This blog looks at what a source-measurement unit is, what it does, why you might want one, how it might simplify your test regimes and what you might use it for. Then, we discuss the specifications of the bench-top style SMUs on the market and their relative strengths and weaknesses.

 

What is an SMU?

A source-measurement unit, or SMU for short, can be thought of as a precision voltage/current source and meter in the one unit. Operating over all four quadrants, the SMU is able to source and sink positive and negative voltages and currents. Some SMUs are even able to generate short high-powered pulses.

 

Compared with a regular bench-top power supply, SMUs offer a number of advantages. Most power supplies operate in one quadrant only – being able to source voltage or current. This makes them unsuited for tasks which require sinking current or voltage. This usually would be the domain of either a two-quadrant power supply or a DC electronic load, but these are not capable of operating in both positive and negative voltages at the same time. Compared to power supplies, SMUs generally operate with higher sourcing and measurement precision, comparable to a dedicated 5.5 to 6.5-digit digital multimeter. They also offer a “real” current source, being able to deliver a stable regulated current as they generally have much faster regulation performance.

 

This means that SMUs are much more flexible compared to conventional power supplies and DC electronic loads, however, this flexibility does come at a cost of a more limited power envelope and a more complex and expensive instrument. In return, it makes performing a number of tests easier due to tight integration and synchronisation, reduces the footprint on the bench or in the rack, improves accuracy and testing speed and reduces the wiring necessary.

 

This has led to the SMU being used in many different applications. These include manufacturing production testing of semiconductor devices of almost every variety (diodes, LEDs, lasers, BJT, MOSFET, IGBT, OLEDs), photovoltaic (solar) cells, batteries, power supplies, thermistors, MOVs, gas-discharge tubes, insulator leakage current, resistances and more. Major applications include tracing the I-V curve of such devices to understand their power-on and reverse-breakdown characteristics, which somewhat overlap with the capabilities of semiconductor curve tracers.

 

SMU Market Survey

In order to understand the positioning of the Keithley 2450 SMU, it is perhaps useful to compare on-paper specifications with other bench-top style SMU units. Please note the disclaimer – while all care has been taken in the preparation of this market survey, I cannot be held responsible for errors or omissions and any damages which may have resulted regardless of how they are incurred. Use at your own risk. Also note that the maximum channel power is sometimes quoted as the rounded value – actual values may be slightly different (e.g. 20W instead of 21.6W). Some features are only available on certain models, indicated by ^, and other parameters come with additional conditions or for pulsed operation only, indicated by *.

 

Keithley can be considered the leading supplier of SMUs, dominating the market when it comes to the sheer number of models on offer. This is no surprise, as they are the vendor of the first standalone SMU in 1989, the Keithley 236. A comparison of the Keithley family alone is shown in the table below:

The family can be broken down into three segments roughly speaking – the first is the latest graphical SourceMeters with all the bells and whistles including a 5” colour capacitive touch LCD screen, onboard TSP/TSP-Link/TriggerFlow, front and rear outputs and inbuilt LAN. The second would be the older benchtop series SMUs of the 2400-series which feature a two-line VFD display and the older TriggerLink system. In-between the two series, there is the 2600-series SMUs which seem to be a hybrid between bench-top and system SMUs, upgrading the 2400-series with two-channel capabilities on some units and onboard TSP/TSP-Link/LAN capabilities.

 

The graphical SourceMeters generally outperform the older units in all metrics of concern, with improvements especially noted on the programming and read-back accuracies for both current and voltage. As alluded to earlier, they offer more flexible connectivity options and superior programmability and user interface options.

 

By comparison with the other graphical SourceMeters, the 2450 is the most affordable of the four models. When compared with the full range, its price is above the less-capable 2401 which is the most affordable unit, but on par with the 2400 while offering a better user interface and improved performance, making it a logical successor to the 2400. The 2450 doesn’t offer the special capabilities found in the more expensive units including the 2460 which sacrifices some voltage for greater current capabilities, the 2461 which is capable of pulsed operation, and the 2470 which extends the voltage through to 1000V. Instead, the 2450 seems to focus on offering better quality low-level measurements, so it is clearly a case of choosing the right SMU for the task at hand and their line-up definitely has different models to cater for different needs.

