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Simultaneous reading of ADC and the best and fastest way to do it.

Natalia Requejo

I am writing to see if someone can give me a hand with the ADC on LPC4337

I'm doing a practice in which I have to sample two signals at 200Hz.
I need to sample at least 500 points for each cycle.
Since I know that the LPC4337 ADC have a sampling frequency of 400 KHz I assumed that is possible.

However the maximum speed that achievement, sampling with DMA Channel 2 and 3 of ADC0 and ADC1 is 33,8kHz. which represents approximately 169 samples per cycle.
(To reach that number, (33,8KHz) I turn on and turn off a exit pin, when finished reading the 2 ADC and I measure it with an oscilloscope)
If I only activated one channel and one ADC, the actual sample frequency is 79.9 KHz. (Which also remains poorly)

all those test was with BURST Mode disabled. If I configurate the ADC in interruption mode instead of DMA, it is slower yet.

Finally I tryed enabling Burst Mode, and remove the DMA and interrupts.
I made a loop and I kept waiting for the data inside a while (1), Channel 2 and Channel 3 ADC0.

/* Waiting for A/D conversion complete */
        while (Chip_ADC_ReadStatus(LPC_ADC0, ADC_CH_SEN, ADC_DR_DONE_STAT) != SET) {}
        /* Read ADC value */
        Chip_ADC_ReadValue(LPC_ADC0, ADC_CH_SEN, &dataADC0);


Now the frequency is 100KHz approx. almost exactly 500 points per cycle . But as I start to make something, this consume cycles and the frecuency falls, obviously I do not get the amount of points I need.
   
The ADC is configured with a 400KHz sampling frequency and with 10-bit resolution.
the test were without interfering with the reading of the ADC.

Questions:
What is the best and fastest way to read the two signals with the ADC?
Is it possible to sample faster than 100KHz?
Is this a limitation?
Are there some extra settings I should do?

I also need to read the two signals simultaneously.
Is this possible?
if I use the Channel 2 on ADC0 and the channel 2 on ADC1,
Are the readings simultaneous ? or sampled first one and then the other.

Any help you can give me I would appreciate it. Sorry for my english.
Thank you so much.
Natalia

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    Note that depending on the resolution you want from the ADC, each conversion requires a specific number of clock cycles. So at a specific resolution, it might need 11 clock cycles. If you then want to do 10kHz sampling rate then you need at least 11*10000 clock frequency to the ADC. Have you made sure that you have configured the ADC to get a fast enough clock frequency? A too low clock frequency to the ADC will limit the maximum sampling rate from the ADC.

    So if the maximum sampling rate is 400kHz for your processor, then it's likely that you would need 5MHz input clock frequency.

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  • 0

    Note that depending on the resolution you want from the ADC, each conversion requires a specific number of clock cycles. So at a specific resolution, it might need 11 clock cycles. If you then want to do 10kHz sampling rate then you need at least 11*10000 clock frequency to the ADC. Have you made sure that you have configured the ADC to get a fast enough clock frequency? A too low clock frequency to the ADC will limit the maximum sampling rate from the ADC.

    So if the maximum sampling rate is 400kHz for your processor, then it's likely that you would need 5MHz input clock frequency.

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