One thing to watch out for in choosing a capacitor for any circuit is =
distortion. This type of distortion occurs, if the capacitor is in the =
signal=20
path. Figure 1 shows a few typical circuits that contain capacitors in =
the=20
signal path.
Figure 1. The first stage of a sixth-order bandpass =
filter=20
(a) and an amplifier driving an R/C pair to an ADC (b).
Figure 1a shows the first stage of a Sallen-Key, bandpass filter. In =
this=20
circuit two key capacitors are exposed to the signal going through the =
filter.=20
Figure 1b shows an amplifier, using an R/C pair to drive a SAR-ADC. The=20
capacitor, CF, is fully exposed to the incoming signal before =
it=20
reaches the ADC.=
This distortion occurs due to the normal capacitor voltage-dependant=20
characteristic. In other words, the capacitance changes with the voltage =
and=20
frequency applied.
An equation that describes this change in one region of the voltage =
curve=20
is:
C =3D C0=20
( 1 + bVCAP),
where
=20
C0 is the nominal capacitance,
=20
VCAP =
is the=20
voltage across the capacitance,
=20
b is the voltage=20
coefficient of the capacitor.
Figure 2 plots a typical curve of this effect across a capacitor.
Figure 2. Capacitor voltage coefficient
The input or output charge on a capacitor travels through adjacent=20
impedances, creating a voltage drop error. Since the charging current =
from a=20
capacitor is voltage dependant, it creates a non-linear error. For a =
sine wave,=20
this error contains harmonics.
The capacitor voltage coefficient characteristic can be more =
pronounced in=20
semiconductor process technologies. Since the ADC input (Figure 1b) has =
an=20
internal input R/C, this distortion-producing phenomena also occurs at =
the=20
converter=E2=80=99s input.
The input signal frequency across a capacitor also can impact the =
accuracy of=20
your conversion. The capacitance value introduces distortion, which also =
changes=20
with frequency (Figure 3).
Figure 3. Capacitor THD+N versus Frequency
This graph shows the =
characteristics=20
of several capacitor technologies and their total harmonic distortion =
plus noise=20
(SINAD) versus frequency performance. The lowest line on this chart is =
taken=20
using a C08 capacitor. The line above the C0G capacitor data shows the =
system=20
measurement. The other lines on this chart are from ceramic caps with =
different=20
dielectrics: Z5U, Y5V, and X7R. Note that these types of capacitors =
introduce=20
significant non-linearity and signal distortion over frequency.
Not shown on this chart is the ceramic NPO-type capacitor. The NPO =
type=20
capacitor closely matches the C0G performance. It is critical that you =
select=20
the right capacitor type for C1 and C2 (Figure 1a) =
and=20
CF (Figure 1b). You will find that a higher quality external=20
capacitor (CF) will not degrade the ADC=E2=80=99s AC =
specifications. The=20
larger voltage coefficient of the smaller internal ADC capacitor=20
(CSH) will be dwarfed by the lower voltage coefficient of the =
larger=20
external capacitor.
Signal distortion can come in many forms, but the capacitors in the =
circuit=20
signal path may be the last thing that you think about if you encounter =
this=20
type of problem. Do you have any war stories when it comes to =
distortion? Please=20
share! Meanwhile, for more information check out these references:
- =E2=80=9CHow=
the=20
Voltage Reference Affects your Performance: Part 1,=E2=80=9D =
Baker, Oljaca, Analog=20
Applications Journal (SLYT331), Texas Instruments, 2Q 2009
- =E2=80=9CHow=
the=20
Voltage Reference Affects your Performance: Part 2,=E2=80=9D =
Baker, Oljaca, Analog=20
Applications Journal (SLYT 339), Texas Instruments, 3Q 2009
- =E2=80=9CHow=
the=20
voltage reference affects ADC performance, Part 3,=E2=80=9D Baker, =
Oljaca, Analog=20
Applications Journal (SLYT 355), Texas Instruments, 4Q 2009