Bypass capacitors when using multiple op amps - Page 1
Forum section: electronics / beginners

author: tedbar
author_rank: Contributor
timestamp: November 15, 2024, 11:55:26 am
content: Best practice for op amp layout always includes a bypass capacitor (or capacitors). That makes absolute sense. However, when a board is laid out with multiple op amps, in close proximity, adding a cap to each op amp will result in them being in parallel and thus increasing the capacitance.What is the best practice for bypass caps when using multiple op amps? Should I add a lower value cap close to each op amp, retain the recommended value cap at each op amp or place a single cap as centrally as possible?

author: gcewing
author_rank: Regular Contributor
timestamp: November 15, 2024, 12:17:38 pm
content: Use a capacitor of the recommended value for each op amp, positioned as close as possible to its power pins. With bypass capacitors, it's not usually a problem to have more capacitance than needed.

author: tggzzz
author_rank: Super Contributor
timestamp: November 15, 2024, 12:47:04 pm
content: It depends, as always.Bandwidth, noise, and output current are key components of the decision tree. In many cases there are more important considerations than simply capacitor value.Fundamentally you need to understand the theory of how decoupling capacitors work (and fail!) in a circuit. Without that, you are left with "cargo cult engineering".

author: Terry Bites
author_rank: Super Contributor
timestamp: November 15, 2024, 01:47:05 pm
content: Yeah, you really need a bypass caap per power pin. Also needed are some bulk storage here and there.I usually use 10-100n film caps per pin and 10-100u polymer elco's on the rails every few amps.How much bulk is required depends on the speed and output current the amps are having to deliver.For high speed opamps be aware that ceramic caps are self resonant. You have to be careful that this self resonant frequency is above your opamps GBWP.To get good performance at higher frequencies a combination of local bypass caps will be needed.I avoid tantalum and ceramics in linear circuits where ever possible. Save the ceramics for digital.Dave does some good tutorials on decoupling.

author: tggzzz
author_rank: Super Contributor
timestamp: November 15, 2024, 04:18:41 pm
content: Quote from: Terry Bites on November 15, 2024, 01:47:05 pmSo are resistors and inductors and PCB wire traces.Plus, of course, oscillation doesn't require any self-resonant behaviour in components. "Amplifiers oscillate, oscillators won't".Summary: understand how the physics will affect your circuit.

author: TimFox
author_rank: Super Contributor
timestamp: November 15, 2024, 05:19:25 pm
content: Ceramic capacitors are typically smaller than film capacitors for the same capacitance and therefore have higher self-resonant frequencies (lower parasitic inductance).The generalization that ceramic capacitors are bad for signal applications dates from years ago where X7R, Y5U, and Z5U dielectrics were the only ones available in reasonably large values (those dielectrics are non-linear).Nowadays, C0G (linear) dielectric capacitors are available up to roughly 100 nF at voltages suitable for solid-state designs at higher price.When I measured different leaded capacitors for self-resonance effects relevant to low MHz frequencies, I found that the parasitic inductance varied roughly with the overall length of the capacitor plus leads, not very sensitive to capacitance or construction (including ceramic vs. film).