And examples can be even more strange: we had DC-DC converter modules
failing to work properly because one of the converter's power pins
feeding an internal linear regulator picked up noise from the same
converter.
Regards,
Istvan Novak
John Woodgate wrote:
I agree. One particular point is keeping a trace connected to an
inverting input very short, even if that means including a low-value
'stopper' resistor close to the chip. That point has quasi-infinite
sensitivity but infinitesimal impedance only within the op-amp pass
band. Above the pass-band, it is an antenna connected to a diode. And
yes, it can pick up power rail noise.
John Woodgate OOO-Own Opinions Only
J M Woodgate and Associates www.woodjohn.uk
Rayleigh, Essex UK
On 2018-04-09 04:27, Doug Smith wrote:
Hi Ken and the group,
Many analog circuits, as well as the analog parts of large ICs that
you mention, do have response to GHz noise even 1 MHz unity gain
opamps! Low frequency op amps can generate a DC offset on their
inputs from GHz digital noise or radio signals, a common problem for
the last 45 years since I first observed it. Power supply rejection
of op amps goes to pot pretty quickly with frequency as well. High
frequency effects therefore are important even to low speed analog
circuits. In some ways, one must lay out the low frequency analog
circuit using microwave techniques to keep RF noise at bay. The
circuit features must be keep really small,, including the bypass
capacitors being kept extremely close (< 100 mils) to the op amp.
45 years ago, faced with opamp problems from RF noise I developed
some techniques for keeping the op amps happy. I discuss these in my
upcoming course.
Doug
University of Oxford, Course Tutor
Department for Continuing Education
Oxford, Oxfordshire, United Kingdom
--------------------------------------------------
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P.O. Box 60941
Boulder City, NV 89006-0941
TEL/FAX: 702-570-6108/570-6013
Mobile: 408-858-4528
Email: [email protected]
Web: http://www.dsmith.org
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