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Old 11th January 2012, 05:40 AM   #1
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Default bypassing for opamp circuits

Ok. So i've got a circuit using opamps. its a balanced line driver. My power supply board is seperate and generating +&- 15V.
The supply rails are well filtered on the power supply side with 100uf caps.
Should 100uf caps be added again on the opamp board from each supply rail to ground?
Also, should I bypass these electrolytics with 100nf caps too?
I did see somewhere that a single 100nf cap from +15V rail to -15V rail was used in another design.
SOOO, what exactly is the general rule here??
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Old 11th January 2012, 06:57 AM   #2
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100nf cap between the rails as close as possible to the op-amp.10uf (or larger) from the rails to ground not necessarily close to the op-amp but on the same pcb.
Regards.
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Old 11th January 2012, 02:21 PM   #3
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thanx!
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Old 11th January 2012, 06:40 PM   #4
AndrewT is online now AndrewT  Scotland
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Does the driver datasheet give the guidance you are seeking?
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Old 11th January 2012, 07:29 PM   #5
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From Op Amp for Everyone 3:

[The Number 1 Design Mistake
I saved the very best and most common mistake for last. And it doesn’t even involve an op amp. It involves support components: the decoupling capacitors!
In Chapter 1, I mentioned some part numbers that are etched in the memory of every design engineer, at least those involved in analog design. There is one other: 0.1 mF.
Need to decouple? OK, everybody knows you put a 0.1 mF capacitor on every power supply input and the job is done, right? I can disprove that truism very easily with two words: cell phone.
Put your cell phone near your prototype circuit, which is bypassed with 0.1 mF, and make a call while monitoring the output on a high bandwidth oscilloscope. You will see horrendous 2.4 GHz leakage!
An alternative version of this problem came from some cellular telephone base
station installers who called in a panic, “We have 90 MHz noise running all over our system—and can’t figure out where it is coming from.” A suspicion on my part asked them to tell me the exact coordinates where they were installing the system, and they provided the exact latitude and longitude. A quick check of the FCC database revealed the problem. I asked them, “Are you anywhere near the tower for W____ 90.5 FM, a 100,000 W NPR station listed at those coordinates?” They told me on the phone that they could see the transmitter 5 ft away—they were colocating with the station!
The point of this is that their board was bypassed with 0.1 mF capacitors. While that worked fine for the digital portions of the board, the analog portions were being clobbered by radiation of the powerful 90.5 MHz FM station. Conventional thinking
is that the lower the value of capacitance, the lower the frequencies it will filter.
So, 0.1uF should get rid of just about everything because it is a very large value (relatively speaking). This conventional wisdom is wrong! The actual case is the exact opposite.
Where did the value 0.1 mF come from, anyway? A high technology store near me used to have antiquated computer boards as a wall decoration. Backlit with white light, the translucent green boards made a pretty sight. But, they were also populated with 0.1 mF decoupling capacitors. A quick survey of the circuitry revealed that the clock rate of the old computer had been 1 MHz.
So, the 0.1 mF capacitor value seems to have come from bypassing TTL logic in the 1960s! Isn’t it time to rethink the issue a bit, in light of op amps and other analog components that can operate to frequencies of 3 GHz, especially when almost every engineer carries a 2 W, 2.4 GHz transmitter into the lab (cell phone)?
The reality of the situation is that a good 0.1 mF capacitor with an X7R dielectric exhibits a resonance in the 10 MHz region. This is due to parasitic inductance creating an LC circuit. Below 10 MHz, its impedance is capacitive, decreasing almost linearly on a logarithmic plot until it reaches the resonant frequency. Above the resonant frequency, the impedance is inductive. Since inductor resists the flow of high frequencies and passes only low frequencies, the decoupling capacitor is useless above its resonant frequency.
Looking at representative plots from capacitor manufacturers, at 100 MHz, the venerable 0.1 mF bypass capacitor has become an inductor with an XL of at least 1 Ohm.
By 2.4 GHz, XL has risen to above 10 Ohm.
A good rule of thumb for effective bypassing is to put several capacitors in parallel. The standard 0.1 mF capacitor does quite nicely for frequencies up to 10 MHz, a 1000 pF NPO dielectric does nicely up to 100 MHz, and 33 pF NPO eliminates frequencies in the 2.4 GHz region. Bulk decoupling of the power supply as it enters the board eliminates low frequency ripple.
Here is a truism to replace the older one: When poor decoupling is suspected, decrease (do not increase) the value of the capacitance.]

Also see the table 2

General Layout Guidelines for RF and Mixed-Signal PCBs - Maxim

Happy days, Raúl
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Old 11th January 2012, 11:13 PM   #6
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I always put 100nf from b+ to gnd and b- to gnd.
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Old 12th January 2012, 08:17 AM   #7
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ive taken to using 10nf lately from each rail to ground, no specific reason other than that i'm running higher clock speeds/bandwidth than i used to. i cant claim a master stroke, i tend to use either a larger 4.7uf x7r or pps film, or the 10nf and sometimes both. they are also slightly cheaper
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Old 12th January 2012, 08:34 AM   #8
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There's no general rule about decoupling - you'd ideally decouple according to the application. Here you say its a balanced line driver - in such cases there's little point in substantial decoupling to ground, provided that the load is indeed balanced. When the load is balanced, no current flows to ground, so why put a cap to it? However I've experienced oscillations when caps to gnd are totally absent on opamp circuits. So use relatively smaller caps to gnd (say 10nF X7R 0805) with series resistors (1-10R determined empiirically - use the highest value which gives consistent freedom from oscillation).
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