On 5/28/2014 4:20 PM, whit3rd wrote:> On Wednesday, May 28, 2014 10:05:31 AM UTC-7, Phil Hobbs wrote: > >>>> Lately I've been using shunt regulators on the base string of cap >> >>>> multipliers. AFAICT the best is the Exar SPX431A. >> The Exar is quieter and has a lower minimum cathode current. > > It's possible to get lower (SPX431A wants over 400 uA, and TLV431 > wants over 50 uA) but there's some cathode-voltage-range issues, too. > > Have you considered direct shunt regulation? > > > > 0--------*---RRRR---*-----*----------0 > | | | > R | | > R | *-----* > R CCC R | > R CCC R C > | | R | > | | *-----* > *--RRRR----* | > | | | > | /---/ | > CCC / \----* > CCC --- | > | | R > | | R > GND | | > GND GND >You're stuck with the wideband noise of the reference, though. I often have to care about nanovolt 1-Hz noise on power supplies. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Benchtop Power Supply Options
Started by ●May 7, 2014
Reply by ●May 28, 20142014-05-28
Reply by ●May 29, 20142014-05-29
On 05/28/2014 01:15 PM, John Larkin wrote:> On Wed, 28 May 2014 13:05:31 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 05/28/2014 10:24 AM, George Herold wrote: >>> On Tuesday, May 27, 2014 8:27:19 PM UTC-4, Phil Hobbs wrote: >>>> On 5/27/2014 7:21 PM, John Larkin wrote: >>>> >>>>> On Mon, 26 May 2014 21:50:17 -0400, Phil Hobbs >>>> >>>> >>>> Lately I've been using shunt regulators on the base string of cap >>>> multipliers. AFAICT the best is the Exar SPX431A. You can adjust that >>>> to lop off the ripple if you like, though of course it isn't as >>>> efficient as using a huge cap. >>> >>> That's interesting, the Exar is "best" because it has the least noise to start with? >>> >>> As you've said, the reference/regulator can be the noisiest part of a design. >>> I've got an LT3080 in an instrument. (cap multiplier after it.) >>> And I've regretted it. Much better to just make your own, >>> (reference->opamp->transistor) >>> >>> George H. >>> >> >> The Exar is quieter and has a lower minimum cathode current. >> >> 0-------*---------RRRR---*---* *-*----------0 >> | | \ / | >> R | \ A | >> R | ------ | >> R CCC | | >> R CCC R | >> | | R R >> | GND R R >> | R R >> | | R >> | | | >> *---RRRR---*--RRRR------* | >> | | | | >> | | | | >> CCC /---/ CCC | >> CCC / \---* CCC *--RRRR--* >> | --- | | | | >> GND | | GND | GND >> | | | >> GND *-------------* >> >> Cheers >> >> Phil Hobbs > > That circuit necessarily increases Vce, which has additional benefits. > > Lots of circuits have good supply rejection at low frequencies and > need help at high frequencies. In those cases, just RC power filtering > works well. Polymer aluminum caps are great for that. > > I've also done closed-loop opamp regulators with a huge RC tau on the > output. That lops off the HF part of the noise.Yup. A two-pole cap multiplier (the above circuit with another RC stage in front of the regulator) can do 100-dB of suppression at SMPS frequencies, which is what I usually want it for. Connecting the cold end of a photodiode to a supply that isn't super-quiet is one good way to blow the signal out of the water. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●May 29, 20142014-05-29
