AD1861 Analog Devices, AD1861 Datasheet - Page 6

• Manufacturer Part Number: AD1861
• Description: 16-Bit/18-Bit/ 16 X Fs PCM Audio DACs
• Manufacturer: Analog Devices

AD1851/AD1861
Analog Circuit Considerations
GROUNDING RECOMMENDATIONS
The AD1851/AD1861 has two ground pins, designated Analog
and Digital ground. The analog ground pin is the “high quality”
ground reference point for the device. The analog ground pin
should be connected to the analog common point in the system.
The output load should also be connected to that same point.
The digital ground pin returns ground current from the digital
logic portions of the AD1851/AD1861 circuitry. This pin
should be connected to the digital common point in the system.
As illustrated in Figure 2, the analog and digital grounds should
be connected together at one point in the system.
POWER SUPPLIES AND DECOUPLING
The AD1851/AD1861 has three power supply input pins. The
portions of the DAC including the voltage reference, output am-
plifier and control amplifier. The V
operate at 5 V.
The +V
ing the input shift register and the input latching circuitry. The
+V
Decoupling capacitors should be used on all power supply pins.
Furthermore, good engineering practice suggests that these ca-
pacitors be placed as close as possible to the package pins as
well as to the common points. The logic supply, +V
decoupled to digital common, while the analog supplies, V
should be decoupled to analog common.
The use of three separate power supplies will reduce feedthrough
from the digital portion of the system to the linear portion of the
system, thus contributing to improved performance.
V
S
L
supplies provide the supply voltages to operate the linear
supply is designed to operate at +5 V.
L
Figure 2. Recommended Circuit Schematic
supply operates the digital portions of the chip includ-
GROUND
DIGITAL
DGND
2
+
+V
AD1851/AD1861
5V
3
L
–
–V
5V
1
S
+5V
+V
16
AGND
S
S
12
supplies are designed to
ANALOG
GROUND
L
, should be
S
,
–6–
However, three separate voltage supplies are not necessary for
good circuit performance. For example, Figure 3 illustrates a
system where only a single positive and a single negative supply
are available.
In this example, the positive logic and positive analog supplies
must both be connected to +5 V, while the negative analog sup-
ply will be connected to –5 V. Performance would benefit from
a measure of isolation between the supplies introduced by using
simple low pass filters in the individual power supply leads.
As with most linear circuits, changes in the power supplies will
affect the output of the DAC. Analog Devices recommends that
well regulated power supplies with less than 1% ripple be incor-
porated into the design of any system using the AD1851/AD1861.
OPTIONAL MSB ADJUSTMENT
Use of an optional adjustment circuit allows residual differential
linearity error around midscale to be eliminated. This error is
especially important when low amplitude signals are being re-
produced. In those cases, as the signal amplitude decreases, the
ratio of the midscale differential linearity error to the signal am-
plitude increases, thereby increasing THD.
Therefore, for best performance at low output levels, the op-
tional MSB adjust circuitry shown in Figure 4 may be used to
improve performance. The adjustment should be made with a
small signal input (–20 dB or –60 dB).
Figure 3. Alternate Recommended Schematic
Figure 4. Optional THD Adjust Circuit
ADJUST
TRIM
MSB
GROUND
15
14
DIGITAL
DGND
470k
2
AD1851/AD1861
+5V
+V
3
L
–5V
–V
1
100k
S
+5V
+V
16
AGND
12
S
200k
ANALOG
GROUND
–V
1
S
REV. A