DAC16_15 Datasheet, PDF (7/12 Pages) Analog Devices – 16-Bit High Speed Current-Output DAC

DAC16
system, such as the DAC16, it is recommended that common
ground tie points be provided at each such device. If only one
+5V
10␮F
system ground can be connected directly to the DAC16, it is
recommended that the analog common be used. If the system’s
AGND has suitable low impedance, then the digital signal cur-
FB
0.1␮F
rents flowing in it should not seriously affect the ground noise.
The amount of digital noise introduced by connecting the two
grounds together at the device will not adversely affect system
performance due to loss of digital noise immunity.
Generous bypassing of the DAC’s supplies goes a long way in
V_CC
DAC16
V_EE
AGND
0.1␮F
reducing supply-line induced errors. Even with well-filtered,
well-regulated supplies, local bypassing consisting of 10 µF tan-
FB
10␮F
talum electrolytic shunted by a 0.1 µF ceramic is recommended.
The decoupling capacitors should be connected between the
DAC’s supply pins (Pin 3 for +5 V, Pin 20 for –15 V) and
the analog ground (Pin 22). Figure 19 shows how the DGND,
AGND, and bypass connections should be made to the DAC16.
+5V
DB0 – DB15
TE ^10␮F
0.1␮F
V_CC
AGND
DAC16
DGND
V_EE
0.1␮F
10␮F
I_OUT
E _{TO OTHER TO}
ANALOG POWER
CIRCUITS GROUND
–15V
L _{Figure 19. Recommended Grounding and Bypassing}
Scheme for the DAC16
Using the Right Capacitors
Probably the most important external components associated
O _{with high speed design are the capacitors used to bypass the}
power supplies and to provide compensation. Both selection
and placement of these capacitors can be critical and, to a large
extent, dependent upon the specifics of the system configura-
S _{tion. The dominant consideration in selection of bypass and}
compensation capacitors for the DAC16 is minimization of se-
ries resistance and inductance. Many capacitors begin to look
inductive at 20 MHz and above—the very frequencies where re-
B jection of interference is needed. Ceramic and film-type capaci-
tors generally feature lower series inductance than tantalum or
electrolytic types. A few general rules are of universal use when
approaching the issue of compensation or bypassing.
O ^{Bypass capacitors should be installed on the printed circuit}
–15V
Figure 20. Using a Ferrite Bead as a High Frequency Filter
Reference Amplifier Considerations
The reference input current buffer is a high performance ampli-
fier optimized for high accuracy and linearity. The design of the
reference amplifier ensures fast settling times by tightly control-
ling the node common to all the current sources internal to the
DAC with an external compensation capacitor (C_COMP). Since
the primary design goal of the DAC16 is to achieve 16-bit per-
formance, proper operation of the reference amplifier requires a
47 µF tantalum electrolytic capacitor shunted by a 0.1 µF
ceramic capacitor, as shown in Figure 21. Increasing the capaci-
tance at this node above the recommended values does not fur-
ther reduce the analog transition current noise spikes at the
output of the reference amplifier. Reducing the value of com-
pensation, however, is not recommended as DAC linearity will
degrade as a result. In most systems, the V_EE supply offers suffi-
ciently low impedance to maintain a quiet return point for the
reference amplifier. If this is not the case, the AGND point can
also be used for the compensation capacitor return, as shown in
Figure 21.
47␮F
0.1␮F
C_COMP
DAC16
V_EE
AGND
I_OUT
–15V
Figure 21a. Recommended Compensation Scheme to V_EE
0.1␮F
C_COMP
DAC16
AGND
I_OUT
47␮F
board with the shortest possible leads consistent with reliable
construction. This helps to minimize series inductance in the
leads. Chip capacitors are optimal in this respect. Where illus-
trated in the applications section, large tantalum electrolytic
capacitors are shunted by low self-inductance ceramic capaci-
tors. This technique reduces the self-resonance of the electrolytic
while shifting the resonant frequency of the ceramics out-of-band.
Some series inductance between the DAC supply pins and the
power supply plane often helps to filter out high frequency
power supply noise. This inductance can be generated using a
small ferrite bead as shown in Figure 20.
Figure 21b. Recommended Compensation Scheme to AGND
In applications where 16-bit multiplying performance is
required, the DAC16 might appear to be a viable solution.
However, the compensation capacitor network would have to
be removed in these applications. The DAC16’s reference am-
plifier was specifically designed for low frequency operation,
with a compensation capacitor network. In fact, this network
serves not only as a charge reservoir for the DAC’s internal
current sources but also as a wideband noise filter for the
REV. B
–7–