On 05/28/2014 04:20 PM, whit3rd wrote:> On Wednesday, May 28, 2014 10:05:31 AM UTC-7, Phil Hobbs wrote: > >>>> Lately I've been using shunt regulators on the base string of cap >> >>>> multipliers. AFAICT the best is the Exar SPX431A. >> The Exar is quieter and has a lower minimum cathode current. > > It's possible to get lower (SPX431A wants over 400 uA, and TLV431 > wants over 50 uA) but there's some cathode-voltage-range issues, too. > > Have you considered direct shunt regulation? > > > > 0--------*---RRRR---*-----*----------0 > | | | > R | | > R | *-----* > R CCC R | > R CCC R C > | | R | > | | *-----* > *--RRRR----* | > | | | > | /---/ | > CCC / \----* > CCC --- | > | | R > | | R > GND | | > GND GND >That only helps within the regulator's bandwidth, and the maximum suppression is limited to its loop gain. The cap multiplier is quiet at all frequencies of interest (usually), and the regulator just keeps its output from wandering around. For running op amps, a barefoot cap multiplier is usually better, because it's simpler and quieter, very stiff at high frequency, and you usually don't care about a few hundred millivolts' worth of supply sag. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●May 29, 20142014-05-29
On Wednesday, May 28, 2014 7:27:09 PM UTC-7, Phil Hobbs wrote:> On 5/28/2014 4:20 PM, whit3rd wrote:> > Have you considered direct shunt regulation?[ AC-coupled example, with some DC stability issues unaddressed]> You're stuck with the wideband noise of the reference, though. I often > have to care about nanovolt 1-Hz noise on power supplies.Classic solution: a string of mercury batteries. You can still do it, with standard cells. Wideband noise in a '431 is likely recombination noise in the base currents, as I understand it. So, maybe one with higher quiescent current would be better. A FET solution (like current-limit diode for a reference and jFET op amp for gain) would help, if things like popcorn/flicker didn't take over. Even the humble follower transistor in the lightly-loaded C-multiplier, might be noisier than a FET (or MOSFET,though I recall a discussion that showed the MOSFET to have no advantage at Zgate under 100k ohms). There's one voltage source that has very low ripple, good filtering, and the same noise output as a low-value resistor. That's a thermopile. Maybe the heatpump items aren't optimized for it, but one could imagine using two-alloy printed wiring to get to some serious potential differences in a compact, rugged, ultrareliable component. You'd just have to allow a bit of warm-up time on that photon detector.
Reply by ●May 29, 20142014-05-29
On Thursday, May 29, 2014 1:44:41 PM UTC-4, whit3rd wrote:> On Wednesday, May 28, 2014 7:27:09 PM UTC-7, Phil Hobbs wrote: > > > On 5/28/2014 4:20 PM, whit3rd wrote: > > > > > > Have you considered direct shunt regulation? > > [ AC-coupled example, with some DC stability issues unaddressed] > > > > > You're stuck with the wideband noise of the reference, though. I often > > > have to care about nanovolt 1-Hz noise on power supplies. > > > > Classic solution: a string of mercury batteries. You can still do it, with > > standard cells. > > > > Wideband noise in a '431 is likely recombination noise in the base currents, > > as I understand it. So, maybe one with higher quiescent current would > > be better. A FET solution (like current-limit diode for a reference and > > jFET op amp for gain) would help, if things like popcorn/flicker didn't > > take over. > > > > Even the humble follower transistor in the lightly-loaded C-multiplier, might > > be noisier than a FET (or MOSFET,though I recall a discussion that showed > > the MOSFET to have no advantage at Zgate under 100k ohms). > > > > There's one voltage source that has very low ripple, good filtering, and > > the same noise output as a low-value resistor. That's a thermopile. > > Maybe the heatpump items aren't optimized for it, but one could > > imagine using two-alloy printed wiring to get to some serious > > potential differences in a compact, rugged, ultrareliable component. > > > > You'd just have to allow a bit of warm-up time on that photon detector.Cap multipliers are totally cool, when it comes to low noise. If you haven't built one and tried it, you should. I needed a low noise ~10 Watt supply. (30V, 0.3A) I tried a couple of TIP31/ 32's. a few (1-3 nV/rtHz.)* (measured in 100Hz-100kHz band width) Done. George (don't argue with success) H. *measured with an 8nV fet opamp
