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George Herold
Guest

Mon Jan 07, 2019 8:45 pm   



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.

Phil Allison
Guest

Mon Jan 07, 2019 8:45 pm   



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)



** Considered using a capacitor ??




..... Phil

John Larkin
Guest

Mon Jan 07, 2019 9:45 pm   



On Mon, 7 Jan 2019 11:31:51 -0800 (PST), George Herold
<gherold_at_teachspin.com> 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.


Well, it's simple, but the resistor tempcos could add a bunch of
drift.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com


Guest

Tue Jan 08, 2019 12:45 am   



On Tuesday, January 8, 2019 at 7:19:19 AM UTC+11, John Larkin wrote:
Quote:
On Mon, 7 Jan 2019 11:31:51 -0800 (PST), George Herold
gherold_at_teachspin.com> 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)

Well, it's simple, but the resistor tempcos could add a bunch of
drift.


You can buy thin film networks on a common substrate, with a very tight guarantee on the ratio drift.

The difference amps you buy tend to have such a network built into the package.

Instrumentation amps are a slightly different breed of cat, and you need to read the data sheets rather carefully to make sure you buy the right part.

--
Bill Sloman, Sydney

bitrex
Guest

Tue Jan 08, 2019 1:45 am   



On 01/07/2019 07:23 PM, George Herold wrote:
Quote:
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>

George Herold
Guest

Tue Jan 08, 2019 1:45 am   



On Monday, January 7, 2019 at 7:00:41 PM UTC-5, bitrex wrote:
Quote:
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.

George Herold
Guest

Tue Jan 08, 2019 1:45 am   



On Monday, January 7, 2019 at 6:41:59 PM UTC-5, bill....@ieee.org wrote:
Quote:
On Tuesday, January 8, 2019 at 7:19:19 AM UTC+11, John Larkin wrote:
On Mon, 7 Jan 2019 11:31:51 -0800 (PST), George Herold
gherold_at_teachspin.com> 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)

Well, it's simple, but the resistor tempcos could add a bunch of
drift.

You can buy thin film networks on a common substrate, with a very tight guarantee on the ratio drift.

The difference amps you buy tend to have such a network built into the package.

Instrumentation amps are a slightly different breed of cat, and you need to read the data sheets rather carefully to make sure you buy the right part.


Re Int amps; Right, near unity gain you have to worry about the specs of the
difference-output amp. AoE3 has a nice section on diff/int amps.

George H.
Quote:

--
Bill Sloman, Sydney


bitrex
Guest

Tue Jan 08, 2019 1:45 am   



On 01/07/2019 02:31 PM, 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.


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

George Herold
Guest

Tue Jan 08, 2019 1:45 am   



On Monday, January 7, 2019 at 3:19:19 PM UTC-5, John Larkin wrote:
Quote:
On Mon, 7 Jan 2019 11:31:51 -0800 (PST), George Herold
gherold_at_teachspin.com> 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.

Well, it's simple, but the resistor tempcos could add a bunch of
drift.

Hmm and the diff/int amp has trimmed R's built in.
I was thinking I can get better opamps than diff/int amps.
I guess at DC I care most about drift... I've never used a
chop amp.

George H.

Quote:


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com


George Herold
Guest

Tue Jan 08, 2019 1:45 am   



On Monday, January 7, 2019 at 2:34:58 PM UTC-5, Phil Allison wrote:
Quote:
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.

whit3rd
Guest

Tue Jan 08, 2019 2:45 am   



On Monday, January 7, 2019 at 4:37:01 PM UTC-8, 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)


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.

Phil Hobbs
Guest

Tue Jan 08, 2019 2:45 am   



On 1/7/19 8:27 PM, George Herold wrote:
Quote:
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.

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 2:45 am   



On Monday, January 7, 2019 at 8:01:28 PM UTC-5, bitrex wrote:
Quote:
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 Wink


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.

George Herold
Guest

Tue Jan 08, 2019 2:45 am   



On Monday, January 7, 2019 at 7:37:48 PM UTC-5, bitrex wrote:
Quote:
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.

bitrex
Guest

Tue Jan 08, 2019 2:45 am   



On 01/07/2019 07:52 PM, 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.


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 Wink

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