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.
** Considered using a capacitor ??
.... Phil
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.
Generating the -2.5 offset for the summer sounds like a job for a DC servo/integrator, servoing the op-amp output
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.
Like a sample and hold? It needs to hold the offset sorta forever, so a pot called for.
George H.
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.
DC blocking capacitor. Y'know, like a high-pass on the input.
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.
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 ;)
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.
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
Re 'signal source'. Right! I was looking at that today. one of these, (I think, may be a different part number)
George H.
Is the 2.5V background really stable to within a fraction of a mV?
Sylvia.
George Herold wrote
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?
An identical sensor outside the field and take the difference?
piglet
round from a few mV signal. I first think about a difference amp, or
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 ca n 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 R
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 amplifi ers at that level, but you could use them if the input glitches are not an issue.
--Spehro Pefhany
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