Current source issue using AD8221/AD8616 - Page 1
Forum section: electronics / projects

author: ECEdesign
author_rank: Regular Contributor
timestamp: June 14, 2016, 06:10:41 pm
content: Hi,I am trying out this circuit from the AoE with a few modifications but it isn't working and I'm not sure why.I have made a few modifications including a swap from the LT1490 to the AD8616 which is also a precision RRIO op-amp but it is limited to a supply voltage of 5V so I am powering it from 5V. See my post-it note for a schematic of how I have my circuit setup. Do you think it is necessary to use the LT1490? I have a LT1492 which also might work.The circuit does not have any current limiting, it just takes whatever I give R3 in terms of current. I have only given it up to 400mA since Im worried something has gone wrong and dont want to blow things up.

author: danadak
author_rank: Super Contributor
timestamp: June 15, 2016, 01:27:18 am
content: AD8221 input connections need to be reversed ? I am thinkingthere is + fdbk in ckt as it now exists ? Unless I am not lookingat this right.Similiar circuit in this ap note -http://www.analog.com/media/en/technical-documentation/application-notes/AN-968.pdfRegards, Dana.

author: rs20
author_rank: Super Contributor
timestamp: June 15, 2016, 01:35:39 am
content: AD8616 has a supply of 5V, so it can't output more than 5V. However, in order to turn off the FET, the gate voltage has to be close to 10V. Hence, MOSFET will never ever come close to turning off with the hand-drawn circuit you've shown.Also, make some debugging effort! Try putting 0 amps and 1 amp through the current measuring resistor, and see how the output of the two op-amps varies. Does it behave the way you expect? Just throwing up your hands and asking a forum is not a very good learning experience for you.

author: ECEdesign
author_rank: Regular Contributor
timestamp: June 15, 2016, 02:19:27 am
content: Quote from: rs20 on June 15, 2016, 01:35:39 amYeah I thought about this so I switched the AD8616 to the LT1492 which has a much higher max supply voltage and I am now powering both the LT1492 and R3 from the same 10V rail although the problem remains.I am sorry I haven't done much more debugging I have played around with the voltage supplies but I am a beginning EE student and I am quite ahead of my coursework in this project and am currently pretty lost!I am looking for some nuggets of information like you gave about trying to do different currents across the sense resistor, I will try that out! Although I must admit I am not sure what I am expecting to see... I will definitely do some more reading but thought it was worth posting here to see if there was something obvious a newbie like me was missingQuote from: danadak on June 15, 2016, 01:27:18 amThanks for the note I will look at it! I will also try reversing the inputs of the AD8221 to see what will happen

author: rs20
author_rank: Super Contributor
timestamp: June 15, 2016, 05:38:06 am
content: Quote from: ECEdesign on June 15, 2016, 02:19:27 amGood stuff! You'll do much better than the students who are just barely fulfilling the requirements of the course.Quote from: ECEdesign on June 15, 2016, 02:19:27 am500 milliamps through the 0.1 ohm should produce a 50 millivolt drop across the resistor (V=IR). The next block has a gain of 10, so after that you should see 500 millivolts. These figures should scale, of course, with the current that you pass through the resistor. EDIT: I agree with Dana's remarks, I don't get why the inputs are that way around. As shown in your diagrams, I'd expect the output after the AD8221 to be-500 millivolts, which is definitely not useful (and indeed, not possible if your negative supply there is not a negative voltage).The AD8616 block is a little bit more confusing, because if you're feeding the current directly through the current sense resistor, the MOSFET has no control over that current, so the AD8616 will go to a hard maximum or hard minimum, turning the MOSFET hard on or hard off in a desperate attempt to change the current going through the sense resistor. Specifically, if the current through the resistor is more than what you've "programmed" with the DAC, the AD8616 should go high, and thus turn off the MOSFET (the AD8616 sees that the current is higher than it should be, so it "tries" to reduce it.)