Hobby Electronics Basics need to measure low freq. DC waveform

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Subject Author Date
need to measure low freq. DC waveform mattyg1884 05-18-08
Posted by on May 18, 2008, 3:07 am
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i need to measure a low frequency waveform from a DC circuit (voltage
up to 200mV). I need a sample rate of around 10Hz. to buy an
oscilloscope for this is a total overkill. I was thinking of using the
soundcard, but then realised it's AC coupled. any suggestions on a
cheap method to find this measurement?

Posted by Rich Webb on May 18, 2008, 9:02 am
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On Sun, 18 May 2008 00:07:28 -0700 (PDT), mattyg1884@hotmail.com
wrote:

>i need to measure a low frequency waveform from a DC circuit (voltage
>up to 200mV). I need a sample rate of around 10Hz. to buy an
>oscilloscope for this is a total overkill. I was thinking of using the
>soundcard, but then realised it's AC coupled. any suggestions on a
>cheap method to find this measurement?

I'm assuming you need an off-the-shelf solution, so recommendations
using dividers, buffers, and ADCs and the like are non-starters.

Several vendors have plug-in USB analog measurement gizmos. The
products at http://www.measurementcomputing.com/ are reasonable and
they include some basic data logging software as well as a DLL that
you could (?) use. The included logging software on the less expensive
models (e.g., USB-1208LS) maxes out at 10 sps (I think, been a while
since I used it) but the interface DLL will allow you to capture up to
the full rated speed of the modules.

--
Rich Webb Norfolk, VA

Posted by Bob Masta on May 18, 2008, 9:49 am
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On Sun, 18 May 2008 09:02:49 -0400, Rich Webb

>On Sun, 18 May 2008 00:07:28 -0700 (PDT), mattyg1884@hotmail.com
>wrote:
>
>>i need to measure a low frequency waveform from a DC circuit (voltage
>>up to 200mV). I need a sample rate of around 10Hz. to buy an
>>oscilloscope for this is a total overkill. I was thinking of using the
>>soundcard, but then realised it's AC coupled. any suggestions on a
>>cheap method to find this measurement?
>
>I'm assuming you need an off-the-shelf solution, so recommendations
>using dividers, buffers, and ADCs and the like are non-starters.
>
>Several vendors have plug-in USB analog measurement gizmos. The
>products at http://www.measurementcomputing.com/ are reasonable and
>they include some basic data logging software as well as a DLL that
>you could (?) use. The included logging software on the less expensive
>models (e.g., USB-1208LS) maxes out at 10 sps (I think, been a while
>since I used it) but the interface DLL will allow you to capture up to
>the full rated speed of the modules.
>

Another possibility is the Dataq DI-194 starter kit. I haven't tested
this myself, but specs look great: 4 channels, +/-10V, 10 bits, up to
240 samples/sec, comes with chart recorder software, and is only
US$25.00. I've seen this advertised for years (maybe 10 years),
so it certainly has staying power. The only downside may be that it
uses a serial port interface. (I actually acquired some of these way
back when I was developing drivers for Daqarta for DOS, but there were
no DI-194 interface docs available back then and I never got around to
figuring it out by spying on the serial port traffic.)

Another alternative, if you have an old computer than can run
real-mode DOS (Win9x or earlier) and has a parallel printer port,
would be Daqarta for DOS with the LPTX driver and an 8-bit ADC
that you can build from a handful of resistors stuffed into a DB-25
connector shell. See <www.daqarta.com/download.htm> and look
at the LPTX driver pages at <www.daqarta.com/lptx.htm> for details.
Note that this only supports one channel, but it does go down to DC.
Daqarta for DOS is free for non-commerical use by requesting a license
at <www.daqarta.com/contact.htm>. Note, however, that the minimum
sample rate is limited to 18 Hz in the software.

Finally, if you want to use a Windows sound card you could consider
chopping your signal at (say) 1 kHz so that the resultant square wave
will pass the AC coupling easily. Depending on what you are trying to
do, this may be all you need. PLUG: Daqarta for Windows will show you
the waveform, and you can use its built-in voltmeter to measure Pk-Pk
or 0-Pk (instead of RMS or dB ranges) to recover the original voltage.
You'll need to calibrate your card using the included auto-calibrate
feature with a loopback cable, plus a single full-range calibration
measurement (full instructions at <www.daqarta.com/dw_00uu.htm>
and following.)

Daqarta for Windows is $29.00 for non-commercial use, but you can try
it for 30 trials/30 days for free, and after that it becomes a free
signal generator (and a whole lot of other stuff... just won't monitor
the input lines any more.)

I'd be glad to discuss your specific needs. Just contact me via the
contact form on the site.

Best regards,


Bob Masta

DAQARTA v3.50
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, FREE Signal Generator
Science with your sound card!

Posted by Rich Webb on May 18, 2008, 11:04 am
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On Sun, 18 May 2008 13:49:10 GMT, NoSpam@daqarta.com (Bob Masta)
wrote:

>On Sun, 18 May 2008 09:02:49 -0400, Rich Webb
>
>>On Sun, 18 May 2008 00:07:28 -0700 (PDT), mattyg1884@hotmail.com
>>wrote:
>>
>>>i need to measure a low frequency waveform from a DC circuit (voltage
>>>up to 200mV). I need a sample rate of around 10Hz. to buy an
>>>oscilloscope for this is a total overkill. I was thinking of using the
>>>soundcard, but then realised it's AC coupled. any suggestions on a
>>>cheap method to find this measurement?
>>
>>I'm assuming you need an off-the-shelf solution, so recommendations
>>using dividers, buffers, and ADCs and the like are non-starters.
>>
>>Several vendors have plug-in USB analog measurement gizmos. The
>>products at http://www.measurementcomputing.com/ are reasonable and
>>they include some basic data logging software as well as a DLL that
>>you could (?) use. The included logging software on the less expensive
>>models (e.g., USB-1208LS) maxes out at 10 sps (I think, been a while
>>since I used it) but the interface DLL will allow you to capture up to
>>the full rated speed of the modules.
>>
>
>Another possibility is the Dataq DI-194 starter kit. I haven't tested
>this myself, but specs look great: 4 channels, +/-10V, 10 bits, up to
>240 samples/sec, comes with chart recorder software, and is only
>US$25.00. I've seen this advertised for years (maybe 10 years),
>so it certainly has staying power. The only downside may be that it
>uses a serial port interface. (I actually acquired some of these way
>back when I was developing drivers for Daqarta for DOS, but there were
>no DI-194 interface docs available back then and I never got around to
>figuring it out by spying on the serial port traffic.)

Got one of those too. ;-) They're not bad for throwing in the laptop
bag to have "just in case." The gain is fixed at +/- 10 V, though.

The Measurement Computing USB-1208LS has a +/- 1 V differential range
that matches up better with the OP's 200 mV need and will sample
(using the supplied logger app) at up to 1200 sps. Had to grab it from
the toolbox and fire it up to verify that, so disragard the "10 sps"
above.

Dataq has some USB gizmos (e.g., DI-158) that do offer programmable
gain settings. From my reading of the datasheets the free (supplied)
logger is limited to 240 sps, though.

--
Rich Webb Norfolk, VA

Posted by Phil Allison on May 18, 2008, 10:12 am
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>i need to measure a low frequency waveform from a DC circuit (voltage
> up to 200mV). I need a sample rate of around 10Hz. to buy an
> oscilloscope for this is a total overkill.


** Anyone who * claims * they have no access to a basic " scope" is as
ass.

Beg, borrow or steal one.

You fuckwit asshole.




..... Phil




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