DISCUSSION OF
SPECIFICATIONS
The PCM67 and PCM69A are specified to provide critical
performance criteria for a variety of applications. The accu-
racy of a D/A converter is described by the transfer function
shown in Figure 1.
V
OUT
= 1.2mA • R
NF
V
OUT
R
NF
V
COM
1.2mA
Digital In
011...11
+FSR
Gain Error
I
OUT
V
OUT
V
COM
(3.5V)
FIGURE 2. I/V Amplifier Circuit.
000...00
111...11
BPZ
Analog
Out
BPZ – 1LSB
S/N RATIO
S/N ratio is defined as the ratio of full scale output and no input
noise level at BPZ point. The PCM67/69A is specified at
110dB typical with “IHF-A” filter.
LEVEL LINEARITY ERROR
Level linearity error is defined as the deviation of actual
analog output level from digital input level. PCM67/69A is
specified at 1dB typical at –90dB output level. The 0.5LSB
quantization error at –90dB of 16-bit conversion is equal to
+1.94dB, –2.5dB.
TOTAL HARMONIC DISTORTION
THD is a key parameter in audio applications, THD is a
measure of the magnitude and distribution of the linearity
error, differential linearity error, and noise, as well as quanti-
zation error. To be useful, THD should be specified for both
high level and low level input signals. This error is unadjustable
and is the most meaningful indicator of D/A converter accu-
racy for audio applications.
THD is defined as the ratio of the square root of the sum of the
squares of the values of the harmonics to the value of the
fundamental input frequency and is expressed in percent or
dB. The rms value of the PCM67/69A error referred to the
input can be shown to be
100...00
–FSR
(V
= 3.5V)
COM
FIGURE 1. Transfer Performance.
DIGITAL INPUT CODE
The PCM67/69A accepts Binary Two’s Complement (BTC)
digital input code (MSB FIRST).The relationship of digital
input to analog output is shown in Table 1.
ANALOG OUTPUT
(VOLTAGE)
+FSR
BPZ
BPZ – 1LSB
–FSR
ANALOG OUTPUT
(CURRENT)
–1.2mA
–0.6mA
–0.59995mA
0mA
DIGITAL INPUT
7FFFFF (HEX)
00003F (HEX)
FFFFFF (HEX)
80003F (HEX)
TABLE I. Digital Code and Analog Out.
GAIN ERROR AND GAIN MISMATCH,
CHANNEL-TO-CHANNEL
Gain error is defined as deviation of the output current span
from the ideal span of 1.2mA (FSR) on each channel. Gain
error of PCM67/69A is typically
±3%
of FSR.
Gain mismatch, channel-to-channel is defined as the differ-
ence in gain error between the left channel and right channel.
THE RELATIONSHIP OF V
COM
AND I/V OUT
The output current range of PCM67 and PCM69A is 0mA to
1.2mA as shown in Table 1.
In the typical application, the non-inverting input of the
external I/V op amp is connected to the V
COM
pin of PCM67
and PCM69A. Accordingly, the output voltage level at FSR
after I/V conversion is V
COM
voltage (+3.5V) as shown in
Figure 2.
ε
rms
=
1
n
i
=
1
∑
n
E (i )
+
E (i )
L
Q
2
(1)
where n is the number of samples in one cycle of any given
sine wave, E
L
(i) is the linearity error of the PCM67 or
PCM69A at each sampling point. THD can then be expressed
as
THD
=
ε
rms
E
r ms
2
1
E
L
(i )
+
E
Q
(i )
n
∑
n
=
i
=
1
E
rms
×
100%
(2)
where E
rms
is the rms signal-voltage level.
®
PCM67/69A
6
BURR-BROWN [ BURR-BROWN CORPORATION ]
