Polarized caps on DAC - Page 1
Forum section: electronics / projects

author: sirhaggis
author_rank: Regular Contributor
timestamp: December 01, 2016, 10:38:36 am
content: I am working with the PCM5121http://www.ti.com/lit/ds/symlink/pcm5121.pdfOn the typical application circuit p55 it species 10uF polarised bypass caps alongside the usual 0.1uF. Is the polarity necessary in this case or would unpolarized MLCC SMD caps be appropriate here? I presume polarity is probably important on VCOM, but the supplies? For SMD polarized caps, would tantalum caps by the way to go in this application? My applications requires small size and low vertical profile caps so electrolytics are not a good option, but I would also obviously like to minimise distortion.ThanksLuke

author: Zero999
author_rank: Super Contributor
timestamp: December 01, 2016, 12:04:18 pm
content: Quote from: sirhaggis on December 01, 2016, 10:38:36 amI think you've got the page numbers mixed up. There's no schematic on page 55. I think you mean page 58.Of course non-polarised capacitors can replace polarised capacitors, in any situation.Beware of MLCC capacitors which suffer from dielectric saturation, meaning the actual capacitance drops, as the voltage on the capacitor rises. Using a much higher voltage rating than the supply voltage (factor of five or more) will help.You could use tantalum but some people here talk about issues with them failing catastrophically and burning up, although I've never encountered this myself. It shouldn't happen if you use decent branded capacitors, overate them and make sure the current surge when the power is first applies isn't too high. If your circuit is powered by a voltage regulator with an output current of under 1A, then this shouldn't happen.Distortion won't be a problem, as the analogue signal doesn't pass through any of the capacitors mentioned above.

author: sirhaggis
author_rank: Regular Contributor
timestamp: December 01, 2016, 01:06:19 pm
content: So do you think overrated 10uF MLCCs would be my best option in this case, for a 0805ish form factor? I'm new to SMD caps so there may be options I'm not aware of...

author: Zero999
author_rank: Super Contributor
timestamp: December 01, 2016, 04:13:16 pm
content: Quote from: sirhaggis on December 01, 2016, 01:06:19 pmYes and to give you an idea about derating, look at the datasheet for a 22µF 25V ceramic capacitor in an 0805 package. With a bias voltage of 5V, its capacitance is already under 50% of the rated value. As this is just for supply rail bypassing, more capacitance is generally better, so at 3.3V it should have a high enough capacitance for what you need.http://uk.rs-online.com/webdocs/13d2/0900766b813d2ca1.pdfhttp://uk.rs-online.com/web/p/ceramic-multilayer-capacitors/7900682/

author: tatus1969
author_rank: Super Contributor
timestamp: December 01, 2016, 06:06:55 pm
content: In many cases the guys collecting the datasheets are just copy&pasting the so-called "best practices". I would skip the 10uF part and test possible impact on analog performance. I would bet a beer that the result is: none. Because- all switching noise is handled by the 100n MLCC anyway- the chip only consumes around 10mA per supply, and for a DAC it should be pretty constant.Unless... you have a significant load at the outputs. Which you shouldn't

author: suicidaleggroll
author_rank: Super Contributor
timestamp: December 01, 2016, 06:52:47 pm
content: Quote from: Hero999 on December 01, 2016, 12:04:18 pmFrom what I've seen, voltage rating has little to no impact on the DC bias of MLCCs. What matters most (other than dielectric type) is package size. The larger the package, the less of an effect DC bias has. Going to a substantially higher voltage rating could help, but only if doing so forced you into a larger package.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 01, 2016, 07:53:13 pm
content: Alu elcap might be used in the supply pin bypassing because it has high ESR, providing necessary damping for the naturally occuring LC filter (from the trace inductance).On a PCB, with short traces from the voltage regulator (which might have a high ESR cap on its output, for the same reason!), it's most likely totally unnecessary, but sometimes it's needed.It's not about polarity, and not about the loss of capacitance of a MLCC with DC bias (because the exact capacitance doesn't matter) - ESR is most likely the real reason the cap has originally been drawn. You can use MLCC with resistor in series (1-2 ohms?), but use a small, low-inductance resistor and keep the traces short. The resistor is not going to dissipate much unless there is huge ripple, so you can use a small package.Tantalum has high ESR too, and is a good replacement candidate, but you need to derate the voltage rating to <50% and only use the tantalum after a current-limited linear regulator, no switches.But most likely, it's OK without it as long as you have the regulator close, not oscillating, with proper output capacitance. In this case, you don't want to replace this elcap with any big MLCC, you just go without it and go with the 100nF MLCC only.This may be a rather old schematic, from a design that is physically large and shares a voltage bus with long cabling. In that case, think about the chip requiring a pulsed current: they need more capacitance than 100nF to store the energy locally, as the wiring inductance slows down energy transfer when it's needed. And then they need AC losses to get rid of the reactive energy when the energy stored in wiring inductance finally flows,afterit's no longer needed - this would be an overvoltage spike with ringing. An alu elcap is the simplest solution, as it provides both high capacitance and high ESR (AC losses) in one package, for low price, and in this kind of application, the lifetime will be just fine.

author: dmills
author_rank: Super Contributor
timestamp: December 01, 2016, 08:36:22 pm
content: The only one I would be a little cautious of is the VCom decoupling cap, as that is quite likely a sensitive analogue high Z node and high K ceramics beget microphony.Other then that ceramics (with the usual precautions) should be fine.regards, Dan.

author: Zero999
author_rank: Super Contributor
timestamp: December 01, 2016, 10:09:30 pm
content: Quote from: suicidaleggroll on December 01, 2016, 06:52:47 pmBut won't a higher voltage rating mean a bigger package, even if the footprint remains the same?Let's compare the datasheets for two capacitors, same dialectic, manufacturer, capacitance, size but different voltage ratings, running at 5V:22µF 25Vhttp://uk.rs-online.com/webdocs/13d2/0900766b813d2ca1.pdfAt 5V, the capacitance will be down by just over 50%22µF 6.3Vhttp://uk.rs-online.com/webdocs/13d4/0900766b813d404c.pdfThe capacitance will be down by 65%

author: Someone
author_rank: Super Contributor
timestamp: December 02, 2016, 12:40:01 am
content: Quote from: Hero999 on December 01, 2016, 10:09:30 pmMurata have a good tool for both obtaining their parts characteristics, and comparing them. While most of the 0805 10uF line has similar DC bias characteristics you can find a wide variety with some outliers: