MAX6325 Precision Reference - Page 1
Forum section: electronics / beginners

author: hgg
author_rank: Frequent Contributor
timestamp: March 04, 2014, 07:26:19 pm
content: Hi,A quick question.I am using the MAX6325 2.5V precision reference and I cannot understand something.Its voltage output is a steady and noise free 2.497V regardless of the power supplyvoltage, from 8V to 15V. When I place a load of 10mA on the power supply, then thereference output will increase by 2mV. I am not placing any load on its output.Can someone explain that behaviour? Is this something that is expected?Thank you.

author: Christopher
author_rank: Frequent Contributor
timestamp: March 04, 2014, 08:09:56 pm
content: line regulation. Measure the input and output voltage with 10mA on the input. Wouldn't have thought it would have effected it too much, though.What capacitance do you have on the input and output?

author: Andreas
author_rank: Super Contributor
timestamp: March 04, 2014, 09:30:42 pm
content: Quote from: hgg on March 04, 2014, 07:26:19 pmDefinitely not. You are doing something wrong.Either GND-Line or R42 are oscillating.With best regardsAndreas

author: hgg
author_rank: Frequent Contributor
timestamp: March 04, 2014, 10:44:22 pm
content: QuoteWITH NO LOAD WITH LOADVcc = 9.008V Vcc = 8.981V 27mV dropVout = 2.497V Vout = 2.499V +2mVI am connecting a 1K resistor with an LED from VCC to Ground.The MAX6325 output is floating.The thing is that if I drop the power supply down to 8V, there is no difference to the output.Why is there with only a 27mV drop? Only if I connect a load to Vcc.Voltage regulation of the IC version I am using is 30ppm/V.I am using the following setup:QuoteI see no oscillation on the oscilloscope.What is R42 ??

author: Kevin.D
author_rank: Frequent Contributor
timestamp: March 05, 2014, 12:14:38 am
content: Are you probing gnd close to the ref or at the V source ? ,have you got a poor gnd and your losing 2mV in the gnd trace when you connect load .

author: hgg
author_rank: Frequent Contributor
timestamp: March 05, 2014, 07:56:58 am
content: Kevin you are right!If I probe the ground from the power supply terminal, then I get a 2mVrisewhen I connect a load.If I probe the ground on the breadboard then I get a 1mV voltagedropwhen I connect a load.Only if I measure the voltage just next to the reference pins I get a steady reading regardlessthe load I connect to the power supply.The breadboard ground rail resistance has to play a role here as well. It has a 3.5R resistance atbest and a steady 3.5M ohms from positive rail to negative.

author: hgg
author_rank: Frequent Contributor
timestamp: March 05, 2014, 09:58:33 am
content: I am trying to build a precision reference for multimeter calibration.Things look good now down to 3 significant digits.It will have a voltage output of 2.5V, a Vishay S102K precision resistor and a 2.5mA current source.I have placed an order for a chopper stabilized op amp LTC1049 for the final setup.Of course it will need to be calibrated at the end by a high accuracy meter which I don't have...One question.Do you think that if I use IC sockets for the chips they will affect its accuracy or drift?(I have some good quality gold plated ones.)

author: Andreas
author_rank: Super Contributor
timestamp: March 05, 2014, 08:42:05 pm
content: Quote from: hgg on March 04, 2014, 10:44:22 pm"42" is the answer to "all questions" (Douglas Adams)Meaning: Too few information that one can answer the questin (no schematic, no picture of the wiring).Now with the schematics:Although the MAX6325 has a minimum supply voltage of 8V you should supply a well stabilized voltage of around 10V.Below 10V the line regulation is much worse than above 10V. Not much more than 10V because of self-heating of the chip.Well stabilized: so that the self-heating of the chip will be constant.Star ground wiring is essential for precision cirquits to avoid GND shifting.And you should never use sockets: In my ageing box I use sockets. It went a long time good. But since a few weeks some of the references (sockets) are drifting away with up to 120 ppm where formerly the drift was below 1ppm/month.With best regardsAndreas

author: hgg
author_rank: Frequent Contributor
timestamp: March 05, 2014, 09:45:10 pm
content: Andreas thank you for your advice.I will be using a 9V battery with the voltage reference circuit because I want it to be portable.It looks pretty stable and noise free down to 8.0V and with a 2.5mA load on the referenceoutput.QuoteWhat about if I use a ground plane in the final PCB design? All the return signals willfollow the path of least resistance. Do you suggest that a star ground wiring will be abetter approach in this case?George.