Reply by ●May 30, 20142014-05-30
On 05/29/2014 01:44 PM, whit3rd wrote:> On Wednesday, May 28, 2014 7:27:09 PM UTC-7, Phil Hobbs wrote: >> On 5/28/2014 4:20 PM, whit3rd wrote: > >>> Have you considered direct shunt regulation? > [ AC-coupled example, with some DC stability issues unaddressed] > >> You're stuck with the wideband noise of the reference, though. I often >> have to care about nanovolt 1-Hz noise on power supplies. > > Classic solution: a string of mercury batteries. You can still do it, with > standard cells.Horrible tempco, short lifetime, expensive, and completely unnecessary. Some circuits require very low DC drift and noise in their power supplies, but not many. Most of the time the low-frequency PSR of op amps and current sources and such like will take care of the low baseband instability. The PSR of op amps falls off pretty badly with frequency, however, and the noise current due to photodiode capacitance differentiating its bias supply rises linearly. So you really want the supplies to be quiet at high frequency, plus you want excellent rejection of SMPS ripple. Cap multipliers are great for that, but normal voltage regulators aren't. Thus the shunt-regulated cap multiplier is a big win.> > Wideband noise in a '431 is likely recombination noise in the base currents, > as I understand it. So, maybe one with higher quiescent current would > be better. A FET solution (like current-limit diode for a reference and > jFET op amp for gain) would help, if things like popcorn/flicker didn't > take over.Bandgaps are inherently noisy, because they have to put 20 dB of gain on the delta-Vbe to get it to cancel out d(Vbe)/dT. Base current shot noise (which is what I think you're talking about) is rarely dominant at low current densities.> > Even the humble follower transistor in the lightly-loaded C-multiplier, might > be noisier than a FET (or MOSFET,though I recall a discussion that showed > the MOSFET to have no advantage at Zgate under 100k ohms).Your average low-sat transistor such as a ZXT11N15DFTA has about a nanovolt 1-Hz noise at 1 mA of collector current. Your average bandgap starts at 40 nV and goes up from there--a difference of 30 dB or more. MOSFETs make horrible cap multipliers, because they're far noisier than BJTs, their operating V_GS is much bigger than a V_BE drop, and they have very low transconductance.> > There's one voltage source that has very low ripple, good filtering, and > the same noise output as a low-value resistor. That's a thermopile. > Maybe the heatpump items aren't optimized for it, but one could > imagine using two-alloy printed wiring to get to some serious > potential differences in a compact, rugged, ultrareliable component.Ultrareliable isn't how I'd describe it. Also its tempco would be on the order of 3000 ppm/K.> You'd just have to allow a bit of warm-up time on that photon detector.Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●May 30, 20142014-05-30
On Friday, May 30, 2014 6:59:20 AM UTC-7, Phil Hobbs wrote:> On 05/29/2014 01:44 PM, whit3rd wrote:> > There's one voltage source that has very low ripple, good filtering, and > > the same noise output as a low-value resistor. That's a thermopile.> Ultrareliable isn't how I'd describe it. Also its tempco would be on > the order of 3000 ppm/K.Not so. That's the tempco of the series resistance, not of the voltage output! You'd be differentially heating two (sets of) thermocouples, and a common-mode temperature doesn't change the voltage at all.
