ADC sampling frequency

Hi,

I’m reading the RSL10_hardware_reference, i cant’ understand the ADC given information.

I want to use the DIO0 to convert an analog value, but I have trouble understanding the configuration of witch mode to use (Continuous or normal) ? also to choose the ADC prescaler.

If we suppose that SLOWCLK frequency of 1.00 MHz, and the ADC_PRESCALE_40H is used, The sampling frequency is :

  • Continuous mode is 1.00 /40 ?
  • Normal mode is 1.00 /320 ?
    Best regards,

I have trouble understanding the configuration of witch mode to use (Continuous or normal) ?

In “Normal” mode, the ADC converts from all 8 channels so the sample rate of any single channel is 1/8th the base sample rate. In “Continuous” mode the ADC converts only a single channel continuously. The decision to use Continuous vs Normal would be based on whether you want to perform conversions on more than one channel in a round-robin fashion or instead more quickly sample a single channel in your application.

If we suppose that SLOWCLK frequency of 1.00 MHz, and the ADC_PRESCALE_40H is used, The sampling frequency is :

  • Continuous mode is 1.00 /40 ?
  • Normal mode is 1.00 /320 ?

Yes, those are the correct per-channel sample rates for ADC_PRESCALE_40H.

Please note that the ADC_PRESCALE_xH values select the “High-Frequency Mode” of operation resulting in 8-bit resolution. If you need more than 8-bits of resolution, choose the a pre-scale of ADC_PRESCALE_200 to ADC_PRESCALE_6400 (valid settings are listed in Table 22 in the Hardware Reference Manual.)

Thank you Jamie for your explications.

what I understood from reading this from data sheet :

High-Frequency Mode:
SLOWCLK is prescaled by a factor of 2, with sampling rates of up to 25 kHz where ADC
measurement results have a resolution of 14 bits, or up to 50 kHz where ADC measurement
results have a resolution of 8 bits

as long as the frequency does not exceed 25Khz, the result have a 14 bit resolution.

Always in data sheet concernng low frequency mode, i found this :

Low-Frequency Mode
SLOWCLK is first prescaled by a fixed factor of 10, with a maximum sampling rate of 5 kHz.
ADC measurement results have a resolution of 14 bits

This maximum rate sampling is given only when the SLOWCLK=1Mhz ? what about SLOWCLK=8Mhz for example ?

Do you have any schematic about the ADC block of the RSL10 ?

Best regards,

The maximum frequency for the SLOWCLK is 2MHz in High-Frequency mode and1MHz in Low-Frequency mode.

I do not have a schematic available for internal blocks other than what is in the Hardware Reference Manual.

Thank you Jamie.

Just to be clear, can we have 25Khz as sampling rate using high frequency mode and have 14 bit resolution ?

Best regards,

Resolution is typical and not guaranteed, but yes you should see up to 14 bits of resolution at 25kHz in continuous mode of operation in high frequency mode.

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Thank you Jamie for your explanations

Best regards,

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@jamie.meacham
Is this true?

The maximum frequency for the SLOWCLK is 2MHz in High-Frequency mode and1MHz in Low-Frequency mode.

I don’t see that limitation specified anywhere in the HW Reference or Datasheet - I only see note like “for a typical SLOWCLK frequency of 1.00 MHz”, but no specified upper frequency limit.

I have been (and would like to) using a SLOWCLK of 4MHz and the ADC configured in Low-Frequency mode with a prescaler of 1600. I seem to be getting credible results, but I’m assuming violating a maximum SLOWCLK frequency will result in a parametric degration (i.e., increased noise, INL, etc). Is that what is likely happening and it isn’t obvious to me without some detail measurements?

Upon further investigation, the SLOWCLK needs to be limited to 1 MHz for guaranteed operation of the ADC. Usage of the ADC with other frequencies is not recommended or supported.

This information will be added to a future version of the Hardware Reference Manual or Datasheet.

We apologize for any information posted previously that suggests otherwise.