Need a solid 3-4 channel DMM - Page 1
Forum section: electronics / testgear

author: Phaedrus
author_rank: Frequent Contributor
timestamp: July 12, 2013, 02:15:57 am
content: Hi y'all!My company is upgrading its lab and I'm looking for a DMM with specific requirements. We do a lot of testing of multi-output (ATX) SMPS. Currently this is all done by hand. We have a few DC loads, a power analyzer, a quad channel Tek scope, a Fluke, and a custom break-out board. I want to replace the Fluke with a bench DMM with at least 3 (preferably 4) inputs, which is reasonably accurate and is LabView compatible. I don't need crazy resolution or anything, 4.5 digit would be more than adequate for our needs, but extra wouldn't hurt. Also it would be nice if it had a touch-hold measurement scheme similar to the 34461A that Dave reviewed, as that would make manual testing a heluva lot less hassle as well.I'm looking at a budget of $1500-$2500, since I'm fairly confident I can swing that with management. I could go a bit higher if I really had to, up to $3500 say? But that would be eating into my budget for other instruments, and I really want a function generator and another scope for prototyping.The end goal is to have a fully automated test rack where I just plug in the PSU, enter the min/max ratings of the PSU into a laptop, then let a LabView program handle the rest with regulation, ripple, and efficiency measurements at every possible load condition.So to reiterate:* Bench Multimeter (rack mount compatible)* 4.5 digit or better* 3 or 4 simultaneous inputs* LabView compatible* Nice touch-hold functionality? Optional* Budget: $2500, absolute max $3500If this doesn't exist I suppose I could just get a 34461A and build a multiplexer board of some sort to switch the input voltages to the probes. But that'd be a hassle; I'd rather just have an instrument that does what I need out of the box.Anyway, thanks for your advice!

author: EEVblog
author_rank: Unknown
timestamp: July 12, 2013, 02:24:20 am
content: The usual no-fuss solution is to use multiple system meters with a single channel each, all networked.Or a bunch of National Instruments multimeter cards:http://www.ni.com/digitalmultimeters/The Agilent meters are cheaper than the National Instruments cards!

author: Phaedrus
author_rank: Frequent Contributor
timestamp: July 12, 2013, 02:34:48 am
content: Multiple DMMs are nice in theory--but at that point we'd practically need two racks I think! At a previous job we had a similar rack setup with just one DMM (manual regulation test only) and the rack was just about full. Floor (and desk) space here is at a premium.The NI cards are a bit rich for us, yes.

author: ve7xen
author_rank: Super Contributor
timestamp: July 12, 2013, 02:43:45 am
content: If you don't need simultaneous or high speed sampling, I think the Keithley 2700 with a single 7700 module satisfies all your needs. 6.5 digit, half-rack and a 20-way mux for around about $2k. For a couple hundred more the 2701 has Ethernet which might save you the cost of GPIB stuff etc.

author: EEVblog
author_rank: Unknown
timestamp: July 12, 2013, 02:49:23 am
content: Quote from: Phaedrus on July 12, 2013, 02:34:48 am4 Agilent meters does not take up much room.Less than 500mm x 200mm x 300mm deep.

author: mtdoc
author_rank: Super Contributor
timestamp: July 12, 2013, 02:54:53 am
content: If you only need to measure ATX range voltages, why not use a multichannel DAQ board? If you're proficient programming LabView it would be trivial to set up. If you don't need high speed acquisition, the DAQ board should be within our budget.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: July 12, 2013, 03:13:22 am
content: A DAQ might work, but I would prefer an actual meter for two reasons.1) I'm actually not a LabView expert, I helped with some programming at my previous work, and I've started programming our Tek, but I do not have gobs of experience. The LabView stuff will be programmed by me in my down-time. I actually do three separate jobs here (product manager, marketing rep, engineering consultant) so it will take some time for me to finish the programming. In the meantime we need good manual measurement.2) We're expanding this lab's function to include R&D, instead of just testing what we get sent from HQ. I don't know what our requirements will be in the future, but having a high-quality bench DMM will probably be useful.The Keithly 2701 and 7700 look ideal. The measurements don't have to be exactly simultaneous by any means, just fast enough to read all four inputs in a few ms. And inside budget. And being a half rack makes life a little easier for me, as I can stash it in next to the power analyzer (also half rack). Beauty.EDIT: Yup, looks like we can fit it all on a 16U rack with this solution.Thanks all!

author: AlfBaz
author_rank: Super Contributor
timestamp: July 12, 2013, 04:15:35 am
content: With the budget you've got you should end up with a nice solution.I scored a HP3457A with a 44491 relay card for $275.Whilst I haven't measured the time between channels at NPLC of 1 I can sample 4 channels (DCV) in under half a second. Makes a hell of a racket with the relays chattering but it does the job.I'm a complete noob at labview but just now I'm using the relay card to check resistance drift of a trim pot compared to a fixed resistor and ambient temperature over time. I'm using a thermistor to measure temperature in 2 wire mode, converting it to temperature, then switching to 4 wire mode to measure the pot and resistor and plotting them on 3 separate XY graphs... Seems to be working a treat.

author: PedroDaGr8
author_rank: Super Contributor
timestamp: July 12, 2013, 05:38:20 am
content: Does it have to look nice?If not, the Keithley 199 should fit you bill. Most come with the 1992 relay card so it can scan 8 channels plus either the front or back inputs. They aren't the most attractive (very late 80s retro) in the brown and tan coloration but they can scan multiple inputs, have lab view control and can typically be had for well under $200. Plus they have the option to zero out the offset, AC measurements are standard, etc.

author: casinada
author_rank: Frequent Contributor
timestamp: July 12, 2013, 06:29:09 am
content: I don't know how close is to be obsolete but they still sell them. You can get the 2000 or the 2001 with the scanning card option. the other suggested options are also valid.http://www.keithley.com/products/dcac/dmm/broadpurpose/?mn=20002000-SCAN

author: Robomeds
author_rank: Frequent Contributor
timestamp: July 13, 2013, 03:14:48 am
content: Quote from: AlfBaz on July 12, 2013, 04:15:35 amI was going to suggest the same thing. The HP cards can be setup to read quite a few channels quite quickly. Certainly worth a look.

author: Unknown
author_rank: Unknown
timestamp: July 13, 2013, 03:23:10 am
content: Unless you need very fast sampling, I would use either a DMM with built-in scanner (the Keithley 2000/2001 is the only current model that comes to mind, the already mentioned HP 3457A or Keithley 199 would be good options on the used market. Another alternative would be dedicated switching units like the HP 3488a. Used these tend to go for cheap since they're fairly useless outside ATE and similar applications. In all cases make sure you get the correct relay cards. These are usually optional and might come in several types. For example one for voltage/resistance, another for thermocouples, another for four-wire resistance, another for RF.A DMM with > 1 actual channel (as in ADC) is going to be very rare outside data acquisition boards, and not really necessary unless you need sampling rate in the kS/s.