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DC background subtraction

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George Herold
Guest

Tue Jan 08, 2019 3:45 am   



On Monday, January 7, 2019 at 8:10:13 PM UTC-5, whit3rd wrote:
Quote:
On Monday, January 7, 2019 at 4:37:01 PM UTC-8, George Herold wrote:
On Monday, January 7, 2019 at 7:00:41 PM UTC-5, bitrex wrote:
On 01/07/2019 02:31 PM, George Herold wrote:
Perhaps a silly simple question. I need to subtract off a 2.5V background from a few mV signal. I first think about a difference amp, or
instrument amp. But I wondered if there is any advantage to a simple
opamp summing amp. (I'll add in -2.5 V)


Wheatstone bridge, blocking capacitor, differential amplifier, even transformer
coupling can do this task. I've seen a series battery with potentiometer to do a DC offset.

For splitting a 5V supply, consider using a two-resistor array (matched tempco)
in addition to a trimpot, the drift and aging will improve.

Most important, if the signal is to be amplified, is to match the value and
drift of the offset source. Second most important, is not to distort the
signal in the process.


Re 'signal source'. Right! I was looking at that today.
one of these, (I think, may be a different part number)
https://www.digikey.com/product-detail/en/allegro-microsystems-llc/A1324LUA-T/620-1432-ND/2728144
Ugly output spectrum, dominate peaks at ~100 kHz and 1MHz...
with harmonics everywhere.
(And that was after gain of 100 opamp (~1MHz GBW).)
The allegro has a chopamp inside.

George H.

Sylvia Else
Guest

Tue Jan 08, 2019 7:45 am   



On 8/01/2019 6:31 am, George Herold wrote:
Quote:
Perhaps a silly simple question. I need to subtract off a 2.5V background from a few mV signal. I first think about a difference amp, or
instrument amp. But I wondered if there is any advantage to a simple
opamp summing amp. (I'll add in -2.5 V)

George H.


Is the 2.5V background really stable to within a fraction of a mV?

Sylvia.


Guest

Tue Jan 08, 2019 10:45 am   



George Herold wrote
Quote:
Oh no we are talking slowly varying signals. Minutes to
an hour maybe..

So it's my bosses idea, you put a hall probe sensor inside
a High Tc super conductor toroid. Induce a super current.
(by cooling down with permanant magnet in the toroid.)
And then observe the field go away as you warm it up.

George H.


Probably .. strange idea,
use Winfield's 24 bit ADC board, subtract digitally 2.500000
do processing digital?

This I cut and paste from his MPX-16H circuit diagram:
24-bit 4ppm delta-sigma ADC
f = 250kHz, quiescent drain 0.64mA
ADC 24-bits 0 to 4.096V FS
133 or 160ms conversion time

??
Ask him?
You do not need speed no?

piglet
Guest

Tue Jan 08, 2019 11:45 am   



On 08/01/2019 00:52, George Herold wrote:
Quote:
Oh no we are talking slowly varying signals. Minutes to
an hour maybe..

So it's my bosses idea, you put a hall probe sensor inside
a High Tc super conductor toroid. Induce a super current.
(by cooling down with permanant magnet in the toroid.)
And then observe the field go away as you warm it up.

George H.


An identical sensor outside the field and take the difference?

piglet

speff
Guest

Tue Jan 08, 2019 2:45 pm   



On Monday, 7 January 2019 19:37:01 UTC-5, George Herold wrote:
Quote:
On Monday, January 7, 2019 at 7:00:41 PM UTC-5, bitrex wrote:
On 01/07/2019 02:31 PM, George Herold wrote:
Perhaps a silly simple question. I need to subtract off a 2.5V background from a few mV signal. I first think about a difference amp, or
instrument amp. But I wondered if there is any advantage to a simple
opamp summing amp. (I'll add in -2.5 V)

George H.


Generating the -2.5 offset for the summer sounds like a job for a DC
servo/integrator, servoing the op-amp output

It's ~1/2 the bias voltage so I'll reference it from that
with a pot and buffer. -2.5 will add an inverter.
(sounds easier to try the int amp first.)

George H.


Divide the bias voltage in half with precision resistors and subtract?

Unless you go to some kind of flying capacitor deal, I think at best you can get away with a 1:1 resistor network as the only precision part.

As suggested, an with excellent DC specs might avoid even that- for example an AD7177-2, 1 ppm INL but that's a $25-ish part.

Depends a lot on your exact requirements, if you can get away with Susumu RM2012B duals (such as an 0805 dual 10K with +/-2ppm/°C TCR matching), it could be pretty cheap and easy using resistor networks and a small trim. If you need foil resistors, tens of dollars.

If the output changes by 20ppm, that's 50uV. You don't need chopper amplifiers at that level, but you could use them if the input glitches are not an issue.


--Spehro Pefhany

George Herold
Guest

Tue Jan 08, 2019 3:45 pm   



On Monday, January 7, 2019 at 8:44:16 PM UTC-5, Phil Hobbs wrote:
Quote:
On 1/7/19 8:27 PM, George Herold wrote:
On Monday, January 7, 2019 at 8:01:28 PM UTC-5, bitrex wrote:
On 01/07/2019 07:52 PM, George Herold wrote:
On Monday, January 7, 2019 at 7:37:48 PM UTC-5, bitrex wrote:
On 01/07/2019 07:23 PM, George Herold wrote:
On Monday, January 7, 2019 at 2:34:58 PM UTC-5, Phil Allison wrote:
George Herold wrote:


Perhaps a silly simple question. I need to subtract off a 2.5V
background from a few mV signal. I first think about a difference amp,
or instrument amp.
But I wondered if there is any advantage to a simple
opamp summing amp. (I'll add in -2.5 V)



** Considered using a capacitor ??




.... Phil

Like a sample and hold? It needs to hold the offset
sorta forever, so a pot called for.

George H.


DC blocking capacitor. Y'know, like a high-pass on the input.

https://www.dropbox.com/s/he7zy2rdhomfveh/IMG_20190107_193244554_HDR.jpg?dl=0

Oh no we are talking slowly varying signals. Minutes to
an hour maybe..

So it's my bosses idea, you put a hall probe sensor inside
a High Tc super conductor toroid. Induce a super current.
(by cooling down with permanant magnet in the toroid.)
And then observe the field go away as you warm it up.

George H.


Got it, I figured you had not forgotten how to use a capacitor to block
DC but just so long as we cover all the bases here and the signal period
wasn't mentioned explicitly ;)

Oh dear no, big C, big R and fet opamp.. about 1 second is the longest
TC I can wait for. (10uF and 100k or 1uF and a meg.)
I was doing that today to look at power supply noise.
I think most of the circuits I build are 'scope preamps. :^)
I've never done the feedback cap thing, (you posted previoulsy),
looks like a Sallen-Key.

George H.


If you want to amplify the signal by N times and the offset by 1x (so
that you're still biased above ground), that's easy to do accurately
with a garden-variety noninverting amp.


I don't really care where the signal level is, I just want to look at
(small) changes. I'm trying to picture how a non-iverting amp
does this, but can't. Can you give me more of a hint?

George H.
Quote:

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com


George Herold
Guest

Tue Jan 08, 2019 3:45 pm   



On Tuesday, January 8, 2019 at 5:09:34 AM UTC-5, piglet wrote:
Quote:
On 08/01/2019 00:52, George Herold wrote:
Oh no we are talking slowly varying signals. Minutes to
an hour maybe..

So it's my bosses idea, you put a hall probe sensor inside
a High Tc super conductor toroid. Induce a super current.
(by cooling down with permanant magnet in the toroid.)
And then observe the field go away as you warm it up.

George H.


An identical sensor outside the field and take the difference?

piglet


Oh that's interesting! This goes down a probe, for which there is always
a lack of wires... but it might work. The good thing is it would take
out any changes in the local B-field. The bad thing is that the sensor
has to be heated to ~130 deg K or it stops working. Two sensors may be
a little hard to squeeze in but it's certainly worth thinking about.

Thanks,
George H.

Phil Hobbs
Guest

Tue Jan 08, 2019 4:45 pm   



On 1/8/19 8:51 AM, George Herold wrote:
Quote:
On Monday, January 7, 2019 at 8:44:16 PM UTC-5, Phil Hobbs wrote:
On 1/7/19 8:27 PM, George Herold wrote:
On Monday, January 7, 2019 at 8:01:28 PM UTC-5, bitrex wrote:
On 01/07/2019 07:52 PM, George Herold wrote:
On Monday, January 7, 2019 at 7:37:48 PM UTC-5, bitrex wrote:
On 01/07/2019 07:23 PM, George Herold wrote:
On Monday, January 7, 2019 at 2:34:58 PM UTC-5, Phil Allison wrote:
George Herold wrote:


Perhaps a silly simple question. I need to subtract off a 2.5V
background from a few mV signal. I first think about a difference amp,
or instrument amp.
But I wondered if there is any advantage to a simple
opamp summing amp. (I'll add in -2.5 V)



** Considered using a capacitor ??




.... Phil

Like a sample and hold? It needs to hold the offset
sorta forever, so a pot called for.

George H.


DC blocking capacitor. Y'know, like a high-pass on the input.

https://www.dropbox.com/s/he7zy2rdhomfveh/IMG_20190107_193244554_HDR.jpg?dl=0

Oh no we are talking slowly varying signals. Minutes to
an hour maybe..

So it's my bosses idea, you put a hall probe sensor inside
a High Tc super conductor toroid. Induce a super current.
(by cooling down with permanant magnet in the toroid.)
And then observe the field go away as you warm it up.

George H.


Got it, I figured you had not forgotten how to use a capacitor to block
DC but just so long as we cover all the bases here and the signal period
wasn't mentioned explicitly ;)

Oh dear no, big C, big R and fet opamp.. about 1 second is the longest
TC I can wait for. (10uF and 100k or 1uF and a meg.)
I was doing that today to look at power supply noise.
I think most of the circuits I build are 'scope preamps. :^)
I've never done the feedback cap thing, (you posted previoulsy),
looks like a Sallen-Key.

George H.


If you want to amplify the signal by N times and the offset by 1x (so
that you're still biased above ground), that's easy to do accurately
with a garden-variety noninverting amp.

I don't really care where the signal level is, I just want to look at
(small) changes. I'm trying to picture how a non-iverting amp
does this, but can't. Can you give me more of a hint?

George H.


A noninverting gain of N amplifier is also an inverting gain of N-1
amplifier, and the 1 is very accurate when the loop gain is high. Thus
if you have an offset of 2.50000V, connecting the input resistor of the
op amp (on the inverting side) to your offset generator and the
noninverting input to signal+offset, you get

Vout = (N+1)(Vos + Vsig) - N Vos = Vos + N Vsig.

This is just like the usual split-supply, single-ended noninverting amp,
except with the ground reference relabelled. Resistor inaccuracy causes
gain inaccuracy, but the offset remains correct.

This move is super common in single-supply data acq circuits, for instance.

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
https://hobbs-eo.com

George Herold
Guest

Tue Jan 08, 2019 4:45 pm   



On Tuesday, January 8, 2019 at 8:42:03 AM UTC-5, speff wrote:
Quote:
On Monday, 7 January 2019 19:37:01 UTC-5, George Herold wrote:
On Monday, January 7, 2019 at 7:00:41 PM UTC-5, bitrex wrote:
On 01/07/2019 02:31 PM, George Herold wrote:
Perhaps a silly simple question. I need to subtract off a 2.5V background from a few mV signal. I first think about a difference amp, or
instrument amp. But I wondered if there is any advantage to a simple
opamp summing amp. (I'll add in -2.5 V)

George H.


Generating the -2.5 offset for the summer sounds like a job for a DC
servo/integrator, servoing the op-amp output

It's ~1/2 the bias voltage so I'll reference it from that
with a pot and buffer. -2.5 will add an inverter.
(sounds easier to try the int amp first.)

George H.

Divide the bias voltage in half with precision resistors and subtract?

Unless you go to some kind of flying capacitor deal, I think at best you can get away with a 1:1 resistor network as the only precision part.

As suggested, an with excellent DC specs might avoid even that- for example an AD7177-2, 1 ppm INL but that's a $25-ish part.

Depends a lot on your exact requirements, if you can get away with Susumu RM2012B duals (such as an 0805 dual 10K with +/-2ppm/°C TCR matching), it could be pretty cheap and easy using resistor networks and a small trim. If you need foil resistors, tens of dollars.

If the output changes by 20ppm, that's 50uV. You don't need chopper amplifiers at that level, but you could use them if the input glitches are not an issue.


--Spehro Pefhany


Thanks Spehro, I have to look at the signals still, but I'm thinking the
drift/ crud on the sensor will be much worse than any other drifts... so
I may be barking up the wrong tree.

George H.

George Herold
Guest

Tue Jan 08, 2019 5:45 pm   



On Tuesday, January 8, 2019 at 10:04:07 AM UTC-5, Phil Hobbs wrote:
Quote:
On 1/8/19 8:51 AM, George Herold wrote:
On Monday, January 7, 2019 at 8:44:16 PM UTC-5, Phil Hobbs wrote:
On 1/7/19 8:27 PM, George Herold wrote:
On Monday, January 7, 2019 at 8:01:28 PM UTC-5, bitrex wrote:
On 01/07/2019 07:52 PM, George Herold wrote:
On Monday, January 7, 2019 at 7:37:48 PM UTC-5, bitrex wrote:
On 01/07/2019 07:23 PM, George Herold wrote:
On Monday, January 7, 2019 at 2:34:58 PM UTC-5, Phil Allison wrote:
George Herold wrote:


Perhaps a silly simple question. I need to subtract off a 2.5V
background from a few mV signal. I first think about a difference amp,
or instrument amp.
But I wondered if there is any advantage to a simple
opamp summing amp. (I'll add in -2.5 V)



** Considered using a capacitor ??




.... Phil

Like a sample and hold? It needs to hold the offset
sorta forever, so a pot called for.

