Recommended 10nA precision DC bench multimeters for around $100  - Page 1
Forum section: electronics / general

author: maiakaat
author_rank: Regular Contributor
timestamp: August 16, 2012, 02:40:00 pm
content: HiI was wondering if anybody had recommendations for Bench multimeters with good general accuracy, and precision/lsb in the 10nA range for Digital signals and about 20,000/40,000 counts minimum, so a minimum range of around 10 microamps or better.My current standard multimeter has drifted, and up to this point I have needed its microamp range on a 4000 counts display, and suspect as I learn more and do better design I am likely to need a little extra precision over the next year or two.(Apologies for the poor thread title and clarity initially, I hope this offers a better description)So far I have identified the Thurlby 1905a as the best value multimeter available with them going at auction for £40 to £50 in the UK, I like its math functions, and I am not really interested in diode tests or frequency counting, as the other multimeter (and oscilloscope for the frequency) can do these tasks.

author: PA4TIM
author_rank: Super Contributor
timestamp: August 16, 2012, 03:40:25 pm
content: And I'm looking for a 100GHz tube scope in a box not larger as 10x10 cm and under 100 dollarYou ask DC nA. That is no problem for most 5,5 or 6,5 digit meters (depends on the resolution and magnitude, 10A with 1 nA resolution is something else as 1mA and nA resolution.) but if you want tracability you need to reserve money for a calibration. But think about the Burden voltage of the meter. the voltdrop over your meter could be to much.But then i hope I misread your message. you ask for 100MHz, digtal signals, DC and 40-50 pound in one sentence, to speak with Airosmith: dream onAnd why you want to measure the current of a digital signal ? To measure the powersupply you are only talking DC. So why the 100MHz question.Contact Keithley or Agilent, if you pay enough the maybe develope one for you. A TRMS meter with very high crest factor and a 100 MHz bandwidth will cost a fortune.

author: maiakaat
author_rank: Regular Contributor
timestamp: August 16, 2012, 04:18:39 pm
content: @PA4TIMHito clarify on this post, the Thurlby 1905a is a good multimeter that I think would do a good job, I have watched these go for around £40-£50, I am just inviting comments on other good multimeters with precision at or below 100nA, and very good accuracy, this isn't really unreasonable, if this is the only multimeter that can do this, that will go for this price second hand then I will make sure I snap up the next one.

author: maiakaat
author_rank: Regular Contributor
timestamp: August 16, 2012, 04:23:35 pm
content: Your second comment I will cut out and answer on post on the thread containing the question, but good brand name RF signal generators do go for under £100 on Ebay... whether they can do the job for me I do not know...(should have made clear I meant I was working in the sub 1A range, digital signals and low power DC distribution for those circuits)

author: free_electron
author_rank: Super Contributor
timestamp: August 16, 2012, 05:51:10 pm
content: It would help if we know what you are trying to measure. 100nA is no problem... The question is are you happy knowing if it is 100 or 200na or do you need to know if it's 100.1 or 100.0 nA.

author: maiakaat
author_rank: Regular Contributor
timestamp: August 16, 2012, 06:34:28 pm
content: HI precision of around 100nA would be minimum, but ideally about 10nA or 20nA per step if possible.Voltages we work with are likely to be be 2V or above in most cases, but usually below 12V, typically around 2.5 to 3.6VI have a very cheap Mastech from a while ago, and an Meterman/Amprobe 35XP which has lost some accuracy, so a bench multimeter with the extra bit of precision would be ideal just to add finer measurement accuracy with the ability to double check the Meterman/AmprobeMy use is small signal circuits with both analog sampling and digital signals and low current power distribution, with the multimeter just to support the high frequency voltage measurements with an accurate current sample now and then, along with a small amount of work on simple DC to DC circuits.I guess suggestions for readers on lower cost, older bench multimeters that have reasonable accuracy precision and reliability with strengths and weaknesses would help a lot of potential readers out who would like a 20000 count bench quality multimeter in the £50 to £120 range.There is a good number of Vicky and other Chinese Multimeters but the precision is terrible and there is no record of reliability.I would spend more if a second hand multimeter could have this with a fast sampling rate at 100nA or 1 microamp (ie 50 per second or more), and an output (which is rare), but really something like the Thurlby looks perfect for the price. Frequency counting is irrelevant to me.But sharing some brand and model knowledge beyond the obvious might make a useful thread, and give me an extra option or two in the process.

