Voltages different across outputs? / Calibration Questions... - Page 1
Forum section: electronics / general

author: jasonbrent
author_rank: Regular Contributor
timestamp: February 03, 2018, 01:14:25 am
content: Folk,First, I'm a noob at the component level, but growing knowledge here has been a goal... to that end, I've recently acquired a Keithley 2015THD with a calibration that was due 11/4/16 and a Keithley 2231A-30-3 power supply (... and a keithley 238 that I'm not quite sure why I bought... or if it even works, but that's another story). I've also got an E3610A I've had for a few years....So my first assumption is that the 2015 is reasonably correct as it's the closest thing I have to precise at the moment. I've had the equipment powered on the last 24 hours so they should be as stable as they are going to be.I've keyed up 2.5volts on each of the 3 outputs of the 2231A and am seeing the following on the 2015:Output 1: 2.497xx - 2.498xx volts. The last two digits are unstable and the flickers to 2.498 volts is infrequent.Output 2: 2.499xx volts. Only the last two digits fluctuate and 2.499 is stable.Output 3: 2.4999x - 2.50000 volts. It's stable to 4 decimal points with only the 5th jumping around and flipping it over to 2.5 volts.My E3610, when reading 2.52 volts, produces a stable 2.499xx on the 2015... I've know the E3610 to be out of calibration, but functional since I got it.. and I think I know how to calibrate it within tolerances.I've considered buying the 10volt reference from voltagestandard.com as a sanity check on the 2015 and then the rest of the bits on my desk.So, questions:1) is it normal for there to be this much variance across outputs?2) I think output 1 is out of spec for the 2231A, and the last 2 outputs are within spec. I assume each of these outputs can be individually adjusted inside of the 2231A, but I'm not quite sure how yet (I've yet to look for service manuals). Any insights?Apologies if this is too newb for metrology.Thank you,-j

author: amspire
author_rank: Super Contributor
timestamp: February 03, 2018, 01:32:53 am
content: First, the 2231A specs say the output voltage accuracy is 0.06% + 20mV, so all those outputs are well within specifications. Forget the power supply as an issue.There will be a similar story with the E3610A.With the recent calibration, betting on the Keithley accuracy is a pretty good bet, but many people have found these convenient references fairly accurate, as long as they come with the actual voltages of the 2.5, 5.0, 7.5 and 10v outputs written on the back.https://www.aliexpress.com/item/AD584KH-4-Channel-2-5v-7-5v-5v-10v-High-Precision-Voltage-Reference-Module/32722959699.htmlThis particular one doesn't seem to have the voltages included. It is not mentioned or in the photos.A number of the other Chinese references like this:https://www.aliexpress.com/item/AD584-High-Precision-Voltage-Reference-Module-4-Channel-2-5V-7-5V-5V-10V/32663163130.htmloften come with a bit of paper with printed calibaration data that does not apply to the board you have just used.

author: jasonbrent
author_rank: Regular Contributor
timestamp: February 03, 2018, 01:58:47 am
content: I missed the +/- 20mV in my reading, doh.Thank you.

author: amspire
author_rank: Super Contributor
timestamp: February 03, 2018, 02:04:43 am
content: This cheap reference does seem to come with actual calibration data:https://www.aliexpress.com/item/The-AD584-high-precision-voltage-reference-has-been-calibrated-with-Agilent-34401/32814692533.htmlThe good thing with these 4 voltage references is you can tell if the calibration is fake. If it is geniune, any difference between the Keithley and the reference will similar for all outputs. So if you disagree with the 10V reference calibration data by +0.001%, you should see a similar difference on the other outputs.If you did get one of these, what I would expect to see is that the reference calibration data says the 10V is 10.00032V. Your Keithley might say it is 10.00025V. In this case, trust the Keithley and the reference is just a confirmation that nothing has gone wrong in the Keithley. The Keithley had a better internal reference then any of the cheap references you can buy.

