This is an initial overview of the Tektronix DMM4020 Bench-top Digital Multimeter.
This overview was originally written for Element-14 and is available
here.
Unit Price: £528.00
I must stress that this is just an initial overview. The DMM4020 is very much a professional piece of test equipment and needs to be used properly in a work context for some time before I can give a thorough in-depth review. Nevertheless, in the short time that I have had it, I have already learned more than enough to give a good overview.
A few month ago, I had considered that a good bench meter should really go on my list of test gear. Less than a week after its arrival, I wonder how on earth I managed without one. After all, hand-held multimeters are really good now and - it has to be said - more than good enough for most work. So why has a bench meter already earned its place in my lab? Read on…
The meter arrived in sturdy double-layer manufacturer’s box measuring 9”x12”x17”. Inside is a pair of vacuum formed foam blocks which sandwich the meter and provided good impact protection on its journey here. It is the sort of packaging that is worth keeping for storage, should you need to move it around or keep it safe.
Unless you have a huge bench or a permanent place in a rack, then this meter does take up quite a bit of space, being 9 inches wide and 13 inches front-to-back (although it is tiny in comparison to the high end calibration lab stuff). Traditionally though, bench gear like this does appreciate being left switched on permanently, to allow the temperatures to stabilise, minimise thermal cycles and prevent any moisture affecting components as the lab cools down over night. Unfortunately the DMM4020 has a fluorescent display, which has a tendency to fade when left on for extended periods (years). I have not yet discovered a way to turn the display off whilst keeping the electronics running. There is a “soft” power button on the front panel which may well perform a similar function. That is definitely something to investigate. Otherwise, though, the VFD is bright, sharp and fast to react, probably better than LCD could give.
However, this is jumping ahead slightly. The very first thing that I saw – a printed manual. Yes folks, real paper, with print on it. Spiral bound. Hallelujah! Only 26 pages of it are in English (the rest is Japanese and Chinese). It is only a “Safety And Installation” Manual, but it covers the main functions and accuracy tables. The full User Manual is 110 pages but unfortunately only available in PDF format. I will go more into the contents of this manual in the full review. The S&I manual states that power consumption averages 10 Watts, with a maximum of 15 Watts. Each side of the case is plentifully supplied with ventilation holes, and having left the DMM4020 on for 6 hours, the temperature stabilised just above ambient. It runs very cool indeed for mains powered gear.
Caution!
There are two things to watch out for before you start. Most importantly, the DMM4020 uses a conventional transformer power supply which is supplied configured for 110V.
It takes until page 10 of the Installation Guide before it mentions line voltage selection. I imagine almost all users will know to check this first, but in a world where people are increasingly used to switched mode power supplies that take 100-240V without configuration, a caution sticker would probably be wise. The actual method of voltage selection is a bit odd but nevertheless functional; You unclip and remove a module which has the mains fuse and a plastic “spider” inside, which is rotated to suit 100, 120, 220 or 240V and re-inserted. Despite having a mains transformer, the unit only weighs a modest 2.1kg
Secondly, the supplied mains cable has a US standard plug. This is not a problem though, as the connector is a standard IEC “kettle lead” and most labs have plenty of those hanging around! The DMM4020 does have a set of stand-off feet moulded into the rear of the case, which allows it to be operated vertically rather than in the conventional horizontal manner. Unfortunately the supplied mains lead is a “straight through” rather than a 90 degree elbow, and so if you try to stand the meter upright, whether against the floor, desk or slotted into a rack, it sits on the cable rather than the feet, and so is unstable. Note to Tektronix : Supply a 90 degree "Elbow" power lead please!
One thing which is really good to see – a proper mains on/off switch mounted next to the power connector. So far as I can see from looking through the ventilation holes, this is a proper switch which isolates all internal components.
Construction
As you can see from the photo, the DM4020 comes in Tektronix corporate colour. That familiar grey-blue colour which is so often seen around labs. The main body is pressed alloy construction, with plastic front panel. The front and rear both have flexible plastic/rubber protective “boots” which can be removed for rack mounting. As usual with this sort of equipment, there is a tilting bail which can be positioned in a number of angles, used as a carrying handle, or removed entirely. The HRC fuses are of the high quality you would expect, and are accessible by removing a small metal panel underneath the unit. A small plastic box covers the fuses themselves, presumably for insulation and blast containment. They don’t seem as well protected as they are in hand-held meters.
Internally
I don’t intend to take the DMM4020 apart, not least because that would entail breaking the calibration seals, which I am loathe to do. Fortunately, however, you can get a pretty good view of the insides just by looking through the ventilation slots. This is very much a unit for a clean lab and not a dusty shop floor which would risk contaminating the sensitive electronics in no time.
Visible in the middle off the board is the heated precision voltage reference, clearly labelled LM399H with a National Semiconductor logo. A well known and well respected precision voltage source, used in several meters in this market segment. Interesting that they didn’t choose an LM199 or LM299 which seem to be 2% initial tolerance versus the LM399’s 5%, although it has to be said that after calibration there is little difference. I suspect that putting the unit into standby from the front panel will keep the heater running inside the voltage reference whilst powering down the display and processor. This would seem an ideal compromise.
In use.
As you can see from the front panel, the Tektronix DMM4020 is a 5 ½ digit unit, with separate connections for low (200mA) and high current (10A) ranges. It is also capable of full 4-wire resistance measurement, although only a pair of Tek TL710 conventional probes are supplied. Supplied probes are usually low budget affairs, but these are really nice sharp Cat III/Cat IV 10A probes with finger shields. However - no ground clip is provided! This is definitely an oversight in a meter of this stature.
