®

PCM54/55

6

error at each sampling point. The THD can then be ex-

pressed as:

(2)

This expression indicates that, in general, there is a correla-

tion between the THD and the square root of the sum of the

squares of the linearity errors at each digital word of interest.

However, this expression does not mean that the worst-case

linearity error of the D/A is directly correlated to the THD.

For PCM54/55 the test period was chosen to be 22.7

µs

(44.1kHz) which is compatible with the EIAJ STC-007

specification for PCM audio. The test frequency is 420Hz

and the amplitude of the input signal is 0dB, –20dB, and

–60dB down from full scale.

Figure 4 shows the typical THD as a function of output

voltage.

Figure 5 shows typical THD as a function of frequency.

THD

n

ii

rms

rms

i

n

LQ

rms

==

+

=

ε

Ε

ΕΕ

Ε

Σ

1

100

1

2

[( )

( )]

•%

INSTALLATION AND OPERATING

INSTRUCTIONS

POWER SUPPLY CONNECTIONS

For optimum performance and noise rejection, power supply

decoupling capacitors should be added as shown in the

connections diagram. These capacitors (1

µF tantalum or

electrolytic recommended) should be located close to the

converter.

MSB ERROR ADJUSTMENT PROCEDURE

(OPTIONAL)

The MSB error of the PCM54 and PCM55 can be adjusted

to make the differential linearity error (DLE) at BPZ essen-

tially zero. This is important when the signal output levels

are very low because zero crossing noise (DLE at BPZ)

becomes very significant when compared to the small code

changes occurring in the LSB portion of the converter.

Differential linearity error at bipolar zero is guaranteed to

meet data sheet specifications without any external adjust-

ment. However, a provision has been made for an optional

adjustment of the MSB linearity point which makes it

possible to eliminate DLE error at BPZ (PCM54 only). Two

procedures are given to allow either static or dynamic

adjustment. The dynamic procedure is preferred because of

the difficulty associated with the static method (accurately

measuring 16-bit LSB steps).

To statically adjust DLE at BPZ, refer to the circuit shown

in Figure 6 or the PCM54 connection diagram. After allow-

ing ample warm-up time (20-30 minutes) to assure stable

operation of the PCM54, select input code 8000 hexadeci-

mal (all bits off except the MSB). Measure and record it.

Change the digital input code to 7FFF hexadecimal (all bits

off except the MSB). Adjust the 100k

Ω potentiometer to

make the audio output read 92

µV more than the voltage

reading of the previous code (a ILSB step = 92

µV).

A much simpler method is to dynamically adjust the DLE at

BPZ. Again, refer to Figure 6 or the PCM54 connection

diagram for circuitry and component values. Assuming the

device has been installed in a digital audio application

circuit, send the appropriate digital input to produce a –60dB

level sinusoidal output. While measuring the THD of the

audio circuit output, adjust the 100k

Ω potentiometer until a

minimum level of distortion is observed.

0.1

0.05

0.02

0.01

0.005

0.002

0.001

100

1k

10k 20k

Frequency (Hz)

–20dB

Full Scale

FIGURE 5. Total Harmonic Distortion (THD) vs

Frequency.

10.0

4.0

2.0

1.0

0.4

0.2

0.1

0.04

0.02

0.01

0.004

0.002

0.001

–60

–50

–40

–30

–20

–10

0

V

0dB = Full-Scale Range (FSR)

16 Bits

14 Bits

FIGURE 6. MSB Differential Linearity at Bipolar Zero Ad-

justment Circuit (optional).

1M

Ω

560k

Ω

100k

Ω

330k

Ω

–V

CC

1

27