Measuring series resistors using constant current source and 16bit ADC - Page 1
Forum section: electronics / projects

author: eddiesomesan
author_rank: Newbie
timestamp: October 15, 2017, 07:36:29 pm
content: Hello. I am working on a project and I`m unclear with something.I`m trying to measure some series resistors using an LM234 as a constant current source which is fed with 5v (see attachment). In the configuration from the attachment I should get about 0.67mA out of the LM234. The voltage reading is made by using some 16(15) bit ADC - ADS1115 from Adafruit in single-ended mode.All the measurements I get are constant, but my issue appears on higher resistance values. If the first resistors (lower values) are measured correctly, as the value of the resistors is increasing, the calculated values are much lower than they really are...or should be. As an example, for the 2490 ohm resistor, I usually get somewhere around 2420 ohm or even lower. Of course, the resistor values are calculated using ohm`s law - constant current and read voltage.All the measured resistors are 1% and in specs.I`ve seenand from 22:12 Dave talks about some voltage buffers which made me think twice about my design.Please give me some hints for improving my design. The measurement accuracy should be +/- 2%.PS. At my roots I`m a mechanical engineer, so don`t shoot meThank you.

author: ebclr
author_rank: Super Contributor
timestamp: October 15, 2017, 08:14:12 pm
content: Are you not be catching by ohms law?V = R * IAt some point, the power supply will not be enough to maintain the current on big resistors

author: technogeeky
author_rank: Frequent Contributor
timestamp: October 15, 2017, 08:21:46 pm
content: I'm a little confused by the diagram. It looks like the ADS1115 has four ADC inputs, but it appears you are using five.Is it really that you are using one at a time, and you're just showing five examples?

author: Benta
author_rank: Super Contributor
timestamp: October 15, 2017, 08:25:13 pm
content: Quote from: ebclr on October 15, 2017, 08:14:12 pmI thought so as well, but the LM234 should be OK for this. Voltage over the resistors is around 2.77 V.

author: eddiesomesan
author_rank: Newbie
timestamp: October 15, 2017, 08:28:31 pm
content: I`m sorry but I don`t understand.For a total series resistance of 4147 ohms (in my case) and a current of 0.67mA, the needed voltage should be at least 2.78 volts. I have 5V. Or maybe I`m missing something?There are 5 ADC`s in the schematic, but I actually use 16. 4 modules with 4 ADC`s each, for 3 series of serial resistors. I have drawn it like this to be easier to explain. Some of the series have only 3 resistors, others have 4 or 5.

author: ebclr
author_rank: Super Contributor
timestamp: October 15, 2017, 08:41:57 pm
content: You have not 5V, the diode takes +- 0,67V, also the Set Current Error is o 2% or more, please check with a meter if the current is really stable as you believe

author: bson
author_rank: Unknown
timestamp: October 15, 2017, 08:55:28 pm
content: I see nothing in the datasheet that states what the compliance voltage is for a given supply voltage, how stable it is with varying loads, whether this device can go all the way to rail and if not how close, or what happens if it gets too close. I'd check with a DMM and a decade resistor box that it's going to do the job...