100mV DC supply

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Hi All,

I have a need to generate +100mV DC to drive a load which is capable of sou
rcing up to ~50mA but will typically see high impedance. Easily available I
 have a clean 24V source and 208VAC. My intent is to drop from 24V to ~1.8V
 using a buck converter then follow it up with a 1.0V precision shunt (ADR5
10 or other) followed by a 9:1 precision resistor divider. Buffer the 100mV
 output by a low noise single supply op-amp such as ltc1014. Although it lo
oks like ltc1014 won't be able to source enough current so possibly followe
d with a BJT?

Voltage drift is more important than precision for this application and +/-
 3mV should be good enough. The environment the board will reside in is ~25
C +/- 5deg. The output voltage must not drift above ~130mV to prevent causi
ng issues in the test chamber.  

The voltage across the load is buffered, low-pass filtered (2-pole active  
= 1kHz) and multiplied by 10x before being sampled. The low pass filter f
requency is chosen to preserve rise times in the ~500uS range.  

I expect that the largest source of noise will not be from the shunt (70ppm
/C), the resistor divider (25ppm/C), or the Op-Amp ~160uV but from the 5 fo
ot run of cable to the test chamber and back and am interested in best prac
tice shielding techniques to mitigate induced noise.  

I'm not much of an electrical designer and haven't designed anything simila
r to this before so I'm interested in hearing about other possible topologi
es that can meet the requirements or how others would approach the problem.
 The solution will be put onto a pcb as part of a larger circuit so i canno
t use a benchtop supply.  

To summarize:

 - Vin = +24V and/or 208VAC  
 - Vout= 100mV DC +/- 2mV  
 - Iout = up to 50mA into short circuit load

Thank you,

Mark  

Re: 100mV DC supply
Marke wrote:

-----------------


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ourcing up to ~50mA but will typically see high impedance. Easily available
 I have a clean 24V source and 208VAC. My intent is to drop from 24V to ~1.
8V using a buck converter then follow it up with a 1.0V precision shunt (AD
R510 or other) followed by a 9:1 precision resistor divider. Buffer the 100
mV output by a low noise single supply op-amp such as ltc1014. Although it  
looks like ltc1014 won't be able to source enough current so possibly follo
wed with a BJT?
Quoted text here. Click to load it
/- 3mV should be good enough. The environment the board will reside in is ~
25C +/- 5deg. The output voltage must not drift above ~130mV to prevent cau
sing issues in the test chamber.  
Quoted text here. Click to load it
 = 1kHz) and multiplied by 10x before being sampled. The low pass filter  
frequency is chosen to preserve rise times in the ~500uS range.  
Quoted text here. Click to load it
pm/C), the resistor divider (25ppm/C), or the Op-Amp ~160uV but from the 5  
foot run of cable to the test chamber and back and am interested in best pr
actice shielding techniques to mitigate induced noise.  
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lar to this before so I'm interested in hearing about other possible topolo
gies that can meet the requirements or how others would approach the proble
m. The solution will be put onto a pcb as part of a larger circuit so i can
not use a benchtop supply.  
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** Using the KISS principle, all you need is a 5V, 1Amp TO220 reg IC and 3  
resistors. The resistors are 120ohm, 82ohm and 1ohm - all 1% types.

The 120ohm & 82ohm go in parallel to make 48.7ohms, then 1ohm in series acr
oss 5V.

This makes a 1:49.7 divider so you get 100mV with a 1 ohm source impedance  
and  a SCC of 100mA.  

A small cap is also needed across the output of the reg IC for stability.
  