George H.


DC blocking capacitor. Y'know, like a high-pass on the input.

https://www.dropbox.com/s/he7zy2rdhomfveh/IMG_20190107_193244554_HDR.jpg?dl=0

Oh no we are talking slowly varying signals. Minutes to
an hour maybe..

So it's my bosses idea, you put a hall probe sensor inside
a High Tc super conductor toroid. Induce a super current.
(by cooling down with permanant magnet in the toroid.)
And then observe the field go away as you warm it up.

George H.


Got it, I figured you had not forgotten how to use a capacitor to block
DC but just so long as we cover all the bases here and the signal period
wasn't mentioned explicitly ;)

Oh dear no, big C, big R and fet opamp.. about 1 second is the longest
TC I can wait for. (10uF and 100k or 1uF and a meg.)
I was doing that today to look at power supply noise.
I think most of the circuits I build are 'scope preamps. :^)
I've never done the feedback cap thing, (you posted previoulsy),
looks like a Sallen-Key.

George H.


If you want to amplify the signal by N times and the offset by 1x (so
that you're still biased above ground), that's easy to do accurately
with a garden-variety noninverting amp.

I don't really care where the signal level is, I just want to look at
(small) changes. I'm trying to picture how a non-iverting amp
does this, but can't. Can you give me more of a hint?

George H.

A noninverting gain of N amplifier is also an inverting gain of N-1
amplifier, and the 1 is very accurate when the loop gain is high. Thus
if you have an offset of 2.50000V, connecting the input resistor of the
op amp (on the inverting side) to your offset generator and the
noninverting input to signal+offset, you get

Vout = (N+1)(Vos + Vsig) - N Vos = Vos + N Vsig.

This is just like the usual split-supply, single-ended noninverting amp,
except with the ground reference relabelled. Resistor inaccuracy causes
gain inaccuracy, but the offset remains correct.

This move is super common in single-supply data acq circuits, for instance.

OK got it. Thanks.


George H.
Quote:
Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
https://hobbs-eo.com


Lasse Langwadt Christense
Guest

Tue Jan 08, 2019 6:45 pm   



tirsdag den 8. januar 2019 kl. 11.09.34 UTC+1 skrev piglet:
Quote:
On 08/01/2019 00:52, George Herold wrote:
Oh no we are talking slowly varying signals. Minutes to
an hour maybe..

So it's my bosses idea, you put a hall probe sensor inside
a High Tc super conductor toroid. Induce a super current.
(by cooling down with permanant magnet in the toroid.)
And then observe the field go away as you warm it up.

George H.


An identical sensor outside the field and take the difference?


or different with two sensors in the field, one of them flipped

George Herold
Guest

Tue Jan 08, 2019 6:45 pm   



On Tuesday, January 8, 2019 at 12:28:04 PM UTC-5, Lasse Langwadt Christensen wrote:
Quote:
tirsdag den 8. januar 2019 kl. 11.09.34 UTC+1 skrev piglet:
On 08/01/2019 00:52, George Herold wrote:
Oh no we are talking slowly varying signals. Minutes to
an hour maybe..

So it's my bosses idea, you put a hall probe sensor inside
a High Tc super conductor toroid. Induce a super current.
(by cooling down with permanant magnet in the toroid.)
And then observe the field go away as you warm it up.

George H.


An identical sensor outside the field and take the difference?


or different with two sensors in the field, one of them flipped


Ohh, that's fun too! Thanks for all the nice ideas everyone,
and to think I wasn't going to ask this 'simple' question.

George h.


Guest

Tue Jan 08, 2019 7:45 pm   



On Monday, January 7, 2019 at 7:52:59 PM UTC-5, George Herold wrote:
Quote:
On Monday, January 7, 2019 at 7:37:48 PM UTC-5, bitrex wrote:
On 01/07/2019 07:23 PM, George Herold wrote:
On Monday, January 7, 2019 at 2:34:58 PM UTC-5, Phil Allison wrote:
George Herold wrote:


Perhaps a silly simple question. I need to subtract off a 2.5V
background from a few mV signal. I first think about a difference amp,
or instrument amp.
But I wondered if there is any advantage to a simple
opamp summing amp. (I'll add in -2.5 V)



** Considered using a capacitor ??




.... Phil

Like a sample and hold? It needs to hold the offset
sorta forever, so a pot called for.

