi) so I can calculate the maximum range as followed....

5V/6.144V * 4096/2 = 1666,66

when I now calculate my Bits (1592) into a Voltage Im getting 4.776 Volts.

What am I calculating wrong?

Im looking forward receiving some support.

Kind regards Felix

over 9 years ago

0 Krunal Maniar over 9 years ago

TI__Intellectual 1610 points

Felix,

Thank you for your inquiry. Your configuration assumptions are correct. You are in continuous conversion mode with +/-6.144V Full-scale range setting.

The mistake is in the calculations.

87 in 8-bit binary: 01010111

112 in 8-bit binary: 01110000

[87, 112] in 16-bit binary > [0101 0111 0111 0000]

Since the last four bits need to be ignored, your final 12-bit binary word is: 0101 0111 0111

Binary to Hexadecimal: 577,

Binary to Decimal: 1399

Now, let's convert this decimal number to voltage below:

For +/-6.144V Full-scale range,

Output Voltage = 1399*(6.144)/(2^11-1) = 4.19905 V

Anyways, there can be other explanation methods too. Please let me know if you have any other questions.

Thanks,

Krunal

0 Felix Pregitzer over 9 years ago in reply to Krunal Maniar

Prodigy 170 points

Krunal, thanks a lot.

do you think its possible to meassure the voltage from 3.2 to 4.2 Volts with this device? which sample rate do I need and how can I change it?

thanks again.

0 Krunal Maniar over 9 years ago in reply to Felix Pregitzer

TI__Intellectual 1610 points

Yes, you can measure the voltage from 3.2 V to 4.2 V with this device using the same configuration setting that you used previously. The data rate selection depends on your application requirement. The device offers data rates from 128 SPS to 3300 SPS. You will need to update the Config Register . Please review the Registers section of the data sheet (http://www.ti.com/lit/ds/symlink/ads1015.pdf) for more details.

Thanks,
Krunal

0 Felix Pregitzer over 9 years ago in reply to Krunal Maniar

Prodigy 170 points

thanks again.

can you explain me why I devide through (2^11-1)??

So for the value 3,3 Volt I should get back the decimal 1066.......but how do I know which bytes do I get back? is there any Rule between the 2 bytes? (if one is full then it starts to fill the other or something like that)???

Many thanks again

Felix

0 Krunal Maniar over 9 years ago in reply to Felix Pregitzer

TI__Intellectual 1610 points

Hello Felix,

Thank you for your inquiry again.

A small correction, the division should be by (2^11) and not (2^11-1) according to DATA FORMAT section.

For your second question, I would highly suggest you to review the DATA FORMAT section as well as the CONVERSION REGISTER section in datasheet. From the ADS1015 device perspective, the data is stored in a 16-bit Conversion Register and not as '2 bytes'. The 16-bits are D11, D10, D9,... D1, D0, 0, 0, 0, and 0. The first 12-bits of data (D11, D10...D1, D0) are in binary twos complement format as shown in the DATA FORMAT section. The last four bits of the 16-bits are always 0.

Thanks,
Krunal

0 Felix Pregitzer over 9 years ago in reply to Krunal Maniar

Prodigy 170 points

thanks again Krunal.

But that means I can always convert the 2 entries of the array to binary, concatenate them together and convert them to decimal to sjow the meassured voltage?

Felix

0 Krunal Maniar over 9 years ago in reply to Felix Pregitzer

TI__Intellectual 1610 points

Yes, for your case since your are doing positive single-ended measurement and far from 0 V. Remember, it's just the first 12-bits of the total 16-bits that you should convert it to decimal. Also, keep in mind that the device output is in binary twos complement format. If you are measuring signals close to 0 V, I would highly suggest you to use the logic associated with binary two's complement format since the internal offset of the device could flip the MSB bit (D11) and the output will be incorrect if you have the simple logic of 12-bit binary to decimal conversion. Again, please review the DATA FORMAT section and you could do an online search for two's complement format for more details.