....  Phil  



Re: 100mV DC supply
On Thursday, July 13, 2017 at 4:00:57 AM UTC-4, Phil Allison wrote:
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 sourcing up to ~50mA but will typically see high impedance. Easily availab
le I have a clean 24V source and 208VAC. My intent is to drop from 24V to ~
1.8V using a buck converter then follow it up with a 1.0V precision shunt (
ADR510 or other) followed by a 9:1 precision resistor divider. Buffer the 1
00mV output by a low noise single supply op-amp such as ltc1014. Although i
t looks like ltc1014 won't be able to source enough current so possibly fol
lowed with a BJT?
Quoted text here. Click to load it
 +/- 3mV should be good enough. The environment the board will reside in is
 ~25C +/- 5deg. The output voltage must not drift above ~130mV to prevent c
ausing issues in the test chamber.  
Quoted text here. Click to load it
ve = 1kHz) and multiplied by 10x before being sampled. The low pass filte
r frequency is chosen to preserve rise times in the ~500uS range.  
Quoted text here. Click to load it
0ppm/C), the resistor divider (25ppm/C), or the Op-Amp ~160uV but from the  
5 foot run of cable to the test chamber and back and am interested in best  
practice shielding techniques to mitigate induced noise.  
Quoted text here. Click to load it
milar to this before so I'm interested in hearing about other possible topo
logies that can meet the requirements or how others would approach the prob
lem. The solution will be put onto a pcb as part of a larger circuit so i c
annot use a benchtop supply.  
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3 resistors. The resistors are 120ohm, 82ohm and 1ohm - all 1% types.
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cross 5V.
e and  a SCC of 100mA.  
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Right, that's what I'd try.  If you need a lower source impedance
maybe buffer with an opamp.  (Is there some dip opamp that does ~100mA  
besides the TCA0372?.. too lazy to troll Digikey.)  

George H.  

Re: 100mV DC supply
On Thursday, July 13, 2017 at 10:13:13 AM UTC-4, George Herold wrote:
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of sourcing up to ~50mA but will typically see high impedance. Easily avail
able I have a clean 24V source and 208VAC. My intent is to drop from 24V to
 ~1.8V using a buck converter then follow it up with a 1.0V precision shunt
 (ADR510 or other) followed by a 9:1 precision resistor divider. Buffer the
 100mV output by a low noise single supply op-amp such as ltc1014. Although
 it looks like ltc1014 won't be able to source enough current so possibly f
ollowed with a BJT?
Quoted text here. Click to load it
nd +/- 3mV should be good enough. The environment the board will reside in  
is ~25C +/- 5deg. The output voltage must not drift above ~130mV to prevent
 causing issues in the test chamber.  
Quoted text here. Click to load it
tive = 1kHz) and multiplied by 10x before being sampled. The low pass fil
ter frequency is chosen to preserve rise times in the ~500uS range.  
Quoted text here. Click to load it
(70ppm/C), the resistor divider (25ppm/C), or the Op-Amp ~160uV but from th
e 5 foot run of cable to the test chamber and back and am interested in bes
t practice shielding techniques to mitigate induced noise.  
Quoted text here. Click to load it
similar to this before so I'm interested in hearing about other possible to
pologies that can meet the requirements or how others would approach the pr
oblem. The solution will be put onto a pcb as part of a larger circuit so i
 cannot use a benchtop supply.  
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d 3 resistors. The resistors are 120ohm, 82ohm and 1ohm - all 1% types.
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 across 5V.
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nce and  a SCC of 100mA.  
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y.  
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I interpret the OP as needing 100mV and holding +/-3mV for currents of
0 <= i.out <= 50mA. Most op-amps would need a buffer for that, yes.

If efficiency doesn't matter, just use an LM317 as the preregulator
with the Vadj terminal grounded.  That gets you to 1.2V with one part.
An LMV431-1.2 with a 50mA-limiting resistor to +24V might even be
simpler (& more accurate) for getting to 1.2V.

Cheers,
James Arthur

Re: 100mV DC supply
On Thu, 13 Jul 2017 07:56:41 -0700 (PDT), snipped-for-privacy@yahoo.com
wrote:

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The buffer elininates opamp self-heating as a function of load
current, which could be a big source of error. Of course, the buffer
has to be inside the loop.

