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| Local op-amp rail bypassing: Best practices? |
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| cvanc:
Hi all- Let's talk about the small capacitor(s) that are typically used for power rail bypassing of op-amps. The usual idea is 100nF from B+ to ground and 100nF from B- to ground, with both caps being placed in close proximity to the chip. I recently read about an alternative to this: Using just one 100nF cap, wired directly from B+ to B-, in other words not connected to ground at all. The idea is this will avoid injecting rail noise into the ground. That argument makes some sense to me, especially if board layout is less than ideal, but I have to say I've never actually seen this implementation in anything I've ever worked on. I've got the usual questions here: What do we think about this? Have you ever seen this done in a design, and did it work well? Are there any particular op-amps that just hate this idea and misbehave? All comments welcome. Thanks. |
| mvs:
--- Quote from: cvanc on December 23, 2018, 02:12:18 pm ---What do we think about this? Have you ever seen this done in a design, and did it work well? Are there any particular op-amps that just hate this idea and misbehave? --- End quote --- Dave has used this approach in µCurrent Gold. I guess to limit capacitive load on rail splitter amp. I am not a fan of this solution, if ground is used as return path for low impedance load. |
| Zero999:
It depends on the op-amp and the application. If it's a low current, low frequency application, then supply bypassing is not critical and may even be unnecessary. |
| David Hess:
For stability purposes, that only works for a load which terminates into one of the supplies. If the load terminates into ground, then it can be a problem. In precision applications, the signal and power grounds might only be connected at one point but this becomes problematical at higher frequencies. Consider exactly where the return currents from each supply pin to the output have to travel. https://www.analog.com/media/en/technical-documentation/application-notes/6001142869552014948960492698455131755584673020828AN_345.pdf |
| spec:
--- Quote from: David Hess on December 25, 2018, 01:56:11 am ---For stability purposes, that only works for a load which terminates into one of the supplies. If the load terminates into ground, then it can be a problem. In precision applications, the signal and power grounds might only be connected at one point but this becomes problematical at higher frequencies. Consider exactly where the return currents from each supply pin to the output have to travel. https://www.analog.com/media/en/technical-documentation/application-notes/6001142869552014948960492698455131755584673020828AN_345.pdf --- End quote --- This is the answer, for two main reasons: frequency stability/response, supply line rejection, especially at medium and high frequencies. Also as DH implies, you, must decouple to a point as close to the opamp as possible but between the input 0V reference point and the 0V line from the PSU. Hash on the supply lines (nearly all opamps have class A/B outputs) feed current through the decoupling capacitors to the 0V line. This hash current generates voltage drops, caused by current flowing through traces/wire, and it is vital that this hash generated voltage does not add to the input signal, and cause all kinds of problems. Bear in mind that the skin effect increases the impedance of traces/wires and that, even the most lowly opamps ike the LM358, have an open loop gain of 100dB (x100,000 voltage gain) up to 5Hz and still have some voltage gain up to 1MHz. And if you are designing with one of the video opamps you have to be very strict with decoupling and physical layout in order to get a flat frequency response with no ringing. |
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