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John Caldwell

Dec 23, 2014

Audio
Operational Amplifier

TI
Designs – Precision

Piezoelectricity

audioopampportal

TIPD147

low
noise amp

audioportal

Precision
Amplifier

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Related

Stress-induced outbursts: Microphonics in ceramic capacitors
(Part 2)

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If you
missed Part 1 in this series, here’s the
background:

While
working on a low-noise amplifier circuit I found that the ceramic capacitors on
my printed circuit board (PCB) were exhibiting piezoelectric effects and ruining
the otherwise extremely low noise of the circuit. This behavior is common in
non-C0G type ceramic capacitors, and to remedy the situation, I decided to test
three alternative capacitor types: soft-termination ceramic, tantalum and film.
All of the capacitors tested were 2.2uF surface-mount types.

Using
some items I found in my cubicle, I came up with a more controlled test than I
used in Part 1. I had the idea that something impacting the
PCB would be a good way to apply a repeatable stress to the capacitors. For
example, a test mass could be dropped onto the PCB. Assuming that the test mass,
drop height and impact location on the PCB remain constant, the stress applied
to the capacitors in the circuit should be similar in all the tests. The
disturbance caused in the amplifier’s output voltage could be used to compare
how sensitive each type of capacitor is to vibration.

My test
mass was an AA battery that would slide down a tri-corner ruler to hit a spot on
the PCB free from components. Besides guiding the battery to the impact
location, the ruler also made it easy to drop the battery from a consistent
height above the PCB (2.25 inches for this test). The basic test setup is shown
in Figure 1.

Figure 1: Test setup for comparing piezoelectric
effects in different capacitor types.

The
amplifier PCB was fixed in a vice and the input was shorted to ground ensuring
that any large output-voltage disturbance was due entirely to capacitor
piezoelectric effects. The output of the amplifier was connected to an
oscilloscope set to be triggered when the amplifier voltage exceeded the typical
noise floor. Figure 2 shows an example of the amplifier’s output when the
battery impacts the PCB. For this waveform, X7R ceramic capacitors were used.
Notice that the disturbance in the output voltage is large -- about
170mVpp!

Figure 2: An example output voltage waveform when the
battery impacts the PCB (ceramic capacitors, impact at t=0s).

The peak-to-peak
measurement function on the oscilloscope was used to quantify the output voltage
disturbance. For each type of capacitor, I performed five impact tests and then
took the average of the runs. The results for each type of capacitor are shown
in Figure 3.

Figure 3: Average peak-to-peak output voltage of the
amplifier from the impact test (by capacitor type)

I find these results
interesting for two reasons.

1) Although the
soft-termination ceramic capacitors (Ceramic X7R, ST) reduce the output voltage
disturbance, it was not enough to be useable in my application. However, this
amount of reduction might be enough in applications with lower gains or higher
noise floors.

2) The tantalum and
film capacitors showed absolutely no sensitivity to vibration! This is the real
take-away from this testing, if you have an application that must be immune to
vibration effects, such as low noise circuits in automotive environments,
tantalum and film capacitors should be used.

Unfortunately, these
types of capacitors also have their drawbacks. Film capacitors are expensive and
typically are larger than ceramic or tantalum capacitors of the same value.
Tantalum capacitors are polarized and will not be useable everywhere in a
circuit. Tantalum capacitors also have higher ESR than similar ceramic types,
but this may be overcome by placing C0G ceramic capacitors in parallel with a
Tantalum capacitor. The right capacitor choice for your application depends on
the requirements for cost, PCB area and the circuit performance. Armed with the
knowledge from this blog series, you’re ready to build circuits that keep their
composure, even in stressful situations!

Additional
resources:

Learn how the TIPD147 works with low-noise design and noise
measurement.

Read other blog
posts about working with low noise amps, including Part 1 of today’s series.

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