PCM78 Burr-Brown Corporation, PCM78 Datasheet - Page 9

• Manufacturer Part Number: PCM78
• Description: 16-Bit Audio ANALOG-TO-DIGITAL CONVERTER
• Manufacturer: Burr-Brown Corporation

FIGURE 2. Block Diagram of Distortion Test Circuit.
DISCUSSION OF
SPECIFICATIONS
TOTAL HARMONIC DISTORTION
Evaluating distortion specifications can be a difficult task,
as distortion is often specified in different ways. Total
Harmonic Distortion (THD) is defined as the ratio of the
square root sum of the squares of the value of rms harmonics
to the value of the rms fundamental and is expressed in
percent or dB. Note that this measurement only includes
energy present in those frequencies which would contain
harmonics, and therefore is less than Total Harmonic
Distortion plus Noise.
The Total Harmonic Distortion plus Noise (THD+N) is
defined as the ratio of the square root of the sum of the
squares of the value of the rms harmonics and rms noise to
the value of the rms fundamental and is expressed in percent
or dB. This is the most meaningful measurement of a
dynamic converter’s performance because it includes all
energy present in the signal that is not fundamental. A block
diagram of the test circuit used to measure the THD and
FIGURE 3. Block Diagram of Histogram Test.
–100
–120
–20
–40
–60
–80
0
0.0
–1
–2
2
1
0
0
Oscillator
25
Digital
1024
Oscillator
B&K
Digital
Frequency (kHz)
B&K
50
Codes
2048
Sync
Sync
75
3072
100
4095
SHC702
SHC702
Analyzer Software
DataPhysics Corp
Convert Command
Digital Distortion
Convert Command
Timing
Computer
Scientific
Timing
HP-330
9
THD+N of the PCM78 is shown in Figure 2. This digital
system is capable of differentiating harmonic energy and
noise; conventional distortion analyzers which operate on a
tracking notch filter principle cannot distinguish this energy,
and therefore only measure THD+N. Unfortunately, in the
past, these systems were used for measuring distortion
performance of converters, and the distortion was often
simply specified as “THD”, when in fact it was really
THD+N. For this reason, it is often confusing to compare
specifications of converters unless one knows exactly what
was being measured.
If we assume that the error due to the test circuit of Figure
2 is negligible, then the rms value of the PCM78 error
referred to the input can be shown to be
where N is the number of samples, E
error at each sample, E
THD N
PCM78
PCM78
N
1
i 1
N
E
Q
L
(i) is the quantization error at each
Serial-To-Parallel
(i) E
Communication
Serial-To-Parallel
IEEE-488
Computer
Scientific
E
HP-330
IEEE-488
Memory
rms
64k
Q
PCM78
(i) E
L
N
(i) is the linearity
(i)
2
X 100%
®