Reply by ●May 30, 20142014-05-30
On 5/30/2014 12:50 PM, whit3rd wrote:> On Friday, May 30, 2014 6:59:20 AM UTC-7, Phil Hobbs wrote: >> On 05/29/2014 01:44 PM, whit3rd wrote: > >>> There's one voltage source that has very low ripple, good filtering, and >>> the same noise output as a low-value resistor. That's a thermopile. > >> Ultrareliable isn't how I'd describe it. Also its tempco would be on >> the order of 3000 ppm/K. > > Not so. That's the tempco of the series resistance, not of the voltage > output! You'd be differentially heating two (sets of) thermocouples, > and a common-mode temperature doesn't change the voltage at all. >A thermopile is a bunch of thermocouples in series. Their output voltage is approximately proportional to the temperature drop across the junction pairs. That means that V = K(T_hot -T_cold). Even if T_cold = 0 kelvin, that scheme can't do better than PTAT (3000 ppm/K at room temperature), and with feasible T_cold values, it's much worse. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●May 31, 20142014-05-31
On Friday, May 30, 2014 2:21:01 PM UTC-7, Phil Hobbs wrote:> On 5/30/2014 12:50 PM, whit3rd wrote: > > > On Friday, May 30, 2014 6:59:20 AM UTC-7, Phil Hobbs wrote: > > >> On 05/29/2014 01:44 PM, whit3rd wrote: > > > > > >>> There's one voltage source that has very low ripple, good filtering, and > >>> the same noise output as a low-value resistor. That's a thermopile.> >> Ultrareliable isn't how I'd describe it. Also its tempco would be on > >> the order of 3000 ppm/K.> > Not so. That's the tempco of the series resistance, not of the voltage > > output!> A thermopile is a bunch of thermocouples in series. Their output > voltage is approximately proportional to the temperature drop across the > junction pairs. That means that V = K(T_hot -T_cold).> Even if T_cold = 0 kelvin, that scheme can't do better than PTATThat is a tremendously improbable case! Thermopiles operated on HEAT SOURCES stabilize to a fixed gradient (established by heat conduction) and one expects (T_hot - T_cold) to be a constant. It is NEVER proportional to absolute temperature in normal circumstances. It's true, by the third law of thermodynamics, that thermocouple coefficients (and other things) do vanish at absolute zero, but at room temperature a thermocouple has no capability to give an absolute temperature reading, and that's because the Seebeck equation contains no absolute temperature sensitivity.
Reply by ●May 31, 20142014-05-31
On 5/31/2014 2:49 AM, whit3rd wrote:> On Friday, May 30, 2014 2:21:01 PM UTC-7, Phil Hobbs wrote: >> On 5/30/2014 12:50 PM, whit3rd wrote: >> >>> On Friday, May 30, 2014 6:59:20 AM UTC-7, Phil Hobbs wrote: >> >>>> On 05/29/2014 01:44 PM, whit3rd wrote: >> >>> >> >>>>> There's one voltage source that has very low ripple, good filtering, and >>>>> the same noise output as a low-value resistor. That's a thermopile. > >>>> Ultrareliable isn't how I'd describe it. Also its tempco would be on >>>> the order of 3000 ppm/K. > >>> Not so. That's the tempco of the series resistance, not of the voltage >>> output! > > >> A thermopile is a bunch of thermocouples in series. Their output >> voltage is approximately proportional to the temperature drop across the >> junction pairs. That means that V = K(T_hot -T_cold). > >> Even if T_cold = 0 kelvin, that scheme can't do better than PTAT > > That is a tremendously improbable case! Thermopiles operated on > HEAT SOURCES stabilize to a fixed gradient (established by heat conduction) > and one expects (T_hot - T_cold) to be a constant. It is NEVER > proportional to absolute temperature in normal circumstances. > > It's true, by the third law of thermodynamics, that thermocouple coefficients > (and other things) do vanish at absolute zero, but at room temperature a thermocouple > has no capability to give an absolute temperature reading, and that's > because the Seebeck equation contains no absolute temperature sensitivity. >You miss my point. The output voltage is proportional to the temperature drop across the junction, and so the tempco goes as 1/(Thot-Tcold), which is huge--generally _much bigger_ than PTAT. (Tcold being zero kelvin was a fictitious best case for your argument, with zero physicsy stuff in it--see the linear approximation above.) Thus there's absolutely no reason to go to all that trouble, when a simple shunt regulated cap multiplier will do much better for under a buck, with a 1-nanovolt noise floor. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
