Cap voltage divider  - Page 1
Forum section: electronics / projects

author: vignesh_shan
author_rank: Contributor
timestamp: December 06, 2019, 11:58:14 am
content: Hi,In one of my project i seen capacitor voltage divider in the 1.2V DC domain. No clear data given in the datasheet. Only application circuit has that cap voltage divider. If anyone know this requirement pls explain.Thanks,VigneshShan

author: Wimberleytech
author_rank: Super Contributor
timestamp: December 06, 2019, 03:23:22 pm
content: What chip is it?

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 06, 2019, 03:36:30 pm
content: Would need to know the context a bit more. For instance, are those VREFx/MVREFx pins outputs or inputs?If you think of them as inputs, this thing would be odd. Whereas the voltage at the middle point of the "divider" will quicly settle to about half of VCC_1P2 (thus ~600mV, obviously modulo the tolerance of both caps), said voltage will drop exponentially if the inputs are drawing any current.So I kind of doubt those are inputs, and thus kind of doubt this is actually a voltage divider, but taking a look at the datasheet of the IC would definitely help.

author: MagicSmoker
author_rank: Super Contributor
timestamp: December 06, 2019, 04:57:04 pm
content: These caps aren't being used as voltage dividers; they provide symmetrical AC bypassing to both Vcc and GND.

author: Wimberleytech
author_rank: Super Contributor
timestamp: December 06, 2019, 05:39:16 pm
content: Quote from: MagicSmoker on December 06, 2019, 04:57:04 pmThat 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...thius the request. Bypassing VREF this way does not make sense. Thus, I want to know more.

author: MagicSmoker
author_rank: Super Contributor
timestamp: December 06, 2019, 08:59:26 pm
content: Quote from: Wimberleytech on December 06, 2019, 05:39:16 pmIt'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.

author: viperidae
author_rank: Frequent Contributor
timestamp: December 06, 2019, 09:12:56 pm
content: Wouldn't make sense to bypass to VCC. That's going to send any power ripple straight to your vref.

author: MagicSmoker
author_rank: Super Contributor
timestamp: December 06, 2019, 09:40:24 pm
content: Quote from: viperidae on December 06, 2019, 09:12:56 pmA 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.

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 06, 2019, 09:50:59 pm
content: 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.pdfhttp://keysight.com/upload/cmc_upload/All/5Micron.pdf

author: langwadt
author_rank: Super Contributor
timestamp: December 06, 2019, 10:06:24 pm
content: Quote from: viperidae on December 06, 2019, 09:12:56 pmmaybe that is the idea? keep vref tracking the supply?