Is there a USB interface with DC coupled input?

[julianv]

I am helping some university researchers build a very simple waveform recording system. We are looking for an ADC that connects via USB to an iMac or Mac laptop. This is not for recording audio. The specs are quite modest:

1) only one input channel needed, for a waveform varying slowly between 0 and 5 volts
2) 8 bit resolution is sufficient, more is OK
3) sample rate is 50 per second, more is OK
4) realtime, or near realtime display of the recorded waveform is needed (Audacity would be quite sufficient)
5) recordings may last as long as 4 minutes

The only difficult spec is that, since the signal is varying slowly, we need response down to 0 Hz (DC coupled input). But all the sound recording devices that I have found seem to be AC coupled, limited to about 10 or 20Hz and higher. Does anyone know of a device which is DC coupled?

I have found quite a few data acquisition and data logging products which can record down to DC, but very few that meet our specs and run on a Mac. Most require Windows or Linux. There are a couple of high-powered DAQ systems which will run on a Mac (National Instruments LabView, AD Instruments, etc.) but this stuff is expensive and overkill for our needs.

[kozikowski]

It's worse than that. If the signal really is 0, +5v, then the device has to have a power supply system that exceeds that, say +6v and -1v at minimum. Or a prescaller to reduce the input range to 1v, 4v. Good luck finding either of those in a cheap device.

Koz

[kozikowski]

Back in the paleolithic, we used to use choppers to turn DC input signals into AC so AC coupled amplifiers and processors could handle them. They were fiddly and hard to manage, but they did work in the face of measurement systems that wouldn't go to DC.

So if you're good with a soldering iron.

Koz

[cyrano]

Buy a dozen cheap C-media USB sticks from different Chinese sellers. I'll bet there's one where the manufacturer left out the DC blocking capacitors.

If you can desolder and bridge these yourself, any audio interface will go down to DC. The simpler/cheaper the better. The question is, what precision is needed?

If it's a relative measurement, no problem. If it needs to be compared to measurements from other systems, you'll need calibration.

There's also things like an Arduino, that have onboard 8 bit ADC's that are calibrated and pretty precise, if you can live with the 8-bit resolution.

Another solution would be a DMM with serial interface, but I have no idea how good/bad the frequency range is. Usually, it's something like 0-5 KHz, but YMMV.

[TwinOak]

If there are more options on Windows or Linux, perhaps getting a cheap laptop is the simplest solution? Or do you need some other OS X specific software for further processing/analysis?

BR
Alexander

[kozikowski]

Another solution would be a DMM with serial interface, but I have no idea how good/bad the frequency range is. Usually, it's something like 0-5 KHz, but YMMV.

My DMM has a serial interface, but there was something about it that caused me to never use it. Even though the instrument has a high frequency rolloff over 1 MHz, the serial data stream was stupid slow, something like a sample per second or something like that.

There it is. Four times a second (attached).

Koz

[kozikowski]

Is there a USB interface with DC coupled input?

In the audio world that would be a defect, so I'm going with no.

Koz