One feature I will certainly be looking at is the ability to take two measurements at the same time, or sequentially. Some meters can perform functions such as AC voltage + Frequency, but not many can perform current + voltage measurements together, or DC+AC, such as might be used to monitor power supply ripple.
You will note that it does not measure inductance or capacitance. I guess at this end of the market you would be expected to purchase a separate LCR meter.
Performance
These are the easiest to quantify in hard numbers. All this data is copied from the official (copious) Tektronix information, boiled down into the main points. I will go into more depth in the main review:
Measurement Speed
The DMM2040 can measure at 2.5, 20 or 100 samples per second. Normally 2.5 is perfectly sufficient, and 20 cannot be distinguished by human eye. However, this is good for capturing short transition events, or for high speed logging via the computer link. In the faster two modes, however, precision is truncated by one digit.
DC Voltage
Maximum input: 1000V
Input bias current: <30pA @ 25C
Input impedance: >10G Ohms (200mV and 2V ranges) 10M Ohms (20/200/1000V ranges)
Resolution on 200mV scale: 1uV
Accuracy: +0.01% of reading + 0.003% of range
AC Voltage (True RMS)
Maximum input: 1000V peak, 750V RMS.
Accuracy (all ranges 45Hz – 20kHz): 0.15% of reading + 0.05% of range
DC Current
Input protection: 11A/1000V and 440mA/1000V fuses
Shunt resistance: 0.01 Ohms (2A and 10A ranges)
1 Ohm (20mA and 200mA)
Resolution: 200uA scale: 1nA (yes, that’s 1 nanoAmp)
Accuracy: typ. 2mA scale 0.015% of reading + 0.005% of range
AC Current:
AC Filter Bandwidth: 20Hz – 100kHz
Maximum Crest Factor 3:1 at full scale
Resolution: 20mA scale: 0.1uA (100nA)
Accuracy: Typ. 200mA scale, 45Hz-2kHz: 0.25% of reading + 0.05% of range.
Resistance
Specification for 4-wire function or 2-wire with REL (relative measurement)
Input protection: 1000V on all ranges
Resolution: 200 Ohm scale : 0.001 Ohm (1 milli-Ohm)
Frequency:
Range: 20Hz – 1MHz, for input >100mV
Accuracy: 0.01%
Continuity:
Threshold: 20 Ohms
Test current: 1mA
Unseen performance
So much for the book data. With less than a week’s use, it is safe to say that I haven’t even scratched the surface on what the DMM4020 can do, but I have found some very nice functionality that most users just wouldn’t notice unless they were looking for. Specifically:
Diode test.
Most diode test functions only work at 1-1.5V, enough to test ordinary signal diodes, but not enough to test LEDs. The DMM4020 reads up to 1.9999V and drives up to 0.8mA, sufficient for the LED to light quite brightly. In theory this is only enough for red /yellow/green LEDs, but in practise the unit will illuminate blue and white LEDs also, which I was not expecting, although it will not give a voltage measurement for these.
Zero-burden Microammeter.
Most engineers will be aware that the usual way of measuring current is to insert a small resistance into the circuit and measure the voltage drop across it. At low currents this can be quite a high resistance, leading to a “burden” voltage drop which is unacceptable and may even upset the circuit under test.
The correct way to do this is with a transimpedance amplifier which provides almost no voltage drop, but can only realistically be performed at low current levels. It is quite a complicated thing to get right in a design and, as a result, is only usually seen in expensive dedicated microammeters. The Tektronix DM4020 is the only meter that I know of (this side of £1000 anyway) which uses this preferred method for measurement. If you often measure low currents, this feature alone may be worth the price of the meter.
Relay Range Switching.
In order to switch in appropriate shunts and multipliers, the DMM4020 uses proper electromechanical relays rather than silicon switches. This slows down auto ranging, but not unacceptably so. Manual override is always available and is probably preferred in most cases. Relay switching is by far the best method in equipment where you are looking for measurement performance and have the luxury of a large PCB real estate and a mains power supply.
“Other Stuff”
The supplied LabView software is a little clunky to install (and partially deactivates after a preview period) but does work pretty well. I am no LabView expert, but I can see how relatively easy it is to automate tasks if you are working in a production environment. This is definitely one thing I will be looking more closely at. Simply logging data in ASCII format to a text file is very handy indeed.
In order to connect the DMM4020 to the outside world, it is fitted with a standard RS232 port at the rear. Tektronix thoughtfully provide an RS232-to-USB adaptor, with a good 2M of cable. This seems to be based on an FTDI chip, which in my experience has proven most reliable in this sort of role. Windows 7 picked it up straight away and downloaded drivers via Windows Update.
Conclusion
So there we are. Four days use and a “quick” look that is already several pages long. That gives you an idea about what sort of machine the Tektronix DMM4020 is, I hope that I have at least been able to give a taste of what this machine can do. I hope to have a much more in-depth review up in a few weeks, after it has had some use in a real world situation. One of the things I have lined up is the development of my LCR meter project. Having a meter this accurate will help immensely.
A Request
I want to shoot some video of the DMM4020 in action if I can. At the moment I only have a webcam and a cheap compact camera that will shoot video, so I am looking to buy myself a reasonable 720 or 1080 digital video camera. Does anyone have any recommendations? I don’t want to spend more than maybe £150 or so, it doesn’t need to be a fantastic spec. Thanks!
Additional: Farnell / Element-14 tell me that UK purchasers will have their meter pre-configured for UK 240V power.
