need to measure low freq. DC waveform

[[email protected]]

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?

 

[Rich Webb]

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.

 

[Phil Allison]

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

 

[Rich Webb]

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.

 

[Yukio YANO]

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?

Boy!, have "We" progressed so far that we've forgotten the purpose of
MODEMs'"

A "Modem" is an acronym for a modulator-demodulator, a pair of circuits
for transmitting a DC signal over communication circuits bandwidth
limited to 300-3000 cps.

By modulating a Sub-carrier eg (1kc) with the DC signal you can transmit
for example an EKG , a low-voltage, very low frequency wave-form over a
telephone line. Or record a DC waveform using a Soundcard. Look for
circuits for "Voltage to Frequency Converters" and Demodulators

We are re-inventing the wheel here !

Yukio YANO

 

[Paul E. Schoen]

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 have heard of using a game port for this purpose, but it seems that it is
not very simple:

http://en.wikipedia.org/wiki/Game_port

The other suggestions for inexpensive A/D data acquisition devices are
good. Many of them use USB, while older ones used parallel or serial
interfaces. If you want to "roll your own", you can get a Microchip FS USB
PICdem board for about $40. It has A/D inputs that are connected to a pot
and a temperature sensor, and a simple GUI that monitors these values.

It is hard to imagine doing anything in electronics without an
oscilloscope, and there are lots of deals on older high performance scopes,
or simple PC based or hand held scopes. But they are generally more useful
for observing waveforms where rough peak values are enough, rather than
actual measurement (such as RMS value), which is best done with data
acquisition and analysis software.

Paul

 

[Jamie]

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?

Get your self a PIC starter kit, PICS have ADC inputs and can operate
with an RC clock. sounds just about right for your application.
The PIC can be set up to do a serial link to the PC if you wish.
Remember to use a inverter driver because the levels need to be
bumped up and inverted. MAX has a line of chips with charge pumps
if you don't have the require voltage on your board.


Or,
Get an off the shelf Voltage to Serial converter.


http://webpages.charter.net/jamie_5"

 

[Bob Monsen]

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?

One way to go would be to use a simple on-off circuit to turn the input into
an AC square wave. You could then measure the input with you sound card. The
output, when filtered at your switching frequency, will be proportional to
the DC input.

You might use an analog switch of some kind, like a 4066. I think there are
much lower resistance ones available, but that should not matter all that
much, because the input impedance of the sound card should be fairly high,
and your application is slow. You could also cobble something together to do
the switching using discrete, but it isn't as simple as just buying an
analog switch IC.

A signal generator would be the easiest way to generate the switching clock,
but you could also generate it from software on your sound card by
outputting a square wave at the right frequency, and using that to clock the
analog switch.

You might then write a simple digital band pass filter to filter the input
at the switching frequency. There exist various filter design websites,
which will write C code for you given the parameters you want to filter. A
10k switching frequency would be right in the middle of your sound card's
range.

Note that you may be able to use a USB port to power the entire circuit,
since USB can supply 5V. The parallel port and game port can also supply 5V.
However, ensure that your device under test doesn't have a ground that is
seriously different than that of the PC, or you can end up passing current
through the ground wires. If you connect them with an ammeter first through
a 1k resistor, you can see how bad it is likely to be.

Regards,
Bob Monsen

 

[Phil Allison]

"Paul E. Schoen"

It is hard to imagine doing anything in electronics without an
oscilloscope,

** That's for sure.

and there are lots of deals on older high performance scopes, or simple PC
based or hand held scopes.

** Or this 10 MHz one for A$88, including 10:1 probe.

http://www.dse.com.au/cgi-bin/dse.storefront/4830f5a10d20925a2740c0a87f9c071c/Product/View/Q1803

But they are generally more useful for observing waveforms where rough
peak values are enough,

** You get to see the peak value, as part of the whole wave shape, and it is
not a "rough" value.

All modern, internal graticule tube scopes can easily achieve 1% accuracy on
voltage - providing you occasionally calibrate the range. This can be
easily done using DC voltages, verified with a DMM.

rather than actual measurement (such as RMS value), which is best done
with data acquisition and analysis software.

** You still need to SEE the waveshape or you have not got a clue what you
are measuring. RMS meters have strict bandwidth limitations and max crest
factors - so you must know the actual waveform or the readings are
meaningless.

A scope allows you to make voltage measurements with certainty.



...... Phil