connecting Voltage Regulators in Series?

[StarFishPrime]

Hi Folks
can anyone opine here?

I am building a fluxgate solar aurora monitor with the magnetic field sensor buried in my garden to minimise temperature variation. I want to provide a very stable 5V supply to the sensor - which draws less than 15mA. It has been suggested to me that I should take a, say 12V supply from a standard wall mounted dc converter, then feed it through a 9V voltage regulator whose output feeds a 5V regulator.

Is there any merit in this approach i.e. double regulation to produce a more stable output or should I just use a single regulator?

Thanks

ian Robinson

[acbern]

Generally, cascading voltage regulators does make sense only if your input varies quite a lot, then the second regulator sees less variations and any variations due to input fluctuations are minimized. Others are not.
Your approach should rather be to determine what fluctuations limits you need (caused by dc input voltage changes, but also e.g. temperature... also what noise is acceptable). Calculate all this over your environmental conditions.
Then select the right solution, you may be fine with just one higher precision device.

[plazma]

What about the benefit of having two linear regulators dissipating the power loss?

[Ian.M]

Also *most* wallwarts are very noisy.  The switched mode ones are notorious, and the linear ones tend to let through HF noise from mains transients, + the bridge rectifier can itself be noisy.  You may well find you need to build a high quality linear supply using a transformer with an earthed interwinding screen, and a choke input filter immediately after the rectifier, with suppression caps accross all the diodes in the bridge.

[forrestc]

Is there any merit in this approach i.e. double regulation to produce a more stable output or should I just use a single regulator?

I'm guessing the theory is that you'll end up with effectively doubling the PSRR (Ripple Rejection).    I guess if you start with a lot of noise, reduce it by 72dB (typical PSRR of a 78xx), and then reduce the resulting noise by another 72dB you'd be ahead... but how much noise are you really needing to reject?   Even if you started with 1V of ripple, 72dB of rejection would knock that down to 250uV.   Add 40uV of output noise, and you're around 300uV.  Is that low enough? Depends on your application.   Running it through a second regulator might reduce it even further, but that seems excessive.

If it isn't good enough,  think you'd be much farther ahead to use an already-regulated (preferably linear) supply with low ripple and then just run it through a single regulator with good quality input and output filters.  Reducing the ripple to 0.1V should get you down to 25uV+40uV.      Much lower, you're going to have to find a better regulator.   For instance, use a LDO with much better noise and PSRR ratings such as a LT3042 (just the first one I found).   It looks like across most of the spectrum it's 10dB better than the 78xx which means you knock that 0.1V ripple down to 8uV, and even better, the noise voltage is only 0.8uV, so you're down to <10uV.

Personally, I'm skeptical that you need that low of noise on the rail.   More importantly, it's going to take very careful design to both achieve this, and also keep the rail that clean.  10uV is nothing.   To put this into perspective, if you have current ripple on the rail of even 1 mA, and you have a rail resistance of even 0.01 ohms, you're going to induce 10uV of voltage ripple just because of voltage drop.

I'd start backwards at this point - start with how much ripple can you tolerate and then build your power supply system accordingly.   One note here is to not underestimate the utility of local power supply filtering - an appropriate ferrite in the right spot can do wonders.   In the few spots where I've really cared about noise on the rail for a part, I've used that approach instead of trying to filter the rail itself to some unobtainable low value.

[vk6zgo]

Hi Folks
can anyone opine here?

I am building a fluxgate solar aurora monitor with the magnetic field sensor buried in my garden to minimise temperature variation. I want to provide a very stable 5V supply to the sensor - which draws less than 15mA. It has been suggested to me that I should take a, say 12V supply from a standard wall mounted dc converter, then feed it through a 9V voltage regulator whose output feeds a 5V regulator.

Is there any merit in this approach i.e. double regulation to produce a more stable output or should I just use a single regulator?

Thanks

ian Robinson

First of all,they are not "in series"---they are in "cascade".
This means the output of one is fed to the input of the following device,& so on.

Cascaded devices are used,but usually to obtain several different voltages.

[Kleinstein]

Using two regulator in series can improve PSRR, but this might not be the best solution:

As the sensor is likely running long time, the supply should be something like a switched mode wallwart. Here there is very little 60/120 Hz ripple but HF noise is more of a problem. Here the linear regulators are not very good anyway. So I would suggest using extra filtering, e.g. a common mode choke and capacitors and only one regulator after that. For the higher Frequencies the layout and choice of capacitors can be more important than the PSRR stated in the datasheet.

It also depends on the sensor / amplifier used how much noise the sensor can tolerate. I don't remember a fluxgate being very sensitive to noise on the supply. The bigger problem is likely people moving iron parts or cars driving by.

[Ian.M]

Its going to be running 24/7 and the chances of a good linear supply surviving long-term are much higher than anything except an $expensive$ branded SMPSU.   Personally, I'd probably run it and the datalogger off an old car battery kept on float charge, because that will not only strip out all ripple, but provides battery backup so you can log right through any *interesting* events that are severe enough to affect the power grid.   

Also there may well be a lot of benefit in doing final regulation to 5V using a shunt regulator at or near the sensor, rather than having to contend with voltage drop and noise pickup on the wiring.  A shunt regulator fed from a constant current source at the supply end of the cable will keep the power dissipation in the instrument constant, and also means that any residual field from the cable is static.   That means you don't have to combine power and data on a coax cable to avoid stray fields, but can simply use direct burial rated Cat5 STP. with power and amplified differential sensor output on separate pairs.

[mmagin]

One of the things I've noticed is that the typical linear regulator doesn't really have the bandwidth to reject the kind of ripple that a switching regulator emits very effectively.  Some are more effective than others, but the more bandwidth the linear regulator has, the more you have stability issues.  One thing you can do is put an LC low-pass filter between the switching regulator and the linear regulator.