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| Cap voltage divider |
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| MagicSmoker:
--- Quote from: Wimberleytech on December 06, 2019, 05:39:16 pm --- --- Quote from: MagicSmoker on December 06, 2019, 04:57:04 pm ---These caps aren't being used as voltage dividers; they provide symmetrical AC bypassing to both Vcc and GND. --- End quote --- That was my first impression as well given that these go to vref pins. But, I want to see what is going on inside the chip...this the request. Bypassing VREF this way does not make sense. Thus, I want to know more. --- End quote --- It's unusual to bypass VREF this way, but assuming this is a microcontroller with an internal ADC then perhaps the goal is more to filter out noise showing up on VREF from the MCU's operation, rather than keeping external noise from getting into VREF. That said, I've never seen this done in anything but precision/sensitive analog circuits, and those using balanced/differential signaling. For example, CAN bus often uses split termination with small capacitors in parallel with the termination resistors for better common mode rejection. If one views the MCU as the source of common mode noise and its own VREF pin as the victim then this kinda sorta almost makes sense. |
| viperidae:
Wouldn't make sense to bypass to VCC. That's going to send any power ripple straight to your vref. |
| MagicSmoker:
--- Quote from: viperidae on December 06, 2019, 09:12:56 pm ---Wouldn't make sense to bypass to VCC. That's going to send any power ripple straight to your vref. --- End quote --- A valid point, though I suppose I should emphasize that I wasn't exactly making a case for split bypassing VREF on an MCU, just trying to come up with reasons why it might be done (besides stupidity... the default reason a lot of things are done). However, I also wouldn't assume that ground is noiseless, especially not in a mixed signal system as seems to be case here. |
| SiliconWizard:
Since we got no further info... But I was still curious, so from the names of the few pins on this piece of schematic, I'm guessing it must be part of a DDR3 interface. Decoupling DDR3 VREF* pins to Vdd is apparently a very common recommendation for DDR3. https://www.nxp.com/files-static/training_pdf/VFTF09_AN111.pdf http://keysight.com/upload/cmc_upload/All/5Micron.pdf |
| langwadt:
--- Quote from: viperidae on December 06, 2019, 09:12:56 pm ---Wouldn't make sense to bypass to VCC. That's going to send any power ripple straight to your vref. --- End quote --- maybe that is the idea? keep vref tracking the supply? |
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