author: jasonbrent
author_rank: Regular Contributor
timestamp: February 03, 2018, 02:08:23 am
content: At that price point, I'll take the gamble... and in 19-39 days, can hopefully remember why I ordered it. =)-j

author: amspire
author_rank: Super Contributor
timestamp: February 03, 2018, 02:13:54 am
content: Quote from: jasonbrent on February 03, 2018, 01:58:47 amThe 2231A has an output resolution on 10mV. What that implies is that it is using an internal digital to analog converter to set the voltage, and 10mV is equal to one single bit on this D/A. It is the lowest resolution it can manage. Even on a good D/A, errors equal to +/- 5mV would not be unexpected. The 20mV is probably the manufacturer being ultra-conservative.Given that, I think the accuracy of the power supplies is actually better then I would have expected.

author: jasonbrent
author_rank: Regular Contributor
timestamp: February 03, 2018, 02:25:09 am
content: I've got a lot to learn...I'm unsure why I would have expected a device that can only present 2 decimal places of precision to be accurate at 3+ decimal places on another device... I think I'm trying to validate that the various components I've purchased are basically doing the right thing... and I'm sure they are for the foreseeable future as I learn.Thank you for taking the time to look at the datasheets What you've explained removes some of my "I'm a noob and unsure of what I'm thinking" fog.-j

author: amspire
author_rank: Super Contributor
timestamp: February 03, 2018, 02:42:56 am
content: For a beginner, you have some very nice gear. How did you manage to get a Keithley 2015THD? I have never seen one before.A high end multimeter that also measures harmonic distortion.

author: jasonbrent
author_rank: Regular Contributor
timestamp: February 03, 2018, 05:51:46 am
content: Quote from: amspire on February 03, 2018, 02:42:56 amI tend to over-fund hobbies; see TEA thread and GAS. I picked most of this up off of eBay over the last few weeks.The 2015 happened to be from a local seller for 5 bones. It wasn't an exceptional deal as there are others similarly priced, it just appeared to be in very good condition relative to other options.The 2231a was a great deal until the last few minutes, but still well under new pricing, and came in original box with unopened parts bags.The 238 was a random acquisition by throwing a lowball offer that was accepted for 4 bones... I didn't realize I would end up spending close to that again for 3lug triax. :/Speaking of that 238, I only have one triax to alligator at the moment, but was able to verify it's voltage outputs with the 2015 just a few minutes ago since it has both triax and banana for the Lo side. It appears stable and accurate through the voltage range.... not been successful with amperage readings from it yet. The 238 was in the worst condition with lots of missing screws and such.Sent from my SM-N950U using Tapatalk

author: jasonbrent
author_rank: Regular Contributor
timestamp: February 03, 2018, 11:12:20 pm
content: I was able to take some voltage measurements between the 2015 and the 238 and was pleasantly surprised. The 238 voltage output is only out of spec at +/- 100volts by a small margin. On the output side side, the 238 voltages exactly matched what the 2015 read throughput the range. I didn't test input voltages beyond 30v, but the 238 read like for like with the 2015. It's kind of beat up, but $400 for the 238 wasn't a horrible price it looks like.

author: amspire
author_rank: Super Contributor
timestamp: February 04, 2018, 01:03:37 am
content: You really have some great bargains. I don't know what the new cost for gear like that is but you probably have at least $10K worth. Hate to think what kind of oscilloscope you will find. The only problem you have now is you cannot blame bad equipment any more. There are no excuses.

author: jasonbrent
author_rank: Regular Contributor
timestamp: February 04, 2018, 01:11:20 am
content: A few years back I picked up a ds2072a but sold it to a friend. I've got a ds1054z at the moment... that will suffice for now. For the "basics", I'm just missing a dc load and a signal generator (i have a 1mhz generator). .... and on the rf side of the house I'm hoping to add a spectrum analyzer at some point (ham radio is also a hobby).I troll government auctions for good scope buys, but haven't ever succeeded there.Anyway.. I'm off topic for metrology at this point. Thank you again!-jSent from my SM-N950U using Tapatalk