Really low-noise amps need low-value feedback resistors to keep
Johnson noise down, which implies a lot of opamp current even with no
load. So sometimes one adds a buffer just to drive the feedback
resistors.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: 100mV DC supply
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Yup.  300-kelvin resistors are a serious inconvenience when your active devices have 0.01-K noise temperatures. (BF862s are about that good in the flatband.)

Cheers

Phil Hobbs


Re: 100mV DC supply
On Thu, 13 Jul 2017 19:09:12 -0700 (PDT), snipped-for-privacy@gmail.com wrote:

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The NMR amps needed PPM-flat current pulses, so we didn't want thermal
tails in the current shunt amp. Hence the buffer inside the loop.

We made our own current shunts, too. That's a whole nother story.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: 100mV DC supply
:
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e of sourcing up to ~50mA but will typically see high impedance. Easily ava
ilable I have a clean 24V source and 208VAC. My intent is to drop from 24V  
to ~1.8V using a buck converter then follow it up with a 1.0V precision shu
nt (ADR510 or other) followed by a 9:1 precision resistor divider. Buffer t
he 100mV output by a low noise single supply op-amp such as ltc1014. Althou
gh it looks like ltc1014 won't be able to source enough current so possibly
 followed with a BJT?
Quoted text here. Click to load it
 and +/- 3mV should be good enough. The environment the board will reside i
n is ~25C +/- 5deg. The output voltage must not drift above ~130mV to preve
nt causing issues in the test chamber.  
Quoted text here. Click to load it
active = 1kHz) and multiplied by 10x before being sampled. The low pass f
ilter frequency is chosen to preserve rise times in the ~500uS range.  
Quoted text here. Click to load it
t (70ppm/C), the resistor divider (25ppm/C), or the Op-Amp ~160uV but from  
the 5 foot run of cable to the test chamber and back and am interested in b
est practice shielding techniques to mitigate induced noise.  
Quoted text here. Click to load it
g similar to this before so I'm interested in hearing about other possible  
topologies that can meet the requirements or how others would approach the  
problem. The solution will be put onto a pcb as part of a larger circuit so
 i cannot use a benchtop supply.  
Quoted text here. Click to load it
and 3 resistors. The resistors are 120ohm, 82ohm and 1ohm - all 1% types.
Quoted text here. Click to load it
es across 5V.
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dance and  a SCC of 100mA.  
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ity.  
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Sure... you know my opamp idea is probably not so good.  
I used an opamp as a voltage reference in a circuit and then
found when I hung a bunch of bypass caps on the output...
It sang for me. :^)  

Your circuit with transistor would be better.  

George H.

Re: 100mV DC supply
On Thu, 13 Jul 2017 09:09:09 -0700 (PDT), George Herold

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Some opamps, especially RRO types, are c-load stable.

Almost any opamp is c-load stable with enough C!


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: 100mV DC supply
On Thursday, July 13, 2017 at 12:13:23 PM UTC-4, John Larkin wrote:
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ote:
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able of sourcing up to ~50mA but will typically see high impedance. Easily  
available I have a clean 24V source and 208VAC. My intent is to drop from 2
4V to ~1.8V using a buck converter then follow it up with a 1.0V precision  
shunt (ADR510 or other) followed by a 9:1 precision resistor divider. Buffe
r the 100mV output by a low noise single supply op-amp such as ltc1014. Alt
hough it looks like ltc1014 won't be able to source enough current so possi
bly followed with a BJT?
Quoted text here. Click to load it
ion and +/- 3mV should be good enough. The environment the board will resid
e in is ~25C +/- 5deg. The output voltage must not drift above ~130mV to pr
event causing issues in the test chamber.  
Quoted text here. Click to load it
le active = 1kHz) and multiplied by 10x before being sampled. The low pas
s filter frequency is chosen to preserve rise times in the ~500uS range.  
Quoted text here. Click to load it
hunt (70ppm/C), the resistor divider (25ppm/C), or the Op-Amp ~160uV but fr
om the 5 foot run of cable to the test chamber and back and am interested i
n best practice shielding techniques to mitigate induced noise.  
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hing similar to this before so I'm interested in hearing about other possib
le topologies that can meet the requirements or how others would approach t
he problem. The solution will be put onto a pcb as part of a larger circuit
 so i cannot use a benchtop supply.  
Quoted text here. Click to load it
IC and 3 resistors. The resistors are 120ohm, 82ohm and 1ohm - all 1% types
.
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eries across 5V.
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mpedance and  a SCC of 100mA.  
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bility.  
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mA  
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.

