Understanding the purpose of ADC_SMPR
- Mark as New
- Bookmark
- Subscribe
- Mute
- Subscribe to RSS Feed
- Permalink
- Email to a Friend
- Report Inappropriate Content
‎2013-10-12 09:46 PM
Hi,
I'm trying to understand what the ADC_SMPR register is used for.According to Reference Manual we have following control on the ADC conversion time:ADC_CR1 Register:Bits 25:24 RES[1:0]: ResolutionThese bits are written by software to select the resolution of the conversion.00: 12-bit (15 ADCCLK cycles)01: 10-bit (13 ADCCLK cycles)10: 8-bit (11 ADCCLK cycles)11: 6-bit (9 ADCCLK cycles)But we still have ADC_SMPR 1/2:Bits 26:0 SMPx[2:0]: Channel x sampling time selectionThese bits are written by software to select the sampling time individually for each channel.During sampling cycles, the channel selection bits must remain unchanged.Note: 000: 3 cycles001: 15 cycles010: 28 cycles011: 56 cycles100: 84 cycles101: 112 cycles110: 144 cycles111: 480 cyclesI understand that CR1 defines a minimal number of ADC clocks required for conversion while SMPR defines actual number of clocks that core will use. If my assumption is correct I want to understand how the extra clocks are used. Is it a simple delay and if yes is conversion done at the beginning or at the end of SMPR interval? If this time is used for better sampling or holding please tell us how and what is the precision improvement with a number of conversion clocks increase?Thanks.- Mark as New
- Bookmark
- Subscribe
- Mute
- Subscribe to RSS Feed
- Permalink
- Email to a Friend
- Report Inappropriate Content
‎2013-10-13 05:05 AM
In other words
this is
the time during
the input voltage
is measured.
- Mark as New
- Bookmark
- Subscribe
- Mute
- Subscribe to RSS Feed
- Permalink
- Email to a Friend
- Report Inappropriate Content
‎2013-10-13 06:05 AM
According to specification 15 cycles is enough. Why in this case should I take more? And if I take more what are the improvements?
- Mark as New
- Bookmark
- Subscribe
- Mute
- Subscribe to RSS Feed
- Permalink
- Email to a Friend
- Report Inappropriate Content
‎2013-10-13 09:15 PM
Will I get any information about purpose of ADC_SMPR register? I'm asking ST representatives here in forum.
Why should I guess !!! This information has to be in Reference Manual and if it is not where at least provide it here please.- Mark as New
- Bookmark
- Subscribe
- Mute
- Subscribe to RSS Feed
- Permalink
- Email to a Friend
- Report Inappropriate Content
‎2013-10-14 12:10 AM
I'm asking ST representatives here in forum.
Why should I guess !!!
ST staff is rarely present here.This information has to be in Reference Manual and if it is not where at least provide it here please.
I don't think so. This is general electronics stuff. Search for some article about the principles of successive aproximation ADCs. ''Sampling time'' is a narrow defined term in this context. Why should ST repeat all the basics in the reference manuals ?
- Mark as New
- Bookmark
- Subscribe
- Mute
- Subscribe to RSS Feed
- Permalink
- Email to a Friend
- Report Inappropriate Content
‎2013-10-14 12:43 AM
Sampling time is needed to charge the sample-and-hold capacitor from the measured source through the multiplexer (multiplexer's resistance). If you want to measure the output of a signal source with significant internal resistance, you need to give time to this capacitor to charge. This is dealt with in the respective chapter of the datasheet, there is even a formula giving the maximum external input impedance in terms of number of sampling periods defined in the ADC_SMPRx register. Note, that this sampling time is settable per input channel, thus you can have signal sources with different internal resistance.
JW