A/D converter needed

[Volkkarijermu]

Hello everybody!

I need A/D-converter to DB9 serial port. Sample rate should be quite fast. Maybe something more than 50000 samples/second. Could somebody give some schematics/pcb layout/website link or more information. I want to use that converter as a oscilloscope. I have some electronic device and I need to see how voltage drops and rises on one cable.

 

[dknguyen]

ADC's are pretty hard to build, particularily fast accurate ones. Some ADCs have a lot of "switching" happening which you really don't want to have to keep track of. And the ones that don't require accurate capacitors/resistors or have thousands of Op-Amps.

Did you already rule out looking at ADC ICs from Maxim?
**broken link removed**

Pay attention to the interface method, whether it be serial or parallel (very, very fast but every control bit to start a sample and every output bit on the ADC reading takes a pin. Not a problem if you are doing an oscilloscope and only need to read from one or two ADCs.)

 

[Volkkarijermu]

Well I found this:
**broken link removed**

But I don't really know hoh fast is it. And I don't know is it possible to use as an oscilloscope?

 

[dknguyen]

APplication

It is an application note- it describes how to use ICs do get something done. The speed depends on the ADC ICs. (the ones in the application note are the MAX#### numbers). You can search for them on the company website.

To use it as an oscilloscope (which I don't recommend building for use, unless it is just for an experimental project) you need to write PC software for it to interpret the data. And unless you use things if FPGAs (expensive, hard to work with, and requires a background in digital logic design)...it may be too slow.

 

[Volkkarijermu]

Ok... maybe this is too hard project for me

 

[dknguyen]

Dmm

Try it, but think of it more as a digital voltmeter (or a graphable voltmeter). You can always read the voltage out to a small LCD screen, than to try to graph it out on a computer monitor. It just depends on how fast you can sample it. If you can get your compute to receive the output from the ADC very quickly, it shouldn't be that much of a problem...just dont read insanely fast signals.

 

[Volkkarijermu]

Ofcourse it's possible to use computer's soundcard as an oscilloscope with this program: http://www.zelscope.com/

But my laptop is so old that it doesn't have souncard. Of course there are PCMCIA-cards but they are 32bit (cardbus?) cards and my laptop doesn't support those. 16bit works (should be) but where I find 16bit PCMCIA soundcard? I have tried to find but...

 

[dknguyen]

HEhe, actually that's a pretty neat idea, to convert the voltage reading to an audio signal and read it into a microphone input in a computer and use existing audio software. It seems to me it would only be able to measure AC signals though, no DC since sound doesn't have a DC component

 

[Nigel Goodwin]

HEhe, actually that's a pretty neat idea, to convert the voltage reading to an audio signal and read it into a microphone input in a computer and use existing audio software. It seems to me it would only be able to measure AC signals though, no DC since sound doesn't have a DC component

There are various free scope software programs you can download (and have been for years), that use the sound card for an interface - however, they are very limited.

 

[phalanx]

Even with the most basic dsPIC devices like the 30F2010 or 30F2011, you get either a 10-bit 500,000 sample/sec or a 12-bit 200,000 sample/sec A/D converter in addition to a hardware UART which allows for easy connections to a serial port.

I forget the exact numbers but I know that a lot of the 16F devices have an A/D converter that can do at least 50,000 samples/sec with 10-bit resolution along with a hardware UART.

Using a PIC will give you a simple solution.

 

[Volkkarijermu]

But I don't know what kind of connection and other components I need. If you give me 30F2010 to me I don't know what to do with it. I'm not expert of electronics. That's why I need pictures of connections (for example like this one: **broken link removed** )

 

[Paul Obrien]

I have always use a rule of thumb that the sampling rate is about 1/10th of the max sampled frequency so with 50000 samles/sec the bandwidth would be 5KHz not exactly a high speed scope. The line input on the soundcard should be able to do that.

 

[Papabravo]

What people are trying to say is that there are canned solutions that you should investigate before going off on a quest to build something that you don't understand and cannot troubleshoot if it does not work. The more you learn about the alternatives the better prepared you will be to attempt more complicated solutions.

In making mirrors for telescopes, bigger is better. The quickest way to make a four foot diameter mirror is to start by making a six inch mirror, and then a one foot mirror and so forth. Get the picture?

 

[Oznog]

OK, a few things here:
1. There is probably no ADC chip which will talk to the RS232 serial port itself and do what you want to do.
2. The serial port is probably not even capable of reading 50kS/sec. At 16 bits/sample, that would need 800 kBaud whereas 57.6kBaud is the best I hear about. So if it's like an oscilloscope then the hardware needs to handle the signal. Trigger, read a screenful of like 256 samples and save it to memory, then just transmit that frame of samples. A microcontroller can do this.
3. Isolation. The controller and its ADC, as well as any external ADC you might add, are going to share a ground with the serial port and the computer. This may or may not be acceptable. If not, you'll need to isolate the ADC section with an optocoupler and come up with an isolated power source for it (battery or an isolated DC/DC). Unfortunately, optocouplers and generally fairly slow. You probably could not capture at a rate of 800KBaud through any opto. You'd need to add a microcontroller on the isolated side with the ADC to talk fast to the ADC, capture a burst of ADC readings, and then feed them through the optoisolater to a microcontroller on talking to the serial port. Or probably you could just make the isolated ADC microcontroller talk through the opto straight to the interface chip (like a MAX232).

 

[Paul Obrien]

There are serial ADCs around TLC548/9 run at this sample rate but why bother!

 

[Paul Obrien]

One more thought, have you looked a sigma-delta converters. While most have a serial output it's SPI,microwire etc.

 

[csaba911]

Hello everybody!

I need A/D-converter to DB9 serial port. Sample rate should be quite fast. Maybe something more than 50000 samples/second. Could somebody give some schematics/pcb layout/website link or more information. I want to use that converter as a oscilloscope. I have some electronic device and I need to see how voltage drops and rises on one cable.

Hey

Is your sereial port can read 50000*9 bit ?

You should buy a cheap scope, or beter a good one !

Csaba

 

[Misterbenn]

you could try a flash converter... there the fastest type of A/D you can get really. tho extreamly complex to build.... ie you need n^2 op-amps where n is the number of output bits. easier just to buy one....

or you could use the successive apporximation method, made out of simple building blocks EG a state machine, D/A converter and a op-amp