Talk:Instrumentation amplifier

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Contents 1 Gain equation 2 Resistor values 3 Two op-amp version 4 Circuit diagram

[edit] Gain equation

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)

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)

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)

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)

BTW, Are we into wiggly resistors or box resistors? -- SGBailey 22:31, 19 September 2006 (UTC)

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)

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)

[edit] Resistor values

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)

[edit] Two op-amp version

We should also cover the two op-amp version:

• schematic - description
• schematic - description
• schematic - description

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

[edit] Circuit diagram

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:

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)

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)

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)

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)

Very good now. Thanks. — Omegatron 01:00, 25 September 2006 (UTC)

"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)

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)

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)

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)

The text size, of course, depends on the size of the image. — Omegatron 12:58, 2 October 2006 (UTC)

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)

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)