author: maiakaat
author_rank: Regular Contributor
timestamp: August 16, 2012, 06:36:33 pm
content: The Thurlby also has some math functions, which make it attractive

author: PA4TIM
author_rank: Super Contributor
timestamp: August 16, 2012, 08:44:33 pm
content: I do still not understand the question but seeing your second reaction I think the topic title misses a word or my crystal ball stopped working.You look for a DC nA meter onder 50 poundsand you forgot to mention the word generator between "and" and " 100MHz" so besides a meter you look a generator that does 100 MHz ?But like Free (small world) asked, we need to know what you want, you mix up definitions.What you want for :- accuracy , that is a % or ppm or counts plus a full scale error. This tells the possible error right after fabrication during a defined periode under defined conditions ( temp, humidity) and if it is a good brand there is a great change you can adjust it to within those specs. If you now buy a used one it can be 5% off or it can be perfect. You will not know. Also tempco is important if you are not in a lab.I have a lot of precision gear and the difference because of temperature between summer and winter on most of my gear is around 40 uV. If it is 5 ppm/C and i calibrate your meter at 25 degrees but it is 18 degrees in your shop the difference in result will be 35 ppm. If 10 nA is the resolution and you measure 30nA 35 ppm will give a big fault.Edit: the specs for the tti are not great. 210 uA range, 210.000 is 1nA resolution but it is 0.1%R + 0,002%FS + 2 counts so if you measure 100 uA the real value will be between 99.894 and 100.106 so you have 1nA resolution but 106 nA is uncertain. So the 1nA resolution is nice for adjusting but not to know a usable value. In that case accurate resolution 1uA. Burden is 220 mV max ( changes with current)To compare a keithley 199 is 0.05% +15 digits but has only 100 nA resolution and 400 mV max burden. but the usable range is about the same 100 uA will be 00100.0 uA that gives a 0.15 uncertain part so 99.85 tot 100.15 ( or 99.9 or 99.8 on the display) so you know as much as the not too accurate TTI. 199 is software calibratable and they are famous fot holding there calibration but 50 pounds ?? You could be lucky.- resolution, the smallest change in value on the display. This depends on the range and number of digits. A 5,5 digit can display f.i. 1.000,00 So the resolution is 10 uV, but 5V will be 05.000,0V a 100 uV resolution. For current the same. Is the lowest range is 20 mA the resolution will be 20.000,0 so 100 nA. The last digit is always uncertain so you will have 100 nA resolution but not 100 nA accuracy. That can be +/- 300 nA of even +/- 1 mA. You calculate that from accuracy.So for 10nA resolution the smallest range must be 2 mA for 5,5 digit. But then the load must not draw more as 2 mA .- range , see above- tracabillity, is it calibrated and until when and against what and what conditions- repeatablity, if you measure 10 times, do you get the same results every time- speed, sample rate, display update, filtering and using all digits will slow it down. My 7,5 solartron using filtering has a display update that is measured in many seconds ( it takes forever)Meters in that age and price can have very good specs, when the were made 20-30 years ago. In the mean time a lot of things can happen from bad repairs to total out of specs but it can also be in topshape. You will not know until you spentd a couple a hundred on calibration an alignment.Besides that, you need low resistance cables, low EMF connections, have to deal with seebeck effects, EMI, if you use coax things like triboelectric effects. Useing farraday cups ect. Not just two el cheapo wires with to much resistanceAnd you have the burden voltage. The internal resistance over the meter causes a voltage drop. steve has a nice video about that. On 2.5 Volt powersupply this can be a real problem.Good alternatives are Keithley 199 ( i have one, it is going up to 399999 and that gives greater range) , 196 ( 6,5 digit i got one, from Free electron) or A HP 3456 ( nice meter) or a fluke 8500 ( but this is a nightmare to calibrate and even new they had problems but accuracy is great) but there are more models around, not very much however. Fluke/Philips made a few models.Problem is that a good one in uncalibrated state will be 150-350 dollar, a calibrated one more. For 50 pound you need a lot of luck.I do not know the meter you talk about so I do not know the specs. Before i look i must know what you want.Genertors: 100 MHz fixed, from 100 up, max 100 ?sinewave, pulseModulation, output power, specs, digital reading, harmonic suppresion, output impedance, sweep options, range ect. You have to tell more for good answers.Under 100 pound you can build one using POS from minicircuits, i am building one that does 25-2000MHz, sweep, AM, FM. +10dBm down to -50 dBm, second harminics 50-70 dBc.There are more people looking for generators as there are offered. Good choises are R&S, Hameg, HP, Marconi, Tektronix ( in a TM5000 rack) ( mostly pulse generators) but also brandslikr GW instek, leader probably made them.Fred, pa4tim

