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PicoScope offset advice

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PicoScope offset advice

Postby padbadger » Mon Apr 28, 2014 12:06 pm

I wondered if anybody could give me some advice on which scope would be suitable for my needs – we currently have a 2205A which is not.
I’m measuring the output response of optical strain gauges used to measure insect shear forces during controlled slides on a test surface (movement controlled using a 3-axis motor stage). Slides last for several seconds. The signals consist of a large background on the order of 100s of mV (due to the shear force produced by the insect pad sliding) that varies gradually over the length of the slide, superimposed on which are oscillations in the range of 50-500Hz with peak to peak voltage of a few mV (due to perturbations made to the surface). The background signal rises from 0mV to the target as the pad begins to slide, then can drift by around 10-20% of the signal. I’m mainly interested in the smaller oscillations and would like to measure in detail a portion of them, at least 10 cycles or so. I do not need to capture the entire signal, including background, but if it is possible to do so that would be preferred. Unfortunately the 8-bit 2205A oscilloscope lacks the step size to monitor the small oscillations when the voltage scale is determined by the larger background signal. The offset function does not allow offsetting before amplification, so does not improve resolution. I’d like to purchase a scope that will allow good enough time resolution to capture the 500Hz signals in detail, whilst allowing either good enough voltage resolution to capture the entire signal without losing the detail in the small oscillations or allow for offsetting the signal before amplification, so that I can set the range to fit the smaller signals and offset to the background signal to zero. Our budget is under £1000, so the 6000 series is not possible.
I understand that moving from 8 –bit t 12-bit will increase the number of steps in a given voltage range from 256 to 4096. If we have a range of +/-500mV (to capture the whole signal with a background of ~400mV), that would change the step size from 3.9mV to 0.24mV. I’m concerned that still may not be enough to capture in detail the smaller oscillations so think that the ability to apply an offset to bring the background close to zero, then alter the range to reduce this step size, may be necessary.
I’m sorry for the long post and appreciate any help you can give.
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Re: PicoScope offset advice

Postby Martyn » Fri May 02, 2014 10:30 am

Have you tried switching to AC coupling to see what effect that has ?

Otherwise can you post some traces showing the type of waveform you are looking to capture, then we may be able to suggest some options. If these are psdata format with the 2205a that would help.
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Re: PicoScope offset advice

Postby alan » Fri May 02, 2014 11:32 am

As Martyn says, lets have a look at the trace and see if we can do the job with the hardware you have.

If not then something like the PicoScope 5242A http://www.picotech.com/picoscope5000-f ... ution.html would have several advantages and is in your budget. Rather than just 8 bits, you can hardware switch the resolution to 12, 14 or 16 bits - note that this is not just software averaging - the hardware contains multiple high resolution ADCs.

Improving the resolution / reducing the step size will help, but you also have a hardware offset feature so you can dial out / remove any constant DC offset before amplification. These two effects together will massively increase the sensitivity to the smaller oscillations.
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