Seismic Accelerometer for Atomic Force Microscope

[mechknoll]

Hi All,

I am designing a seismic accelerometer using a MEMS capacitive acceleromter in order to measure room vibration for our Atomic Force Microscope (AFM) setup.  I decided to use the Colibrys SF1600S.A for its low noise and wide dynamic range, data sheet attached.  My question is now I am deciding on which ADC to use then a USB interface.  I have been looking at using a sigma delta ADC for their superior DC resolution, also needs to be signed.  Does anyone have any recommendations?  Also what about a USB interface? 

A little about myself...  I have worked in a biomedical imaging lab for years.  I have moderate experience with electronics, more with lasers and PhotoMultiplier Tubes and labview.  I am now moving towards biological physics and AFM measurements of proteins.

Here is the data sheet....

http://www.colibrys.ch/files/pdf/products/30S%20SF1600S%20A%2011%2011.pdf

[fcb]

Do you need to build the unit from scratch (ADC chip, microcontroller and USB driver?) or can you just buy a *suitable* DAQ from someone like NI (www.ni.com) that has it all built in.

[mechknoll]

Yes I know I can use a DAQ from NI...  I have been using the NI-6009 and NI-6220 for a long while now and am very familiar with the interface with labview.  I just now have to opportunity "Funding" to experiment with building my own. 

[Marco]

Am I missing something or can you just use a single ended supply for that sensor? It tells you to use +/- 6-15 Volt, but I don't see any ground connection so 0/12-30 would work just as well wouldn't it? A differential signal around a raised common mode is a common input for ADCs, although it obviously has to be attenuated a bit first.

Not a recommendation, but someone on the open source multimeter thread made a short list of a low noise 24 bit ADCs. Analog and Linear have 50$ range evaluation modules.

[Jon Chandler]

I'm wondering if the accelerometer you've selected is the best choice.  When I did some vibration training at a semiconductor fab, they used a Wilcoxon Research 731A seismic accelerometer.  This is a huge beast of a thing, about the size of a coffee cup.  The sensitivity is 10v/g – almost ten times greater than the accelerometer you listed, and the broadband noise is almost ten times lower.  This would give you a better signal to start with, making digitizing easier.

Powering the 731A is simple.  It uses a constant current supply of 2 – 10 mA with a maximum voltage of between 18 – 30 volts on the signal line.  A capacitor blocks the DC bias voltage from the input of your ADC.

[ejeffrey]

You might also look at geophones.  I have used them to measure sub-nm vibration levels.  They are velocity sensors instead of accelerometers, at least above their resonance frequency (1-10 Hz, depending on the model).  I used the oyo geosphace GS-11D sensors with a resonance frequency around 4.5 Hz.  You can deconvolve the resonance and get useful data down to below 1 Hz, probably still with greater sensitivity than those MEMS devices.

@Marco:  You can't use it single ended, they have just helpfully labeled the ground pins "return" 

[Marco]

@Marco:  You can't use it single ended, they have just helpfully labeled the ground pins "return" 

Oops, oh well ... you'll probably still want to make it single ended for the ADC voltage range wise, just differential.

PS. what kind of voltages were you working with with such small vibrations?

[rossmoffett]

When I was working in a lab with AFMs and STMs the air conditioner would cause my readings to get sour, so I feel your pain.  I'm working on some research at work with an accelerometer package that includes a USB interface and on-board datalogging for 2GB IIRC.  Does your budget support purchasing something like that?  If so, I'd forget about the custom hardware..  We're using this model, available in different sensitivities.

If you're going custom though, cut out the ADC.  There are lots of accelerometers out there with on-board MCUs and digital outputs.  To save time, you can use a hobbyist platform that has built-in USB capabilities.  Arduino and Propeller come to mind.  In my research prior to my R&D project kickoff I was looking into an Analog Devices accelerometer that was so sensitive that it could be substituted for a microphone.  You may have to do some audio frequency filtering, is that why you're considering an analog signal?

[mechknoll]

I have decided to proceed forward with the SF1600S.A mems accelerometer and use a TI ADS1255 for the ADC with an Arduino to communicate over the SPI and USB port.  Thanks a lot for all of your input.  My rough estimates of ambient building modes is in the range of tens of micro g's.  Rossmoffett, does this sound like an acceptable range for room vibrations?  I have read that humans are able to detect milli-g, .001g.  Quite impressive! We already have a NI-DAQ but my adviser is going to fund my venture into designing this PCB.  Im using Altium and am planning on sending out to OSH park for my board fab.