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DC background subtraction

Started by George Herold January 7, 2019
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. 
 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
On Mon, 7 Jan 2019 11:31:51 -0800 (PST), George Herold
<gherold@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. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
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@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
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
On Monday, January 7, 2019 at 3:19:19 PM UTC-5, John Larkin wrote:
> On Mon, 7 Jan 2019 11:31:51 -0800 (PST), George Herold > <gherold@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.
> > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
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.
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.
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>
On Monday, January 7, 2019 at 6:41:59 PM UTC-5, bill....@ieee.org wrote:
> 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@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.
> > -- > Bill Sloman, Sydney