AD1868
DAC, the AD1868 can continue to function at supply voltages
as low as 3.5 V. Because of its unique design, the power require-
ments of the AD1868 diminish as the battery voltage drops, fur-
ther extending the operating time of the system.
AD1868
18-BIT
DAC
V
V
V
L
L
L
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
B
–
+
18-BIT
SERIAL
REGISTER
LL
DL
S
POWER
SUPPLY
V
O
V
REF
NRL
AD1868
CK
V
V
V
L
16
B
1
2
L
0.1µF
V
AGND
NRR
LL
DL
CK
DR
15
14
S
18-BIT
SERIAL
REGISTER
L
O
3
4
4.7µF
4.7µF
V
REF
LR
NRL 13
0.1µF
12
5
6
DR
AGND
+
–
V
R
DGND
O
18-BIT
DAC
NRR 11
LR
V
R
10
9
DGND
O
7
8
V
V
R
S
B
V
S
V
R
B
Functional Block Diagram
Figure 7. Recom m ended Circuit Schem atic
ANALO G CIRCUIT CO NSID ERATIO NS
GRO UND ING RECO MMEND ATIO NS
NO ISE RED UCTIO N CAP ACITO RS
T he AD1868 has two ground pins, designated as AGND (Pin
12) and DGND (Pin 7). T he analog ground, AGND, serves as
the “high quality” reference ground for analog signals and as a
return path for the supply current from the analog portion of the
device. T he system analog common should be located as close
as possible to Pin 12 to minimize any voltage drop which may
develop between these two points, although the internal circuit
is designed to minimize signal dependence of the analog return
current.
T he AD1868 has two noise reduction pins designated as NRL
(Pin 13) and NRR (Pin 11). It is recommended that external
noise reduction capacitors be connected from these pins to
AGND to reduce the output noise contributed by the voltage
reference circuitry. As shown in Figure 7, each of these pins
should be bypassed to AGND with a 4.7 µF or larger capacitor.
T he connections between the capacitors, package pins and
AGND should be as short as possible to achieve the lowest
noise.
T he digital ground, DGND, returns ground current from the
digital logic portion of the device. T his pin should be connected
to the digital common node in the system. As shown in Figure
7, the analog and digital grounds should be joined at one point
in the system. When these two grounds are remotely connected
such as at the power supply ground, care should be taken to
minimize the voltage difference between the DGND and AGND
pins in order to ensure the specified performance.
USING VBL AND VBR
T he AD1868 has two bias voltage reference pins, designated as
VBR (Pin 8) and VBL (Pin 16). T hese pins supply a dc reference
voltage equal to the center of the output voltage swing. T hese
bias voltages replace “False Ground” networks previously required
in single-supply audio systems. At the same time, they allow dc-
coupled systems, improving audio performance.
Figure 8a illustrates the traditional approach used to generate
False Ground voltages in single-supply audio systems. T his cir-
cuit requires additional power and circuit board space.
P O WER SUP P LIES AND D ECO UP LING
T he AD1868 has three power supply input pins. VS (Pins 9 and
15) provides the supply voltages which operate the analog por-
tion of the device including the 18-bit DACs, the voltage refer-
ences, and the output amplifiers. T he VS supplies are designed
to operate with a +5 V supply. T hese pins should be decoupled
to analog common using a 0.1 µF capacitor. Good engineering
practice suggests that the bypass capacitors be placed as close as
possible to the package pins. T his minimizes the inherent induc-
tive effects of printed circuit board traces.
–V
+V
S
1
2
3
16
16-BIT
LATCH
S
16-BIT
DAC
DGND
15 TRIM
MSB
+V
L
SERIAL
INPUT
14
ADJ
REGISTER
I
I
NC
CLK
LE
4
5
6
7
13
12
OUT
OUT
VL (Pin 1) operates the digital portions of the chip including the
input shift registers and the input latching circuitry. VL is also
designed to operate with a +5 V supply. T his pin should be by-
passed to digital common using a 0.1 µF capacitor, again placed
as close as possible to the package pin. Figure 7 illustrates the cor-
rect connection of the digital and analog supply bypass capacitors.
AGND
CONTROL
LOGIC
11 SJ
R
DATA
10
9
F
V
NC
8
OUT
An important feature of the AD1868 audio DAC is its ability to
operate at reduced power supply voltages. T his feature is very
important in portable battery operated systems. As the batteries
discharge, the supply voltage drops. Unlike any other audio
AD1851
NC = NO CONNECT
Figure 8a. Schem atic Using False Ground
REV. A
–5–
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