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ADS1115 PROBLEMS
Moderators: adafruit_support_bill, adafruit

Please be positive and constructive with your questions and comments.

Re: ADS1115 PROBLEMS

by adafruit_support_bill on Tue Jun 27, 2017 5:10 pm

The non-linearity may be due to slight variations in the actual values of your resistors. Voltage dividers are inexpensive and simple, but not always the best solution.

I will check with our analog circuit expert. He should have some good suggestions for your circuit.

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Re: ADS1115 PROBLEMS

by Stamos on Tue Jun 27, 2017 5:29 pm

Hello Dear Bill, i just worked with single ended mode on the emitter and i got the same values i need!!

Thank you for your patience!! if you have any other suggestions, about my issue, it will be welcome!!

Thanks a lot

Stamos

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Re: ADS1115 PROBLEMS

by adafruit_support_mike on Wed Jun 28, 2017 1:01 am

The 2.7 ohm emitter resistor gets all the current through the laser diode and all the current through the 3.3v Zener diode. That was probably a part of the nonlinearity in your earlier measurements.

Your original idea of measuring the voltage across the 5.6 ohm resistor below the laser diode was correct. This circuit will convert that voltage to one you can measure with an ADC:

level-shifter.jpg
level-shifter.jpg (30.01 KiB) Viewed 249 times
The op amp controls the PNP transistor to make the voltage across the 5.6k resistor equal the voltage across the 5.6 ohm resistor. When 444mA flows through the 5.6 ohm sense resistor, 444uA will flow through the 5.6k.

That 444uA then flows through the two 10k resistors, whose parallel resistance is 5k. The voltage across the resistors will be 2.22v, which makes it easy to calculate the current through the laser diode.

The 5.6k resistor steals 0.1% of the current that flows through the laser diode, but that error will remain constant across the 0v to 2.22v output range.

The PNP transistor will steal some of the 444uA as base current, so it's best to use a high-gain transistor whose current gain remains stable across the 0uA to 444uA range. A 2n5087 would be a good choice. Its average current gain is around 400 in that range, and doesn't change much.

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Re: ADS1115 PROBLEMS

by Stamos on Wed Jun 28, 2017 3:18 am

Dear Bill

Thank you for your imidate answer and a very nice scematic..

I would also like to ask if the Amp is ok to be another TLC271

I will keep it as i have to give it to the university for the presentation (not enough time)
And i will work on the new one

Thank you
You will have news from me

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Re: ADS1115 PROBLEMS

by Stamos on Wed Jun 28, 2017 10:50 am

Hey Bill

is TLC271 ok for the AMP for the scematic you sent me??

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Re: ADS1115 PROBLEMS

by adafruit_support_bill on Wed Jun 28, 2017 11:18 am

Mike is our analog expert and he provided the schematic. Mike works the 'late shift' here. I will leave a note for him to get back to you on op-amp recommendations.

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Re: ADS1115 PROBLEMS

by adafruit_support_mike on Thu Jun 29, 2017 12:59 am

The TLC271 should work. It will work from a 12v supply, and it has high input impedance so the error from current flowing into the + and - inputs will be very small.

It looks like the TLC271's highest output voltage will be about 1.5v below its supply voltage, but that shoudn't cause any problems. In the circuit above, the output voltage will always be lower than the bottom of the 3.3v Zener diode.

Make sure you measure the input offset voltage though (the voltage between the + and - inputs). According to the datasheet, the offset can be as high as 10mV, which would create a 1% error when the voltage across the 5.6 ohm sense resistor is 1v.

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Re: ADS1115 PROBLEMS

by Stamos on Thu Jun 29, 2017 4:51 am

Do you recommend something better, did your schematic work with another Amp for the voltages you show me on the schematic Above?? i am about to make an order, thats why :D

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Please be positive and constructive with your questions and comments.