 

But what if you’re not interested in a Keithley SMU – what are your options?

I’m afraid there aren’t that many to choose from otherwise. Other contenders include models from Rohde & Schwarz/ADCMT, Yokogawa and Keysight.

 

Unfortunately, it seems the Rohde & Schwarz/ADCMT units are virtually “unobtanium” as I could not find any listed pricing for any one of the four units. Regardless, it can be seen that their specifications generally show them to be more limited in voltage range and power capabilities and with poorer accuracies and more primitive user interface made of segment VFD displays. The only exception is the 6541 which offers four channels, however, I would naturally assume its price is well above that of the 2450.

 

The Yokogawa units also seem to be more limiting on voltages with slightly poorer best accuracies, no LAN connection (it is an add-in option) and a similarly limiting VFD-based user interface. The GS820 offers two channels, however, at almost twice the cost. The GS510 is more affordable than the 2450, but the 2450 offers better specifications and flexibility making it the more natural choice.

 

This finally leaves the Keysight units. Four units are on offer, two-single channel units and two dual-channel units. The only difference between the specs is the provision of a 10nA range on the B2911A/B2912A units, which almost doubles the cost (surprisingly). In terms of price, the B2901AB2901A is slightly cheaper than the 2450 but it misses out on the smaller voltage and current ranges. In return, it offers higher overall power and pulsing capabilities, while also offering comparable programming and read-back accuracies on most of the ranges. The 2450 hits back with its in-built features, rear triax connectivity and larger touch-screen LCD compared. Based on this, I would still give the edge to the 2450, although if I had to settle for the Keysight B2901AB2901A, I wouldn’t feel like I had lost either.

 

Conclusion

A source-measure unit, or SMU for short, can be thought of as a precision voltage/current source and meter in the one unit. Operating over all four quadrants, the SMU is able to source and sink positive and negative voltages and currents. This is in contrast with conventional power supplies and DC electronic loads which operate in one quadrant only. Source-Measure Units improve flexibility at a cost of a more limited power envelope and a more complex and expensive instrument. In return, it makes performing many tests easier due to tight integration and synchronisation, reduces the footprint on the bench or in the rack, improves accuracy and testing speed and reduces the wiring set-up necessary. Applications include I-V curve testing of semiconductor devices of almost every variety (diodes, LEDs, lasers, BJT, MOSFET, IGBT, OLEDs), photovoltaic (solar) cells, batteries, power supplies, thermistors, MOVs, gas-discharge tubes, insulator leakage current, resistances and more.

 

In order to understand the positioning of the Keithley 2450 SMU, on-paper specifications for bench-top style SMUs were compared. The Keithley family was the largest, consisting of the latest graphical SourceMeters, the older 2400-series and 2600-series. Of these, the graphical SourceMeter range improves on the specifications of the older series while improving flexibility through more connectivity options and programmability, while also incorporating a superior user interface through the use of a 5” capacitive touch LCD. Other vendor alternatives included Rohde & Schwarz/ADCMT, Yokogawa and Keysight, however, some of these appeared to be “unobtainium”, they generally offered lower specifications and more limited user interfaces.

 

As a result, it seems clear that the Keithley 2450 SMU seems appropriately positioned in the market, with leading specifications and capabilities where it matters, at a price that sits well with the Keithley family of SMUs and those from other vendors. The Keithley SMU range has different units for different needs, making choosing a Keithley SMU feel quite natural. Other vendors have a more limited range of SMUs available by comparison, with the closest match on price and specs perhaps being the Keysight B2901AB2901A which doesn’t present any compelling advantages over the 2450.

 

---

This blog is part of the Keithley 2450 SMU with I-V Tracer RoadTest.