®

PCM78

9

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

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

FIGURE 3. Block Diagram of Histogram Test.

Sync

Timing

B&K

Digital

Oscillator

Convert Command

SHC702

PCM78

Serial-To-Parallel

IEEE-488

Communication

HP-330

Scientific

Computer

Digital Distortion

Analyzer Software

DataPhysics Corp

0

–20

–40

–60

–80

–100

–120

25

Frequency (kHz)

50

75

0.0

100

Sync

Timing

B&K

Digital

Oscillator

Convert Command

SHC702

PCM78

Serial-To-Parallel

64k

Memory

IEEE-488

HP-330

Scientific

Computer

2

1

0

–1

–2

Codes

2048

4095

0

1024

3072

THD

+N =

1

N

E

i

=1

N

∑

E

rms

X 100%