author: Zad
author_rank: Super Contributor
timestamp: August 17, 2012, 05:23:12 am
content: You can make yourself a nA (and pA) meter for $5 + whatever Voltmeter you want to use on it. Just make a transimpedance amplifier. Once the signal is in the voltage domain, all the processing becomes a lot easier. No burden voltage either.http://en.wikipedia.org/wiki/Current-to-voltage_converterhttp://www.national.com/assets/en/appnotes/national_AN-1803.pdf

author: free_electron
author_rank: Super Contributor
timestamp: August 17, 2012, 10:50:35 am
content: Looks like the TS doesnt need a nA meter at all.A nanoampere meter for me would be a device that on 100nA RANGE gives me 5 1/2 or 6 1/2 digits.He needs a machine where the lsd is 10na.... So for 4 digit meter his range is actually 10mA ... For a 5 digit machine he is in the 100mA range....Keep you eyes peeled for a used 3478a or a 3456a. Those will do fine.True microa pere meters use a transimpedance system and have a wide powersupply, however, they are only usable for slow signals as they are bandwidth limited. The supply cant track....

author: maiakaat
author_rank: Regular Contributor
timestamp: August 17, 2012, 12:50:16 pm
content: Hisorry I mixed up two posts in the title, ignore the 100Mhz thing

author: maiakaat
author_rank: Regular Contributor
timestamp: August 17, 2012, 12:59:23 pm
content: @PA4TIMI only just saw the 100Mhz, its not relevant to this post, I probably had a bad hair day yesterday.Some good info in there,I'm going to mod the post title again, and as mentioned further down I am looking for a meter with a least significant digit of around 10nA, I'll have to remember to be less vague with engineers, anyway, its a shame I can't clean the thread up.

author: T4P
author_rank: Super Contributor
timestamp: August 17, 2012, 02:08:13 pm
content: Can you look at the rigols? They're value for money.Suggest you don't look at the TongHui's

author: maiakaat
author_rank: Regular Contributor
timestamp: August 17, 2012, 03:17:09 pm
content: The Rigol are well out of the price range new (especially in the UK) for a multimeter, although I am curious about the new Signal generator range which are affordable when were ready to commit to a purchase.I got a UK DS1102E for months old with receipt and cal for £286, which for the UK is quite good, but their price in the UK new is much higher than their price in the US (sometimes double)Staying clear of Vichy, Mastech, Owon, and Uni-T, very cautious of Atten and Hantek

author: maiakaat
author_rank: Regular Contributor
timestamp: August 17, 2012, 04:46:04 pm
content: The specs on many for bench use are crap, and since this is a post about bench units, they are best avoided.Some of the oscilloscopes have also received borderline negative reviews, and there is no well established long term reliability, which means to me, it is my opinion that the same money on a second hand HP/Agilent/Hameg/other is money better spent.Why so aggressive, its a differing opinion I didn't just kill your family?

author: T4P
author_rank: Super Contributor
timestamp: August 17, 2012, 05:00:31 pm
content: No. 2nd hand digital scopes are going to be shit and that's what it is.Nah, a owon is money well spent.

author: maiakaat
author_rank: Regular Contributor
timestamp: August 17, 2012, 05:14:25 pm
content: I'm not interested in digital scopes second hand,.On memory depth limitations you mentioned, Agilent revamp involved less than or close to 1Mpts of memory, its not all about depth of memory, or level of current calibration, oscilloscope design is so complex this comment is as baffling as some of my original post content.Also a second hand digital multimeter or non ARB signal generator doesn't have the same limitations due to age of design.Mastech and the others may go on to develop a good reputation for reliability beyond the 6 month purchase period, but the specs for bench multimeters seem very poor, and paying for calibration is a better option IMHO.I don't want to get into a flame war, and I have no reason to attack Chinese brands that haven't developed a solid reputation, likewise... as open as I am to information, I am entitled to my opinion, which is probably much more flexible in admitting when I am wrong than some people out there in the world. I also try not to take a differing opinion personally, but see it if I can as a sharing of knowledge, as collaboration is better than conflict and competition. It is difficult not to take your input as overly aggressive though.