Multiple analogue inputs to microcontroller

Hi all,

I am doing a project in which I need to get readings from 4 microphones. I decided to use Pic18f2550 with 10 bit 10 channel ADC and I am not sure whether it is possible to get 4 simultaneous samples from all 4 micros at a time by using just one microcontroller. The signal to be sensed changes very rapidly, consequently any delays beetween taking samples might cause a large error. Waiting for your replies.

well many microcontrollers (including 18f25550) have only 1 ADC, with multiple input channels (10 in this case...) so...

You can convert one channel at a time...if it takes around 25us to convert one sample, you will need at least 100us to convert all 4 channles...

if you really need 4 channles at a time, you must consider using external a/d converters... which will begin conversion at the same time...

You may want to look at some of the higher performance devices - particularly the 16-bit families. Some have multiple A/D's, multiple sample and holds and much faster conversion rates. For example, members of the PIC24HJ series can sample 2, 4 or 8 channels simultaneously at 1.1 Msps with 10-bits, or 500ksps with 12 bits.

Hi!
You could use 4 external sample and holds to sample the four analog signals simultaneously.
e.g. the good old LF358..
**broken link removed**
Please provide more technical data about your project.
BR

Basically what I have to do is to sample inputs from four microphones, then amplify the signals and pass them to microcontroller(s). Microcontroller should convert analogue signals into digital form and then pass it to bluetooth serial module, which should send stream of binary data to Laptop, where data is to be analyzed. I have just started the project, so I haven't bought any components yet. I was planning to use Pic18f2550 microcontroller, however after reading through the datasheet I doubt that it is possible to use it for simultaneous sampling of four analogue channels. There is another option I have in mind, that is to have 4 synchronized microcontrollers, but that would be quite troublesome. In one of the previous posts it was mentioned that there are PICs which can sample multiple analogue inputs at a time, but I havent found any info about that yet. If anyone could suggest me a controller that is capable of doing the job, I would be really grateful.

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It is possible to use it for simultaneous sampling of four analogue channels with external SH or external ADC with simultaneous sampling.

“To have 4 synchronized micro controllers” – It is possible but it depends on the acceptable synchronization errors.

the signal I need to sample has a frequency of 22KHz, frequencies which are higher can be filtered off. Measurements don't have to be extremely accurate, so I think that ADC with 8 bit resolution should be enough. And just like in all other applications it is desired to keep all other errors as small as possible.

Please note that with Fsignalmax = 22 kHz you have

Fsampling = 4*Fsignalmax = 88kHz

in order to guaranteed the band width of the sampler not worse than 3 dB.

Also You should take into consideration the characteristic of the filter before SH (if any) and the ADC when you calculate the bandwidth of the entire sampling and conversion system...

In that case you are in need of Tsampling approx 10 microseconds and that means - external ADC in most of the cases.

Also relatively fast MCU.

I think that it is a good idea to test your system with sinusoidal signal (SS) and to apply the paper

https://www.ieindia.org/pdf/88/88ET104.pdf

But you should give more parameters about your system.

I hope that will help a bit.

x3.exe said:

In one of the previous posts it was mentioned that there are PICs which can sample multiple analogue inputs at a time, but I havent found any info about that yet. If anyone could suggest me a controller that is capable of doing the job, I would be really grateful.

Click to expand...

As I indicated earlier, many of the PIC24HJ series devies have multiple sample & holds. For example:
PIC24HJ64GP210A
PIC24HJ128GP306A
PIC24HJ256GP206A

Also the dsPIC33FJ series has parts with this feature:
dsPIC33FJ64GP206A
dsPIC33FJ128GP706A
dsPIC33FJ128GP306A

If you search the Microchip web site (www.Microchip.com) for "simultaneous sample" it will display dozens of possible chips.

Nice, I have just read the specifications of PIC24HJ64GP210A and it seems that it really has everything I need to do the first stage of the project. Thank you very much GSM Man

Hi, it's me again.
Has anyone actually tried to sample 2 analogue inputs to microcontroller simultaneously? I have managed to sample several inputs sequentially and now I want to get simultaneous sampling working. I have configured all adc registers for it, but I have no idea how to get the sample from corresponding SH to be converted and stored in adc1bufx. The pic I am using is 33FJ12GP202. There are no examples of how to use the simultaneous sampling feature on microchips site and I couldn't find any relevant helpful info anywhere else on the internet. Can anyone please help me with that?

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