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Starting Member. ADC maximum bandwidth Hello, new here What I'm trying to do is converting analog voltage signals to USB output, so the analog signal must be first converted to digital data through an ADC.
But I want to be able to enter signals of 1 MHz without problems, i. I still don't get very well what acquisition time or TAD mean, but checking the formulas for Taqc calculation I'm guessing I need an acquisition time much lesser than 1 us in order to be able to handle 1 MHz input signals.
Anyway, assuming I configure the PIC to be using the maximum Fosc of 48 MHz TAD of 64 Tosc according to datasheet , can I reduce conversion times or acquisition times by choosing the right impedance and capacitance values? In order to handle 1 MHz input signals Or if it's not possible to reach 1 MHz, what would be the maximum input frequency the PIC can handle?
I know I'm very lost and I'm badly mistaking terms, I still don't understand how MSPS, conversion time or acquisition time are related. But if you can help, I'll be very grateful.
Super Member. With practical choices of crystal frequencies a TAD of 0. If you can live with a 2. Do not use the AVSS as a ground return path for high current loads or transient switching loads. It will be in the 8 to 10 KHz range. It depends on the waveform you want to sample.
The less like a sine wave to more sample point are needed so the lower the frequency the waveform can be. They can also do automatic successive conversions and store the result in one 16byte or 2 8byte alternated buffer s and give you an interrupt after up to 16 samplings. Circuit breaker. Thanks all very much. Guess I won't be free from getting an external ADC then First of all the target MCU should be choosen properly. Yeah, as MBedder says! Sorry, but I wasn't expecting the last 2 messages, and I'm having trouble understanding them The software used would be LabView, obviously using the Real Time module.
Hope I've finally explained myself. But then what are you saying? How many bits of ADC resolution do we need? How many ADC samples per second do we need? There are many clever ways to get more accuracy in amplitude and time with slower analog to digital converters. The above statements are just the most elementary view of the measurement problem. As you, Dan, said, those may be some of the most basic concepts, and I apologize if I'm asking for simple things, but I'm just starting Electronic Engeenering studies and I haven't seen that much until now, just starting with basic science stuff.
And all the stuff I came here with I had to look in a book a friend lent me I'm sailing mostly unknown seas. But, if you're really talking about USB capacities directly, then no matter if I had the fastest ADCs and operational amplifiers, there would be always an ugly bottleneck. The second to last thing you said, do you mean there are ways to achive my desired frequencies even using slower and common chips?
What I am trying to explain is the sampling required to convert an arbitrary waveform from the analog domain to the digital domain. What do actual data rates on USB look like? Typically, the most important ones are the target device's ability to source or sink data, the bandwidth consumption of other devices on the bus, and the efficiency of the host's USB software stack. In some cases, PCI latencies and processor loading can also be critical. Assuming only the target endpoint consumes a significant amount of bus bandwidth-and both the target and the host are able to source or sink data as fast as USB can move it-the maximum attainable bandwidth is a function of the transfer type and signaling rate.
These bandwidths are given in chapter 5 of the USB Specification. In practice, most hosts can reach the maximum isochronous and interrupt bandwidths with a single target endpoint. The PIC32 has up to kB of ram and with 10 bits per sample you could put 3 samples in one 32 bit dword if you need or just use one 16 bit word for each sample.
One 16 bit word per sample, and a 64k buffer will give you 32ms continuous data at 1Msamples per second. This will not give you a continuously sampling stream but chunks of 32 ms each transfer to the PC. Not sure how long it will be until you can send the next chunk of 32 ms sampling data but probably several seconds. Next problem will be to tell when you should start your sampling. If it is a signal that always looks the same or you don't care then it is no problem but if you want to start the 32 ms sampling at a certain 'look' of the input signal, you also need to handle triggering.
You can also use an external analog triggering circuit which will provide you with a digital output. The advantage with using everything in one chip is that there will be no need for external communication to shift large amount of data in to and out of buffers. What type of signal will you be looking at and what is your start sampling triggering needs? Yeah, what Ruben says is the way to go in such cases, I'd say.
I expect to enter any kind of signal sinus, triangle, square If I decide to stick with more common and slower chips and PIC, what way would you suggest for getting more accuracy in time with slower chips? Because the idea would be then limiting my direct input signal to, let's say, KHz, and for higher frequency signals apply the trick Heck, I'm really in trouble What you want to call the bandwidth is dependent on what you intend to do with this data.
If you are doing a Fourier on the data, then your maximum spectral bandwidth is 48 KHz, bounded by the Nyquist limit. If you want to see the waveforms in time domain, like an o'scope, then you would want at least 8 data points per wave to see what it is, so then the "bandwidth" falls down to about 10 KHz.
The other "bandwidth" to worry about is amplitude flatness. Higher frequencies than 48 KHz will show up as false frequencies. Dan covers this all pretty well in his first post, and his post assumes you are making an o'scope.
What are you going to do with the data when you get it into the PC? Well, yes, I'm trying to "make" an "osciloscope", in the sense that I need indeed faithful reading of the analog signal on the PC. The final application is a bit different, but this is indeed the core idea, yes.
My "bandwidth" concept goes that way. Now that I mention it, I was told more than once here in the thread that even if I had a super fast ADC I could get little to no benefit from it, but I still don't get why.
Regarding USB, can I calculate its maximum "bandwidth" from its transfer rate? Then why does the datasheet says "USB 2. What people are saying is that if you decided to go for a kick-ass external ADC then the poor little pic18 will not be able to keep up with it. You need to move the data out as fast as it is being digitized and the pic18 isn't good for that. The bandwidth of the USB is not a big issue. I don't think you would want to constantly stream the data, right?
Grab a frame, show a frame, grab a frame, etc. So the USB bandwidth is irrelevant. You could use RS if you don't mind a slow refresh rate. But if you want these high speeds above 10 KHz, you really need to move to a processor with fast adc and big ram, like the pic If the answer is positive, what PIC32 would you suggest? I'm barely able to get a in my country, a bit PIC will never be possible here!!! Latest Posts.
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