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Old 13th April 2010, 11:24 AM   #31
marce is offline marce  United Kingdom
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Some background reading worth looking at is the asrticle:
"The physical basis of EMC -Part 3" it shows where a lot of noise comes from, and why PDS and decoupling etc are so important in todays electronics.
Article found in current edition of mag found here.
EMC Information Centre - The EMC Journal (Free in the UK)
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Old 20th April 2010, 06:07 PM   #32
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Quote:
Based on what Naz?
4562 has feedforward compensation, has two ref. points - and + PSU
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Old 20th April 2010, 08:07 PM   #33
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Quote:
Originally Posted by abraxalito View Post
Yes, but decoupling also has the role of creating a nice low impedance power source at higher frequencies for the opamp. Its the optimisation of both of these which we try to achieve.



I take the view that the purpose of such caps is the lowering of the power supply impedance at HF. If there is enough inductance in the power wires then it will help to keep HF current loops small, which is what we want. So no, its secondary function is preventing noise transfer to the next (or more importantly, the previous) stage.



It keeps the HF current loop area small - if the opamp isn't putting out appreciable current to ground, then its all we need. In the case where the opamp does put appreciable current to ground, then it turns the noise on the power supply into common mode noise, affecting both rails equally. Here it might be helpful to refer to an opamp datasheet graphs of PSRR and CMRR. They're closely related but normally the CMRR is better than the PSRR. The PSRR is measured for each supply separately, but the CMRR shows what happens when each supply wiggles up and down the same.



Actually, it just shows that the ground gets as noisy as the power rails at high frequencies because they're both in effect wires with about the same series inductance. The noise goes to the other rail, rather than to ground in my preferred decoupling arrangement. I don't think the intention of people putting two caps around their opamp is to end up with a noisy ground - its an unintended consequence because they didn't quite understand enough circuit theory.

We do need to distinguish between sources of noise though - mainly it comes in via the (toroidal) mains transformer and 'leaks' through the regs as they have insufficient HF rejection. Some is generated in the rectifier. And some noise comes onto the supply from the opamps themselves, depending on the application.
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