AD876
Table I. Output Data Format
Approx.
AIN (V)
>4
4
3
2
<2
X
THREE- DATA
STATE D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
0
0
0
0
0
1
1
1
1
0
0
Z
1
1
0
0
0
Z
1
1
0
0
0
Z
1
1
0
0
0
Z
1
1
0
0
0
Z
1
1
0
0
0
Z
1
1
0
0
0
Z
1
1
0
0
0
Z
1
1
0
0
0
Z
1
1
0
0
0
Z
For DRV
DD
= 5 V, the AD876 output signal swing is compat-
ible with both high speed CMOS and TTL logic families. For
TTL, the AD876 on-chip, output drivers were designed to
support several of the high speed TTL families (F, AS, S). For
applications where the clock rate is below 20 MSPS, other TTL
families may be appropriate. For interfacing with lower voltage
CMOS logic, the AD876 sustains 20 MSPS operation with
DRV
DD
= 3.3 V. In all cases, check your logic family data
sheets for compatibility with the AD876 Digital Specification
table.
THREE-STATE OUTPUTS
A low power mode feature is provided such that for STBY =
HIGH and the clock disabled, the static power of the AD876
will drop below 50 mW.
GROUNDING AND LAYOUT RULES
As is the case for any high performance device, proper ground-
ing and layout techniques are essential in achieving optimal
performance. The analog and digital grounds on the AD876
have been separated to optimize the management of return
currents in a system. It is recommended that a printed circuit
board (PCB) of at least 4 layers employing a ground plane and
power planes be used with the AD876. The use of ground and
power planes offers distinct advantages:
1. The minimization of the loop area encompassed by a signal
and its return path.
2. The minimization of the impedance associated with ground
and power paths.
3. The inherent distributed capacitor formed by the power
plane, PCB insulation, and ground plane.
These characteristics result in both a reduction of electro-
magnetic interference (EMI) and an overall improvement in
performance.
It is important to design a layout which prevents noise from
coupling onto the input signal. Digital signals should not be run
in parallel with the input signal traces and should be routed
away from the input circuitry. Separate analog and digital
grounds should be joined together directly under the AD876. A
solid ground plane under the AD876 is also acceptable if the
power and ground return currents are managed carefully. A
general rule of thumb for mixed signal layouts dictates that the
return currents from digital circuitry should not pass through
critical analog circuitry. For further layout suggestions, see the
AD876 Evaluation Board data sheet.
DIGITAL OUTPUTS
The digital outputs of the AD876 can be placed in a high im-
pedance state by setting the THREE-STATE pin to HIGH.
This feature is provided to facilitate in-circuit testing or
evaluation. Note that this function is not intended for enabling/
disabling the ADC outputs from a bus at 20 MSPS. Also, to
avoid corruption of the sampled analog signal during conversion
(3.5 clock cycles), it is highly recommended that the AD876
outputs be enabled on the bus prior to the first sampling. For
the purpose of budgetary timing, the maximum access and float
delay times (t
DD
, t
HL
shown in Figure 15) for the AD876 are
150 ns.
THREE-STATE
t
DD
D0–D9
ACTIVE
t
HL
HIGH IMPEDANCE
Figure 22. High-Impedance Output Timing Diagram
Each of the on-chip buffers for the AD876 output bits (D0–D9)
is powered from the DRV
DD
supply pins, separate from AV
DD
or
DV
DD
. The output drivers are sized to handle a variety of logic
families while minimizing the amount of glitch energy gener-
ated. In all cases, a fan-out of one is recommended to keep the
capacitive load on the output data bits below the specified 20 pF
level.
REV. B
–11–
AD [ ANALOG DEVICES ]