Thanks,
Krunal

0 Felix Pregitzer over 9 years ago in reply to Krunal Maniar

Prodigy 170 points

Hey Krunal,

I have tested the Device with the adafruitads1015 Breakout board and everything works fine.

Now I want to bring it to a PCB by myself. My question is if i can directly use the 5V and GND or do I have to use the Ferrites and the capacitor as in the attached PDF?

When I only use A0 do I have to connect the outers to GND? What about ADDR and the Alert? to GND or leave them open?

Many thanks again.

Felix

0 Krunal Maniar over 9 years ago in reply to Felix Pregitzer

TI__Intellectual 1610 points

Felix,

Were you trying to send me a PDF? In general, we highly recommend to use a 0.1 µF bypass capacitor between VDD and GND.

What do you mean by 'outers'? The other unused inputs can be tied to GND.

ADDR pin sets the I2C address and can be tied to GND, VDD, SDA or SCL. This address is required to communicate with the device.

The ALERT/RDY pin can be configured as a digital comparator output or a conversion ready pin. If ALERT/RDY pin is unused, the pin should be left open.

Thanks,
Krunal

0 Felix Pregitzer over 9 years ago in reply to Krunal Maniar

Prodigy 170 points

sorry I missed the PDF. So i connect all the other inputs to GND and the ADDR to GND with a 10k Resistor as in the PDF?

Thanks a lot.Adafruit_ADS1015_12Bit_I2C_ADC.pdf

0 Krunal Maniar over 9 years ago in reply to Felix Pregitzer

TI__Intellectual 1610 points

Felix,

Please connect the ADDR pin as well as the other pins directly to GND without a 10k Resistor.

Thanks,

Krunal

0 Felix Pregitzer over 9 years ago in reply to Krunal Maniar

Prodigy 170 points

ok thanks. So laso without the ferrites? and you would use a 0.1uF capacitor between VDD and GND instead og 10uF?

Thanks a lot for your help

0 Krunal Maniar over 9 years ago in reply to Felix Pregitzer

TI__Intellectual 1610 points

Felix

It all depends on the noise on your supplies. Would you know the rms noise on your supplies? I guess, looking at your schematic, you can keep the ferrite and use a 0.1 uF capacitor on the supplies instead. You already have a 10 uF there.

Thanks,

Krunal

0 Felix Pregitzer over 9 years ago in reply to Krunal Maniar

Prodigy 170 points

I dont think its very noisy. But anyway they dont cause any problems right?

Krunal thanks a lot for your help.

0 Krunal Maniar over 9 years ago in reply to Felix Pregitzer

TI__Intellectual 1610 points

Felix,

You should be good as the device offers some power supply rejection as well.

Regards,
Krunal

0 Felix Pregitzer over 9 years ago in reply to Krunal Maniar

Prodigy 170 points

thanks Krunal.

Im a little bit confused. Im doing the reading as before but now im getting back different bytes and when I do the calculation as before Im getting back a different voltage (battery has 3.7 and im getting back 4.5)

do you have any suggestion what happened?

0 Krunal Maniar over 9 years ago in reply to Felix Pregitzer

TI__Intellectual 1610 points

Hello Felix,

Few things to try:

1. Please make sure your analog battery voltage at the input is at 3.7 V.

2. Since you were able to capture the data correctly previously, I would check your conversion from 12-bit binary to decimal first and then decimal to output voltage. Also, force the same voltage, 4.2 V that you forced previously and see if you can verify the results again.

3. If there are no concerns with 2, please probe all the digital pins with an oscilloscope and evaluate the waveform to confirm that it follows the data capture sequence as specified in the datasheet.

4. Also, please check the TIMING REQUIREMENTS section in datasheet and ensure that you are not violating any timing requirements to communicate with the device.

Thanks,
Krunal

Data converters

Data converters forum

ADS1015 Convert bytes to voltage