Is ripple rejection additive for regulators wired in series? - Page 1
Forum section: electronics / projects

author: SharpEars
author_rank: Unknown
timestamp: October 22, 2014, 05:13:13 pm
content: If I wire two linear regulator ICs in series (e.g., a 12 V regulator feeding say a 5 V regulator) and each has 60 dB of ripple rejection, can I sum up the ripple rejection from each to get something ridiculous like 120 dB of ripple rejection?

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: October 22, 2014, 05:54:49 pm
content: formally the answer is yespractically there are other ways for things to couple, may limit your circuit - even making the measurement at that level may be more difficult than you expectand internal Vref noise on most regs outputs will be another practical limit even if a narrow band measurement could show the ripple component was 120 dB downbut such performance is in fact not unusual to achieve in precision instrumentation circuits

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 22, 2014, 08:45:55 pm
content: To be perfectly precise, rejection in dB is additive. The gain ratios are multiplicative. But yes, all things equalTim

author: vk6zgo
author_rank: Super Contributor
timestamp: October 22, 2014, 10:26:36 pm
content: Quote from: SharpEars on October 22, 2014, 05:13:13 pmI assume you are referring to two regulators wired incascade.----wired in "series" is a different connection altogether!

author: DanielS
author_rank: Frequent Contributor
timestamp: October 23, 2014, 02:49:57 am
content: Quote from: vk6zgo on October 22, 2014, 10:26:36 pmUnless you are doing some fancy stuff, a cascade of linear regulators ends up with their series pass terminals in series.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 23, 2014, 04:50:14 am
content: Quote from: DanielS on October 23, 2014, 02:49:57 amThe definition of a series circuit is that both components have the same current flowing through them.With discrete regulators,you could control their pass characteristics from an entirely different voltage/current source,& maybe say the pass devices were "in series".With packaged regulators,that is not normally the case.& they draw some current to operate,which is not passed on to the following stage.Instead of tying ourselves in knots to use an incorrect term,how hard is it to use the correct one which has been around since the early days of amplifying devices?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 23, 2014, 02:03:39 pm
content: Or even more accurately / precisely / pedantically,Series is correct,but cascade is "more" correct.Tim

author: vk6zgo
author_rank: Super Contributor
timestamp: October 24, 2014, 03:04:13 am
content: In everyday speech we may quite correctly say "a series of regulators",but "in series" is incorrect if we are discussing Electronic circuitry in a technical context."In series" has a simple definition,which is not satisfied by "one regulator followed by another".Even if we split hairs & say "the regulator's pass devices are in series",this can't always work,as there are such things as "Shunt regulators".This is not pedantry (you can go to jail for that!)."Cascade" isn't an esoteric term,it has been in common use for many years.

author: andrija
author_rank: Regular Contributor
timestamp: October 30, 2014, 10:10:52 pm
content: I'd say yes in practice. I did some audio designs in the past that had two regulators between input and the rails, not for this specific purpose though. I never needed anything better than wall wart linear DC adapter to get the 60Hz and its harmonics to be barely noticeable on FFT. Granted I was only doing that with a high end sound card but the same "test" setup with a LM317/337 would end up having much higher ripple on FFT and switching to battery input would barely detect as well. It seems like a good idea in theory and practice. With two regulators you push direct ripple enough that you start seeing the effect of PCB layout, grounding scheme, noise pickup and such more easily.