Reply by Jerry Avins July 29, 20102010-07-29
On Jul 29, 4:39�am, "PhilipOrr"
<philip.orr@n_o_s_p_a_m.eee.strath.ac.uk> wrote:
> >Aside from compensating for offsets, why go differential? > > >Jerry > > It allows me to suppress the other signal that is strong and common to both > polarisations (happens to be strain in this sensor). Common drifts in the > two measurements are removed. Also, there is that [weak] increase in > SNR... > > That's about it I suppose. > > Are you implying I shouldn't bother? I'm using the lame 'some benefits > outweighs no benefits' argument here as you can tell.
It has a cost that is probably justified by the strain and drifts. The cost is forgoing the extra samples you could average. A sharper low- pass filter would likely be better than simple averaging. Jerry -- Engineering is the art of making what you want from things you can get. &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;
Reply by PhilipOrr July 29, 20102010-07-29
>Aside from compensating for offsets, why go differential? > >Jerry
It allows me to suppress the other signal that is strong and common to both polarisations (happens to be strain in this sensor). Common drifts in the two measurements are removed. Also, there is that [weak] increase in SNR... That's about it I suppose. Are you implying I shouldn't bother? I'm using the lame 'some benefits outweighs no benefits' argument here as you can tell.
Reply by Jerry Avins July 28, 20102010-07-28
On 7/28/2010 2:00 PM, Fred Marshall wrote:

 > ... which may get rid of 8X the noise
> bandwidth. Many noises add (subtract) as the sqrt of the bandwidth. So, > it appears, you'd get 9dB I believe, if the noise is truly spread across > the higher bandwidth.
I was thinking 8x voltage ratio when I wrote 18 dB. A silly goof. Jerry -- Engineering is the art of making what you want from things you can get. &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;
Reply by Fred Marshall July 28, 20102010-07-28
PhilipOrr wrote:
> If anyone is still interested: > > What I have done to improve the noise level is sample at 16 times the > switching frequency, to give me 8 samples per switch state (where > previously I had only a single sample). > > Then, I use the mean of these 8 to get the differential measurement. The > noise is vastly improved. > > Is this known as 'oversampling'? And is it, in effect, providing > anti-aliasing before producing the low-rate differential signal? > > Philip
Calculating the mean is a form of lowpass filtering. So, you're getting a lowpassed version of what's coming out of the switch. That it improves the result suggests that there's indeed higher frequency components and you've been undersampling. One might ask, "What's the issue with undersampling noise anyway?" Well, as you've experienced here, it removes the ability to filter out some of the noise. In this case, you've increased the bandwidth by a factor of 8 and then lowpassed - which may get rid of 8X the noise bandwidth. Many noises add (subtract) as the sqrt of the bandwidth. So, it appears, you'd get 9dB I believe, if the noise is truly spread across the higher bandwidth. And, by not sampling at the higher rate, you were aliasing the noise down into a narrower bandwidth - thus increasing the noise power. And, particularly in this case, the noise is likely quite random. So, differencing accentuates the higher frequency components by 3dB. ... well, this is pretty much arm-waving and I may have made a mistake or two but the idea is there. Fred
Reply by Jerry Avins July 28, 20102010-07-28
On 7/28/2010 9:56 AM, PhilipOrr wrote:
> If anyone is still interested: > > What I have done to improve the noise level is sample at 16 times the > switching frequency, to give me 8 samples per switch state (where > previously I had only a single sample). > > Then, I use the mean of these 8 to get the differential measurement. The > noise is vastly improved. > > Is this known as 'oversampling'? And is it, in effect, providing > anti-aliasing before producing the low-rate differential signal?
The result shows that (at least much of) the noise is there before the switch. If most of the noise power is at higher frequencies than your signal of interest, there are more effective ways to reduce its effect than simple averaging. If the noise has a Gaussian distribution, averaging provides an 8:1, or 18 dB SNR improvement. Aside from compensating for offsets, why go differential? Jerry -- Engineering is the art of making what you want from things you can get. &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;
Reply by PhilipOrr July 28, 20102010-07-28
If anyone is still interested:

What I have done to improve the noise level is sample at 16 times the
switching frequency, to give me 8 samples per switch state (where
previously I had only a single sample).

Then, I use the mean of these 8 to get the differential measurement. The
noise is vastly improved.

Is this known as 'oversampling'? And is it, in effect, providing
anti-aliasing before producing the low-rate differential signal?

Philip
Reply by PhilipOrr July 27, 20102010-07-27
>On 7/26/2010 12:45 AM, Michael Plante wrote: >> Jerry wrote: > >>> ...that precedes ADC. (You wrote DAC, but I don't think you meant >>> that.) ... >> >> Perhaps a nitpick, but he wrote DAQ, not DAC, and I've seen that in the >> context of Labview/NI to mean "Data AcQuisition unit". I.e., a
packaged
>> ADC. That term threw me off a bit too when I first heard it, because it
is
>> pronounced like "DAC", but really means "ADC". Fun stuff... > >Thanks for the heads-up. I apologize to P.O. for my poor reading. > >Jerry >-- >Engineering is the art of making what you want from things you can get. >
No problem, you have all been immensely helpful :-)
Reply by Jerry Avins July 26, 20102010-07-26
On 7/26/2010 12:45 AM, Michael Plante wrote:
> Jerry wrote:
>> ...that precedes ADC. (You wrote DAC, but I don't think you meant >> that.) ... > > Perhaps a nitpick, but he wrote DAQ, not DAC, and I've seen that in the > context of Labview/NI to mean "Data AcQuisition unit". I.e., a packaged > ADC. That term threw me off a bit too when I first heard it, because it is > pronounced like "DAC", but really means "ADC". Fun stuff...
Thanks for the heads-up. I apologize to P.O. for my poor reading. Jerry -- Engineering is the art of making what you want from things you can get. &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;
Reply by PhilipOrr July 26, 20102010-07-26
>Perhaps a nitpick, but he wrote DAQ, not DAC, and I've seen that in the >context of Labview/NI to mean "Data AcQuisition unit". I.e., a packaged >ADC. That term threw me off a bit too when I first heard it, because it
is
>pronounced like "DAC", but really means "ADC". Fun stuff...
Yep, sorry for the confusion. I wrote DAQ deliberately to mean data-acquisition not DAC. Must be a LabVIEW term only.
Reply by Michael Plante July 26, 20102010-07-26
Jerry wrote:
>On 7/21/2010 8:12 PM, PhilipOrr wrote: >> Hi everyone - this is my first post here. It's about time I joined a
DSP
>> forum. >> >> I need some advice related to a measurement system. The system samples
at 1
>> kHz but between every sample the input to the DAQ is switched between
two
>> inputs. The result is that the acquired signal, at 1 kHz, is two >> interleaved signals at 500 Hz each. >> > >...that precedes ADC. (You wrote DAC, but I don't think you meant >that.) ...
Perhaps a nitpick, but he wrote DAQ, not DAC, and I've seen that in the context of Labview/NI to mean "Data AcQuisition unit". I.e., a packaged ADC. That term threw me off a bit too when I first heard it, because it is pronounced like "DAC", but really means "ADC". Fun stuff...