I once put a 100,000uF low e.s.r. capacitor on the output of a SMPS.
It cleaned up the load-step response nicely.

Cheers,
James Arthur

Re: 100mV DC supply
On 07/13/2017 12:21 PM, snipped-for-privacy@yahoo.com wrote:
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BFC superpowers to the rescue.

Cheers

Phil "use a bigger hammer" Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: 100mV DC supply
On Thursday, July 13, 2017 at 6:16:34 PM UTC-4, Phil Hobbs wrote:
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Yep. I had to drive through a 300A load step without losing more than
about a volt, IIRC.

Hey, Wikipedia's disambiguation page for "BFC" lists one possible
meaning as "Engineering slang for a supercapacitor." :-)

Cheers,
James Arthur

Re: 100mV DC supply
On Thu, 13 Jul 2017 18:42:06 -0700 (PDT), snipped-for-privacy@yahoo.com
wrote:

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It must be French. Beaucoup Farads.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: 100mV DC supply
On Thursday, July 13, 2017 at 11:22:37 PM UTC-4, John Larkin wrote:
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Wouldn't that have to be Beau Farads Coup?

It was a monster computer-grade electrolytic, an absolute beast.

My task was helping a COTS Mean Well 24V supply weather a 300A peak impulse
without losing voltage, and without modifying the Mean Well. That didn't
leave many options, so out came the BFC.

I tested the impulse response and monitored the SMPS' switching--the Mean Well
seemed to like it just fine.

Cheers,
James Arthur

Re: 100mV DC supply
Thank you everyone for the input, very much appreciated.

I am not overly concerned about efficiency in this design so dividing down a linear regulator does seem like a much easier approach.

James: Thank you for the hint to view in courier font, i had wondered how the hell everyone on here was parsing this style of schematic. I think this would work well  

Bill: I definitely misspoke in saying that I need a load which can source 50mA, it is the simpler issue of driving a resistive load with +100mV and having it draw up to 50mA.

John: Thank you for the input. Adding two extra wires for for a kelvin arrangement doesn't complicate matters too much so I'll throw together two test setups and see how much of a difference it makes.  

My plan at this point is to build several different topologies and compare. Probably overkill but I get to build up a few different quick test beds and I'm sure will learn more in the process.  

Mark  

Re: 100mV DC supply
Not to completely derail all the helpful discussion but google groups seems like a horrible way to view these topics. I assume most people use a usenet reader of some kind?  

Re: 100mV DC supply
On Thu, 13 Jul 2017 09:33:45 -0700 (PDT), Marke

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I use an old version of Agent. It works much better than GG.


--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: 100mV DC supply
On Thursday, July 13, 2017 at 12:33:51 PM UTC-4, Marke wrote:
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Oh, topics get 'derailed' all the time.  With google groups if you click
on the little triangle in the upper RH corner, there is an option for  
'show original' and that lets you see the post in Currrier.. and thus  
the ascii art.

George H.  

Re: 100mV DC supply
On Thursday, July 13, 2017 at 6:33:51 PM UTC+2, Marke wrote:
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Thunderbird works as a traditional usenet reader. Google groups does rather more for you, so I tend use it, rather than Thunderbird.

--  
Bill Sloman, Sydney

Re: 100mV DC supply
On 07/13/2017 12:33 PM, Marke wrote:
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Thunderbird.  Other folks like knode, Forte Agent, or one or two others.

Cheers

Phil Hobbs
--  
Dr Philip C D Hobbs
Principal Consultant
We've slightly trimmed the long signature. Click to see the full one.

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