Analog input to USB output for LabView use (medical)

[hineybush]

Hi all, first post here so I'm sorry if I'm doing something wrong- I just got involved with a medical research project at my university where I'm studying electrical engineering and I'm pretty much in charge of it. My only issue is that I've only completed the first-semester courses of basic circuit analysis and digital logic. I'm taking a lab course this summer and I've done a bit with hardware on my own, but I'm not sure where to start with this project.

Project details: A small (handheld hopefully) handheld device that has an analog input, the data will be read in LabView and graphed. I'm not worried about the LabView portion, but I'm at a loss of which ADC's and serial>USB converters to look at. I've read that SAR ADC's are best for medical applications, is this true? As far as channels go I would only need one, correct?

Thanks!

 

[(*steve*)]

Essentially you're after some sort of data logger that can interface to labview, possibly in real time (I don't know).

To decide what you want, you need to know something about the signals you want to capture.

 

[hineybush]

Yep, I'm looking to be able to view the data in real-time. The input I'm monitoring will be from a airflow meter (nebulizer-type device) and I assume the device has some sort of generator/motor (I haven't seen this device yet, but I'll have a better idea of it by the end of the week) which will output a current, I'll run the current across a resistor and measure that voltage with the converter.

 

[(*steve*)]

Questions you'll need to think about are:

• How often do you need to take measurements (once per minute, once per second, ten times per second, 1000 times per second, 1,000,000 times per second)?
• What is the voltage range? (0 to 1V, -100mV to 100mV, etc.)?
• What accuracy/precision do you require (e.g. 0.1mV precision with +/- 0.2mV accuracy)

 

[BobK]

You could build this with a microcontroller that has A/D and a USB interface. You might need an opamp or two to condition the signals.

Bob

 

[hineybush]

Questions you'll need to think about are:

• How often do you need to take measurements (once per minute, once per second, ten times per second, 1000 times per second, 1,000,000 times per second)?
• What is the voltage range? (0 to 1V, -100mV to 100mV, etc.)?
• What accuracy/precision do you require (e.g. 0.1mV precision with +/- 0.2mV accuracy)

Thanks! I'm looking to take measurements between 30 and 100 times per second. I forgot to mention that I'm measuring breathing airflow. After looking at some other commercial applications I figure the best way to go about doing that would be to use a low-resistance fan which the patient would breathe through, and then measuring the voltage across a resistor that the current from the fan is output to, so the voltage would be relative to which resistor I would use. I'm planning on using an Arduino for development since it has ADC and USB features, and the final hardware piece would use either USB power or a dedicated 5v source, so the max voltage measured would correlate to that.

 

[(*steve*)]

You need to have a reliable transducer before you even think about how to interface it. If not, you're putting the card before the horse.

I suspect a fan won't work.

I would suspect that commercial units have a low mass turbine fitted to some sort of optical sensor. A fan (with a motor) is going to have too much interia and resistance for accurate (or possibly *any*) measurements at low flow rates, and certainly won't respond quickly to changes in flow rate.