author: Andreas
author_rank: Super Contributor
timestamp: March 06, 2014, 07:57:46 pm
content: Quote from: hgg on March 05, 2014, 09:45:10 pmHello,if you know what you are doing -> with a star ground design you will get better results than with a ground plane for low frequency analog. Additionally the thermocouple effects will be more symmetrically without ground plane.But its a question to which precision (stability) level you want to go. (0.1%, 0.01% (thermally controlled), or below)With best regardsAndreas

author: hgg
author_rank: Frequent Contributor
timestamp: March 08, 2014, 07:09:44 pm
content: QuoteOk, I see. Thank you Andreas.The MAX6325 uses a low power temperature compensation scheme to achieve temperaturestability. It does not require to be heated. I would be more than happy if I can achieve a 0.01%precision.That is why I've also bought the LTC1049 chopper and zero drift op amp to use it as a voltage followerfrom the 2.5V reference through a 0.01% precision Vishay 1K resistor, in order to have a precision currentsource of 2.5mA as well.http://cds.linear.com/docs/en/datasheet/1049fb.pdfPreviously in my test circuit I was using the LM358N op amp and it was working fine. The 2.500V fromthe reference was very stable and remained that way when I connected the 1K load at its output.But when I replaced it with the LTC1049 I could not make it to work. I doubled checked all theconnections.I removed the LTC1049 to test it on a separate board. I used voltage divider to feed its non invertinginput (pin 3), and connected pin 2 to 6, for a voltage follower setup. I get strange results.Vcc+ = 9.013VInput from Reference = 7.785VOp Amp Output Pin 6 = 8.012VI've connected its output to the scope and I get horrible oscillation:In the datasheet it is mentioned that the op amp does not need external capacitors because theyare inside. But worst than that, when I connect the 1K ohm load to its output and ground, the outputdrops to 0.8V ! So either I got a faulty op amp or I am doing something terribly wrong.How can I test the LTC1049 or any other op amp to make sure that its not faulty?Any ideas of what might be happening?Thank you.p.s. this reminds me of an ancient Greek saying: "The enemy of the good is the better"...

author: Andreas
author_rank: Super Contributor
timestamp: March 08, 2014, 09:00:34 pm
content: Hello,The LT1049 is not specified for a 1K load. (only above 10K)The maximum output current is in the range of 1mA.The precision version of the LM358 is the LT1013 (or OPA2234)You could also try a LTC2057 which has more output current than the 1049with best regardsAndreas

author: hgg
author_rank: Frequent Contributor
timestamp: March 09, 2014, 07:39:20 am
content: Quote from: Andreas on March 08, 2014, 09:00:34 pmHello Andreas.I am beginner in electronics. I may have chosen the wrong op-amp for a reference buffer...I am looking at the datasheet and I cannot explicitly find its current output capability. Howdid you find that its maximum current is 1mA? The 10K is mentioned in the Maximum Outputvoltage swing but this value is just a test condition. One of its applications mentioned in thedata sheet is 4mA to 20mA Current Loops. (If 1ma is the maximum maybe I can use a BJTat its output.)But that is not my problem to start with.Before I even connect a load at its output, I get a value slightly higher than the input anda terrible oscillation shown in the graph above.Is this normal?Thank you.

author: hgg
author_rank: Frequent Contributor
timestamp: March 09, 2014, 09:46:00 am
content: Andreas you are right.I don't know where you found the 1mA specification but this is indeed the maximum it can sourcewith a 15V Vcc. 0.70mA at 8V Vcc. I connected a variable resistor as a load and its output startedto drop at 1mA/15V.I managed to find one of the few op amps that cannot source more than a 1mA...

