Thursday, October 04, 2007

Challenges in measurement

This post is only going to be relevant directly for those people working on the same kind of stuff that my group does. Still, it gives a flavor of the challenges that can pop up unexpectedly in doing experimental work.

Often we are interested in measuring the electronic conductance of some nanodevice. One approach to doing this is to apply a small AC voltage to one end of the device, and connect the other end to something called a current preamplifier (or a current-to-voltage converter, or a glorified ammeter) to measure the amount of current that flows. It's possible to build your own current preamp, but many nanodevice labs have a couple of general purpose ones lying around. A common one is the SR570, made by Stanford Research. This gadget is pretty nice - it has up to a 1 MHz bandwidth, it has built-in filter stages, it is remotely programmable, and it has various different gain settings depending on whether you want to measure microamps or picoamps of current.

Here's the problem, though. One of my students observed that his devices seemed to fail at a surprisingly high rate when using the SR570, while the failure rate was dramatically lower when using a different (though more expensive) preamp, the Keithley 428. After careful testing he found that when the SR570 changes gain ranges (there is an audible click of an internal relay when this happens, as the input stage of the amplifier is switched), spikes of > 1V (!) lasting tens of microseconds show up on the input of the amplifier (the part directly connected to the device), at least when hooked up to an oscilloscope. Our nanoscale junctions are very fragile, and these spikes irreversibly damage the devices. The Keithley, on the other hand, doesn't do this and is very quiet. Talking to SRS, this appears to be an unavoidable trait of the SR570. We're working to mitigate this problem, but it's probably good for people out there in the community using these things to know about this.


Anonymous said...

The venerable DS345s probably do this too. I hear really violent clunks in all of mine as I'm ramping through amplitude or offset or whatever.

Anonymous said...

I also experienced this annoyance with the SRS 570. And yes, it does have a tendency to obliterate the poor unsuspecting molecules. One can work around this problem by choosing an arbitrary gain setting, examining the current level, unplugging the preamp, changing the gain, re-plugging in the preamp, and measuring again. This is, of course, painful.

If I remember correctly, there are specific gain changes where this happens (something like 3 or 4 total on the instrument), so as long as you're not switching between those levels, it isn't an issue. It's been a while, but I think the problem occurs when switching between the 5x multiplier and the next set of units (for example 500 pA to 1 nA), but I could be wrong about that.

Douglas Natelson said...

Anon. - close; it's when switching between 1nA and 2nA, but you've basically described it. The problem seems to be a combination of relay bounce and input stage charging. As far as I'm concerned, "Just don't have it hooked up to your device when you change gain" is not an acceptable solution. I'm tempted to do surgery on one of my 570s and try to replace the relay with a make-before-break or solid-state relay of some kind. Any thoughts on this?

I think the only people who would ever notice this are the molecule/atomic-scale-junction community, and the AlOx tunnel barrier device community. We're the ones with the most fragile systems.

Andrew said...

We had the same problem (and all kinds of other, similar issues) when I worked on electromigrated break junctions as a grad student. I got very frustrated with instruments that failed to control large transient noise. Those atomic-scale junctions are obviously incredibly sensitive to things like this.

I was quite happy with the Ithaco (DL instruments) 1211 current preamp. It doesn't have the remote programmability of the SRS, and I think the maximum bandwidth was only like 50 kHz, but it got down to 1.5 fA/rt Hz, and we never had trouble switching gains and bandwidths. I've never tried the Kiethley preamp, though the Kiethley source meter we used to break the wires had some issues when switching ranges.

The equivalent SRS voltage preamp also gave us some troubles. It's a shame, because their products are otherwise quite nice-- I'm in love with their high bandwidth (350 MHz) preamp.

Anonymous said...

Hi Doug,

We used to ground all devices before changing the gain using make-before break switches (from Electroswitch). One of their gigantic switches with 20 poles or so will do nicely. We used the Ithaco DL1211 preamp that is less likely than the SRS to spike, although it is not completely fail safe. This solution would require somebody to manually flip the switch each time the gain is changed.

I also did make a doodad to automatically switch between devices. I used reed relays (from Coto technology) to do the switching, they do not spike. To control the reed relays I used a regular FET demux chip. If you have only one current line then you can put in a reed relay between the device and the current preamp, and switch the relay off before changing the gain. This could all be automated fairly easily. You would have a little extra noise in the pA range though.


Douglas Natelson said...

Thanks for all your suggestions and recommendations. It's funny - in our usual old mode of operation we hadn't had any problems. However, for some new experiments we've been doing that require a different measurement scheme (involving a fair number of gain changes), we've run into these issues that many of you have seen before.

Schlupp said...

Sometimes, I'm just so glad to be a theorist.

Unknown said...
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Unknown said...

Just my two cents. With digital SRS I had similar problems. I remember I had to issue a command through gpib, or manually set the range. Since some virtual instrument drivers do the autorange before measurement, might need to change that setting, e.g., set input GND before changing the range.

Besides the spike issue, digital SRS input stage seems do produce more RF noise than anolog ones.

A shunt box is useful, can also add a pot resistor to protect your device. And as Abhey said in previous comment, may choose "Contact Style: Shorting" to avoid spikes.

Douglas Natelson said...

Thanks. I'm well familiar with make-before-break switches. I'm just surprised that it wouldn't occur to SRS that it might be a bad thing to have big voltage spikes on the input channel of a device designed to measure sub-pA currents. Ahh well.

Anonymous said...

An example from experience with DL 1211: it spikes only when the output is close to the saturation and the gain is lowered. If the gain is switched at ~1/2 of full scale or below, it behaves. You might need to do a similar investigation for SRS preamp.

Unknown said...

You might be able to mitigate the problem with a purely reactive LP filter (like a minicircuits 1.9 MHz)... or really just an rf choke.

There is a comparable current preamp, however, that's much much cheaper than the ithaco 1211... check out the version. I recall it being under $2k when I bought it. Or roll your own... it's difficult to do single-ended with any of the commercial solutions.

virender singh said...
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virender singh said...

We have SRS570 instrument ant its working weirdly but by some reason its manual has been misplaced .I search a lot in NET for its manual but I dint get any thing Then I read Your Blog in nanoscale blog and I also wanted to solve its problem but for that I want semantic diagram of its circuit which is not available in Internet manual of SRS570 .I only want last 10 pages of manual so It is requested to you that please send its scan copy to me It is really appreciated my email address is

Thanks and warm regards

- virender singh