Talk:Instrumentation amplifier

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Gain equation[edit]

PNG version

The article states Vo/Vi = -(1+2.R1/Rg).R3/R2. Since it is a diff amp, why the negative. Surely Vo/Vi = (1+2.R1/Rg).R3/R2 is cleaner. -- SGBailey 09:23, 18 September 2006 (UTC)[reply]

Just the way it was drawn. We can change the diagram if you really think it's necessary. — Omegatron 13:05, 18 September 2006 (UTC)[reply]
Having seen the reply, I think it would be better to redraw the circuit. I'll try and edit it later this week. -- SGBailey 14:17, 18 September 2006 (UTC)[reply]
SVG version
There's already an SVG, which is great except:
  • We should flip it over, as discussed above
  • Text is a little small for a thumb
  • Wastes a little too much space/components should be closer together
  • The plus and minus in the op-amp symbols are not symmetrical; the minus sign looks like it's hiding in the corner
If you're going to modify one, modify that one, but talk to Alessio first. — Omegatron 15:44, 18 September 2006 (UTC)[reply]
BTW, Are we into wiggly resistors or box resistors? -- SGBailey 22:31, 19 September 2006 (UTC)[reply]
I know of no consensus. I believe wiggly is US and box is Europe, but that doesn't make a difference here, and I see both here. — Omegatron 01:47, 20 September 2006 (UTC)[reply]
Having seen the SVG file, I may not edit it because it is on Commons and for some reason I have difficulty with that. -- SGBailey 22:36, 19 September 2006 (UTC)[reply]
Excuse me but I am new to this. I could expand this article with a derivation of the gain equation to show where it came from but it would need a new diagram with some points labeled to illustrate points in the derivation. I could easily draw this new diagram as a .SVG file. Would this be acceptable? The only problems I would have is putting formulas into a wiki page, at first glance this does no seem straightforeward but perhaps I am missing something. Alternatively I could hand over the derivation and diagram to someone more experienced at this and they could put them in. Drummond047 (talk) 23:22, 7 February 2011 (UTC)[reply]

Resistor values[edit]

If building from 3 op-amps, is there any guide as to how much gain to get from 2R1/Rg and how much to get from R3/R2? Should they ideally be equal or is one term better as unity or ...? -- SGBailey 09:23, 18 September 2006 (UTC)[reply]

All the commercial instrumentation amplifiers I've seen have R2=R3 -- in other words, all 4 resistors connected to the output op-amp are equal.
I could speculate that the designers decided that it is easier to make matched resistors if they are all the same.
Or I could speculate that the designers realized that whenever you have one amplifier followed by another amplifier (a cascade amplifier ?), it is best to try to put the most gain on the first amplifier, because of the Friis formulas for noise.
Or I could speculate that the first popular instrumentation amplifier had arbitrarily-picked minimum gain of 1 for no good reason, and all the later ones mimic it to reduce the switching cost.
Please tell me if you find some way of calculating optimum resistor values. --68.0.124.33 (talk) 18:11, 28 August 2008 (UTC)[reply]
Optimum resistor values vary from one circuit to another, if bandwidth is your main consideration then equal gains is the optimum, if noise is your main consideration (the usual case) then the optimum is to have much more gain in the first stage, hope this helps. Drummond047 (talk) 23:27, 7 February 2011 (UTC)[reply]

Two op-amp version[edit]

We should also cover the two op-amp version:

And here's a good comparison. — Omegatron 13:37, 18 September 2006 (UTC)[reply]

Circuit diagram[edit]

I made the diagram again from scratch according to what Omegatron said. If you watch the small thumb version above it doesn't look great, but the standard size I would expect to place in a article looks fine:

this is a test

I might consider increasing the size of the fonts and switching to Arial-like fonts, not Times-like. What do you think?? is this ok or should I improve it more?? Alessio Damato 16:24, 20 September 2006 (UTC)[reply]

