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John Caldwell

Dec 19, 2014

Audio
Operational Amplifier

TI
Designs - Precision

Piezoelectricity

TIPD115

OPA211

audioopampportal

low
noise amp

audioportal

Precision
Amplifier

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Related

Stress-induced outbursts: Microphonics in ceramic capacitors
(Part 1)

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While working on a
low-noise amplifier circuit for an upcoming TI Designs reference design, I was caught
off-guard by some interesting behavior. Casually moving the printed circuit
board (PCB) on my workbench caused the output voltage to jump! Fascinated, I
decided to perform a highly scientific test: I tapped on the PCB repeatedly
while observing the output voltage on an oscilloscope.

Figure 1: Circuit output produced by tapping on the
PCB

The seven
spikes in the output voltage shown in Figure 1 are the result of my tapping on
the PCB. Many physical interactions with a PCB can cause the output of a circuit
to change. For example, placing stress on the package of an op amp can change
its offset voltage. However, this circuit was EXTREMELY sensitive to vibration,
and op amps typically do not show this level of sensitivity. With this in mind,
I turned my attention to the ceramic capacitors on the PCB.

Multi-layer ceramic
capacitors are incredibly useful. They offer a unique combination of low
equivalent series resistance (ESR) and equivalent series inductance (ESL) and
high volumetric efficiency. Their construction consists of multiple layers of
metal electrodes inside a ceramic dielectric material as shown in Figure
2.

Figure 2: The
typical structure of a multilayer ceramic capacitor

Barium Titanate
(BaTiO3) is commonly used in the dielectric of ceramic capacitors because it can
have a relative permittivity greater than 3000 [1]. Typically, as you shrink the
physical size of a ceramic capacitor, increasing the capacitance requires a
larger quantity of BaTiO3 to be used in the dielectric. Aside from high
permittivity, BaTiO3 has another interesting property: it is highly
piezoelectric. This makes it an excellent choice for piezoelectric microphones
and guitar pickups!

The piezoelectric
effect is the creation of a voltage due to an applied mechanical stress [2].
Figure 3 shows a ceramic capacitor soldered to a PCB. When downward pressure is
applied (red arrow) the PCB deforms, causing the dielectric to be stretched or
compressed by the end caps (blue arrows). When I tapped on my PCB, I applied a
mechanical stress to the ceramic capacitors, causing a piezoelectric response in
the dielectric and producing an output voltage.

Figure 3: Mechanical stress on the PCB is coupled to
the dielectric through the capacitor end caps

Piezoelectricity can
be a major problem for electronics installed in high-vibration environments. In
such applications the need for high capacitance, low ESR and ESL, and small size
may lead an engineer to select a high-k ceramic capacitor (X7R, Y5V, Z5U, etc.),
which contains a large percentage of BaTiO3 [3]. A common example is the
capacitor placed on the reference input to an ADC. The circuit may work fine in the lab, where it isn’t
being shaken vigorously. Once it is installed in an environment with vibration,
significant errors in the ADC readings might appear. Power supply designers are
also aware of the converse piezoelectric effect, where the ripple voltage
across the capacitor causes it to “sing” or vibrate.

For my low-noise
amplifier circuit, I chose to investigate a few different solutions to this
problem:

Soft-termination
ceramics: These are ceramic capacitors with a flexible material inside the
end-caps for stress relief. They were introduced for automotive applications,
where PCB flexing can cause capacitors to fail.

Tantalum
capacitors: Tantalum capacitors reportedly do not exhibit microphonic
effects [4]. However, they do have some disadvantages. They are polarized and
typically have higher ESR and ESL than ceramics of similar size and
capacitance.

Film
capacitors: Some customers have reported using film capacitors in high
vibration environments with satisfactory results. The downside is that film
capacitors are typically larger than ceramic or tantalum types and can be very
expensive.

These solutions are
component-level and don’t include possible modifications to the PCB, such as
strain-relief cutouts. In my next post, I’ll test each of these capacitors in
the same circuit and compare how sensitive they are to
vibration.

Additional
resources:

Read more about an
ADC TI Designs reference design, TIPD115.

Download a datasheet on OPA211, a low-noise and low-power
precision operational amplifier

Footnotes

[1] Kahn, M.,
Multilayer Ceramic Capacitors – Materials and Manufacture, http://www.avx.com/docs/techinfo/mlcmat.pdf

[2]
Piezoelectricity, http://en.wikipedia.org/wiki/Piezoelectricity

[3] Caldwell, J.,
More about understanding the distortion mechanism of high-K MLCCs, http://www.edn.com/design/analog/4426318/More-about-understanding-the-distortion-mechanism-of-high-K-MLCCs

[4] Cain, J.,
Comparison of Multilayer Ceramic and Tantalum Capacitors, http://www.avx.com/docs/techinfo/mlc-tant.pdf

Neil Albaugh

This behavior of high- titanate ceramic capacitors is not widely known and
it can create problems that are puzzling; but knowing about this will help
designers to avoid these types of capacitors in applications where
piezoelectric effects can create microphonics.

By the way-- high- titanate ceramic capacitors are also pyroelectric,
responding to sudden changes in temperature.

Dec 28, 2014

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John Caldwell

That's a really interesting point Neil! I may need to write another blog
post in the future on that effect. The challenge will be to separate the
pyroelectric effects from the piezoelectric ones in any experiment I run
because a rapid change in temperature will produce mechanical stress on the
dielectric as well. I suspect that attaching long leads to the capacitor and
keeping it off the PCB would help reduce this effect. Thanks for your
comment!

Dec 29, 2014

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Mike Partridge

Very interesting discussion.  Do you have a reference for the
sensitivity of an op-amp's bias voltage to stress?  You mentioned this as
an aside.  Also, speaking of IC performance, any comments on these
effects with switch-capacitor filters?   I had a problem that I
attributed to these for a high-shock instrumentation application.  And
finally, I presume your 2.2uF cap was in the signal chain and not just power
supply decoupling.

Jan 5, 2015

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John Caldwell

Hi Mike, I'll have to do some digging for a document on offset voltage and
stress, but this is a fairly well-known effect, especially in precision
voltage references. Any mechanism which applies pressure to the die can cause
a shift in offset voltage (even the mold compound of the op amp package!).
Limiting this effect requires proper layout of the PCB by locating sensitive
circuits in low stress areas like the outside edges of the PCB or including
strain relief cut-outs on the board. Chopper and auto-zero op amps are much
more resistant to these effects because they are cancelling their offset in
real time. I haven't personally seen microphone effects in switched-cap
filters, but the basic principles hold true. Rather than the capacitor acting
as a variable resistance in the filter, it will act like a variable resistance
and a voltage source due to the microphonic effects. And yes, the 2.2uF caps
were definitely in the signal path and not the power supply. Although the
power supply is always part of the signal path, in this application the PSRR
of the amplifier would suppress any microphonic effects in the bypass
capacitors.

Jan 6, 2015

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Frank Rothoff

The pyroelectric sensitivity is indeed an interesting point, thanks Niel.
As John mentioned already it will be a challenge to seperate this parameter
from mechanical stress artifacts. I've already some ideas which might be
helpful in this.

Anyway I wanted to point out an article from our colleague who looked at
this phenomenon from the other perspective. He experienced audible noise from
MLCC's which is basically the same phenomenon. Here is the link: http://www.analog-eetimes.com/en/reducing-mlccs-piezoelectric-effects-and-audible-noise.html?cmp_id=71&amp;news_id=222903370

Jan 7, 2015

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