George H.


DC blocking capacitor. Y'know, like a high-pass on the input.

https://www.dropbox.com/s/he7zy2rdhomfveh/IMG_20190107_193244554_HDR.jpg?dl=0

Oh no we are talking slowly varying signals. Minutes to
an hour maybe..

So it's my bosses idea, you put a hall probe sensor inside
a High Tc super conductor toroid. Induce a super current.
(by cooling down with permanant magnet in the toroid.)
And then observe the field go away as you warm it up.

George H.


If you're talking about that kind of thing, go digital. You'll get much better performance than analog. There are scopes that will do a "rolling" acquisition of very slowly varying signals like this.


Guest

Tue Jan 08, 2019 8:45 pm   



On Monday, January 7, 2019 at 7:52:59 PM UTC-5, George Herold wrote:
Quote:
On Monday, January 7, 2019 at 7:37:48 PM UTC-5, bitrex wrote:
On 01/07/2019 07:23 PM, George Herold wrote:
On Monday, January 7, 2019 at 2:34:58 PM UTC-5, Phil Allison wrote:
George Herold wrote:


Perhaps a silly simple question. I need to subtract off a 2.5V
background from a few mV signal. I first think about a difference amp,
or instrument amp.
But I wondered if there is any advantage to a simple
opamp summing amp. (I'll add in -2.5 V)



** Considered using a capacitor ??




.... Phil

Like a sample and hold? It needs to hold the offset
sorta forever, so a pot called for.

George H.


DC blocking capacitor. Y'know, like a high-pass on the input.

https://www.dropbox.com/s/he7zy2rdhomfveh/IMG_20190107_193244554_HDR.jpg?dl=0

Oh no we are talking slowly varying signals. Minutes to
an hour maybe..

So it's my bosses idea, you put a hall probe sensor inside
a High Tc super conductor toroid. Induce a super current.
(by cooling down with permanant magnet in the toroid.)
And then observe the field go away as you warm it up.

George H.


There should be some handheld VOMs that will do statistics on signals like this. And they produce downloadable acquisition files for USB. Probably something available at HomeDepot Smile

Phil Hobbs
Guest

Tue Jan 08, 2019 8:45 pm   



On 1/8/19 1:53 PM, bloggs.fredbloggs.fred_at_gmail.com wrote:
Quote:
On Monday, January 7, 2019 at 7:52:59 PM UTC-5, George Herold wrote:
On Monday, January 7, 2019 at 7:37:48 PM UTC-5, bitrex wrote:
On 01/07/2019 07:23 PM, George Herold wrote:
On Monday, January 7, 2019 at 2:34:58 PM UTC-5, Phil Allison wrote:
George Herold wrote:


Perhaps a silly simple question. I need to subtract off a 2.5V
background from a few mV signal. I first think about a difference amp,
or instrument amp.
But I wondered if there is any advantage to a simple
opamp summing amp. (I'll add in -2.5 V)



** Considered using a capacitor ??




.... Phil

Like a sample and hold? It needs to hold the offset
sorta forever, so a pot called for.

George H.


DC blocking capacitor. Y'know, like a high-pass on the input.

https://www.dropbox.com/s/he7zy2rdhomfveh/IMG_20190107_193244554_HDR.jpg?dl=0

Oh no we are talking slowly varying signals. Minutes to
an hour maybe..

So it's my bosses idea, you put a hall probe sensor inside
a High Tc super conductor toroid. Induce a super current.
(by cooling down with permanant magnet in the toroid.)
And then observe the field go away as you warm it up.

George H.

There should be some handheld VOMs that will do statistics on signals like this. And they produce downloadable acquisition files for USB. Probably something available at HomeDepot Smile


Big waste of bits though. A nice noninverting chopamp with high DC
open-loop gain will do an excellent job of preserving those bits. Two
30-dB gain stages, or a 60-dB composite amp with a nice fast 40-dB stage
inside the loop is good medicine for that sort of job. An OPA188 for
the outer loop and an LM6171A inside would probably be a decent choice,
and yield a bandwidth of around 200 kHz. Using the composite amp
improves the low frequency characteristics on account of eliminating the
loading on the outer amp and increasing the DC loop gain. (Making the
inner stage a lead/lag amp, i.e. putting a capacitor in series with the
feedback resistor, will greatly increase the DC gain again.

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

Goto page Previous  1, 2, 3, 4  Next

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