We were talking about flipping the differential amp upside-down (ground on top, output on bottom) so that the equation doesn't have a minus sign in it.
The thumb size is set in Special:Preferences#prefsection-3. Mine is set to 300px.
I don't have a preference about the fonts. It would be nice if all our diagrams were consistent, but I don't know we'd do that or which is better. — Omegatron 18:14, 20 September 2006 (UTC)[reply]
since the fonts are exactly the same ones of the other diagrams (I just copied and pasted), I'll keep them for consistency. If we'll ever realize that another font is better, we'll change them for all the SVG diagrams (it's quite easy using only text editors). I'll move it in the article Alessio Damato 16:26, 21 September 2006 (UTC)[reply]
You can't just flip the op-amp. You need to flip the ground and Vout connections, too. It needs negative feedback to function; not positive feedback. — Omegatron 17:47, 21 September 2006 (UTC)[reply]
Very good now. Thanks. — Omegatron 01:00, 25 September 2006 (UTC)[reply]
"Very good now" what? It still isn't flipped which is where I though that this all started. -- SGBailey 21:01, 1 October 2006 (UTC)[reply]
It no longer needs to be flipped, as the inputs are now V1 and V2 instead of V+ and V-. — Omegatron 12:58, 2 October 2006 (UTC)[reply]
Oh yes, and the text is too small for the size of the diagram. In the two images top right, the top one is readable whilst I have to "Open in ne window" the 2nd one to read Rgain and to be sure of R1 R2 etc. -- SGBailey 21:03, 1 October 2006 (UTC)[reply]
I've just done a test on my PC where I tried 100pt arial for the main characters and 72pt Bold Arial for the subscripts. That seemed readable on the third image (1778 * 1157 pixels) on the page when viewed at the size of the second image on the page. -- SGBailey 21:12, 1 October 2006 (UTC)[reply]
The text size, of course, depends on the size of the image. — Omegatron 12:58, 2 October 2006 (UTC)[reply]
When printing Opampinstrumentation.svg‎ I get no circuit diagram just all-black box. Not very nice when it comes to printing off the article. Perhaps the background should be white instead of transparent. 62.197.96.231 12:53, 31 January 2007 (UTC)[reply]
It's a bug of your web-browser, it does not depend on the image. I think older versions of firefox had this problem, for example. Upgrade your browser and it should be fine Alessio Damato 16:40, 31 January 2007 (UTC)[reply]
Drawing checking is very important if replacing the schematic diagram. Make sure the polarities of op-amp inputs are correct, and that the legends for all the resistors are consistent with the formulas in the text. --Wtshymanski (talk)

Gain calculation[edit]

The wiki page is incorrect. it has a minimum -2 gain not 2, as said on the referenced page (the diagram caption says 2, the text says -2 (or less)). i have not edited it because the Gain page is not clear enough to do the gain calculation (i tried +2 gain and i did not get +6dB, i got +3.01dB). Charlieb000 (talk) 08:03, 4 February 2013 (UTC)[reply]

Are you talking about, An instrumentation amp can also be built with 2 op-amps to save on cost and increase CMRR, but the gain must be higher than 2 (+6 dB).? -—Kvng 14:20, 7 February 2013 (UTC)[reply]
Whether the gain figure comes out positive or negative depends on how you define your input voltage. Typically with a differential setup one would define the input voltage so that the gain came out positive but it's kinda arbitary. As for dB the gains we are talking about here are voltage gains so you need to use 20 instead of 10 in your dB formula. — Preceding unsigned comment added by 130.88.154.105 (talk) 17:18, 13 August 2014 (UTC)[reply]

Comparison with 2 OP Circuit[edit]

The article and the ref. given mentions a higher CMRR for the 2 OP circuit. However [1] link from analog.com explains the opposite: a lower CMRR for the 2 OP Circuit, at least for AC, even at rather moderate frequencies like 100 Hz. Matching of the two R1 resistors is not at all critical, so does not introduce an extra error.--Ulrich67 (talk) 18:19, 3 November 2014 (UTC)[reply]

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