author: Kevin.D
author_rank: Frequent Contributor
timestamp: March 09, 2014, 12:34:12 pm
content: The oscillation is probably your crappy breadboard striking again (parasitic capacitance) . If you look at the phase/gain graph plot for that opamp in the data sheet it's barely stable (only 15 deg phase margin and thats with no load capacitance), so it's not really suited to be used at a gain of 1 without some compensation, it will overshoot even without any load cap ,and it would only take a small output capacitance to eat up a 15 deg phase margin and cause it to oscillate .It will be stable though at higher gains ,try at a gain of 10 and see if it's stable then .

author: hgg
author_rank: Frequent Contributor
timestamp: March 09, 2014, 12:55:36 pm
content: QuoteLOL!Yes probably, because it decreased today. A 0.1uF capacitor at its output made it a lot morestable with 5mVpp ripple. Very touchy op amp. I managed to create a steady 2.5mA current source witha BJT at its output and the feedback sensing its emitter, but the precision resistor has to be tied to groundat all times. I need the current source for multimeter calibration, so I need the op amp output to be floatinguntil you short to ground the load resistor with your multimeter, in order to take the reading. So I thinkI will have to choose a different op amp.QuoteI need more reading on that because I am not sure I understand it well.Thanks.

author: hgg
author_rank: Frequent Contributor
timestamp: March 09, 2014, 03:23:17 pm
content: Ok,I am between three op-amp candidates in the same price range:1)ICL7650S by Intersil.http://docs-europe.electrocomponents.com/webdocs/0e27/0900766b80e27842.pdf2)TLC2654A by Texas Instruments.http://docs-europe.electrocomponents.com/webdocs/0c90/0900766b80c90e79.pdf3)LTC1250 by Linear Technologyhttp://docs-europe.electrocomponents.com/webdocs/10ed/0900766b810ed836.pdfOk, I already don't like Linear Technology...(Please state source current explicitly guys. Its not difficult...)Output source current, Io of TLC2654A is+/-50mAmax and of ICL7650S4.5mAtypical.I would be sourcing2.5mA.The TLC2654A looks like a better option for a precision current source. What do you think?To avoid further mistakes any suggestions are appreciated.Thank you.George.

author: Andreas
author_rank: Super Contributor
timestamp: March 09, 2014, 08:35:39 pm
content: Quote from: hgg on March 09, 2014, 03:23:17 pmYou are looking at the wrong data. The maximum ratings are those where the chip may be destroyed.Take a LT1013. Your resistors will be much worse than this chip.With best regardsAndreas

author: hgg
author_rank: Frequent Contributor
timestamp: March 09, 2014, 08:53:36 pm
content: QuoteIntersil is more honest and lists the typical value (4.5mA) of the output source current.There is only a maximum value of +/-50mA in the TI datasheet so I think its safeto assume that it can easily source 2.5mA.QuoteThe 1K resistor is the Vishay metal foil S102K series with 1ppm/C and tolerance of ± 0.005 %http://www.vishaypg.com/docs/63001/63001.pdfIn what respect they are much worse?Thank you.

author: hgg
author_rank: Frequent Contributor
timestamp: March 12, 2014, 04:32:54 pm
content: Hi,Still haven't decided on which chopper op amp to use for the 2.5mA current reference...I might buy at the end the LTC1250 which explicitly says that it can drive 4.3V into 1K load from a single 5V supply.http://docs-europe.electrocomponents.com/webdocs/10ed/0900766b810ed836.pdfbut,The TLC2654A by Texas Instruments has a 0.003uV/mo offset long term drift and a much lower Idd.http://docs-europe.electrocomponents.com/webdocs/0c90/0900766b80c90e79.pdfI am still not sure if it can drive a 1K load @ 2.5mA though.Can somebody help me by having a look at the datasheet above and see if he can datamine... that info?Thanks.