0
\$\begingroup\$

Probably a rookie question and I'm overthinking it.

If I have an 8 bit ADC on a microcontroller sampling at 20 MHz, and the ADC operates at a 132 MHz clock, the voltage isn't communicated every period is it? Does how long it takes to communicate the voltage depend on how big the word is? So in this case, it would take 8 cycles which would be ~ 60 ns (8/132E6) since its an 8 bit ADC? I know 8 bit gives the resolution so the (voltage range/256). I know the 20 MHz sampling just means it can measure changes to the pin at a 10 MHz rate (nyquist)

Thanks

\$\endgroup\$
5
  • \$\begingroup\$ Is the output of the ADC serial or parallel? If it's internal to the uC, it's probably parallel? And if it's parallel, then it probably doesn't take 8 cycles to "communicate" the voltage. \$\endgroup\$
    – SteveSh
    Mar 4, 2022 at 20:44
  • \$\begingroup\$ If the ADC can take a new sample every 50 ns (20 MHz), then a new output from the ADC most be available at that same rate. If that wasn't the case, then ADC samples would be thrown away. \$\endgroup\$
    – SteveSh
    Mar 4, 2022 at 20:46
  • \$\begingroup\$ What's a "period?" MCU clock period? ADC output rate? ADC sample rate? Sample rate and output rate can be different because some ADCs take multiple samples to produce just one output reading. MCUs aren't really designed for applications where you output almost every ADC sample it takes. Usually the I/O interfaces aren't fast enough. It's designed to mostly use the readings internally. \$\endgroup\$
    – DKNguyen
    Mar 4, 2022 at 21:32
  • 3
    \$\begingroup\$ Your question is severely lacking info on chipset, structure , assumptions , specs and focus on question. DMA, BW, SNR, Oversampling tradeoffs \$\endgroup\$ Mar 4, 2022 at 21:51
  • \$\begingroup\$ @SteveSh I'm not sure. I was looking at the data sheet/reference manual and it doesn't really specify. I know its a 32 bit MCU. The ADC is internal to the MCU so does that mean it probably just takes one cycle? \$\endgroup\$
    – bchang32
    Mar 7, 2022 at 13:10

1 Answer 1

1
\$\begingroup\$

I don't know about over-thinking it but you've certainly gone in some very strange directions.

An 8-bit ADC 'sampling at 20 MHz' really means an ADC doing 20 million conversions per second, each of which produces an 8-bit sample from the current input voltage.

So your MCU's ADC is running at 20 Msps (Mega samples per second).

Internally, the ADC would then be producing 20 MB of data per second. That needs processing, storing or communicating out, using the CPU, DMAC or a comms port/parallel bus, respectively. That's a lot of data to handle if sustained and will most likely overwhelm your MCU pretty quickly, depending on what you're doing.

You don't state which MCU you have. The ADCs in MCUs are, by volume sold, most often successive approximation ADCs. These require a higher clock frequency than its conversion rate. That's usually 1 clock per bit for a conversion which would be 160 MHz for an 8-bit 20 Msps ADC.

\$\endgroup\$
4
  • 1
    \$\begingroup\$ "The ADC will require a higher clock frequency than its conversion rate" Not necessarily. There such things as flash converters, which take one clock per conversion, and half-flash converters, which need 2 or 3 clocks per conversion. \$\endgroup\$ Mar 4, 2022 at 20:58
  • 1
    \$\begingroup\$ @WhatRoughBeast, yes I was thinking that hadn't come out right once I'd posted it. Have clarified what I was trying to say. Flash converters have been around since the dawn but are rarely (never?) found in low-end/cheaper MCUs, which is what most MCUs are by volume (rather than by product range). \$\endgroup\$
    – TonyM
    Mar 4, 2022 at 21:12
  • \$\begingroup\$ " Flash converters have been around since the dawn" - Well, yes, but ... I once built a video flash converter which was better than anything commercially available. Used Am686 comparators and 74S encoding logic, and produced 4 bits. So, yeah, "the dawn" pretty much covers it. \$\endgroup\$ Mar 5, 2022 at 15:10
  • \$\begingroup\$ @TonyM So I'm currently looking at the MPC 5775E from NXP and comparing it to an IC, the ADS7961. It seems like the MCU ADC can sample everything at 33 MHZ and is parallel, but the IC is SPI but samples at 1 MHz and communicates at 20 MHz. Since SPI is serial, I'm guessing it takes 5ns * x to communicate where x is the bit resolution of the IC? \$\endgroup\$
    – bchang32
    Mar 7, 2022 at 13:19

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.