AD677
in Figure 3. In this circuit BUSY is used to reset the circuitry
which divides the system clock down to provide the AD677
CLK. This serves to interrupt the clock until after the input sig-
nal has been acquired, which has occurred when BUSY goes
HIGH. When the conversion is completed and BUSY goes
LOW, the circuit in Figure 3 truncates the 17th CLK pulse
width which is tolerable because only its rising edge is critical.
11 3Q
4 1D
12.288MHz
SYSTEM
CLOCK
9 CLK
2Q 7
3D 12
CLR 1
BUSY
Table I. Serial Output Coding Format (Twos Complement)
V
IN
<Full Scale
Full Scale
Full Scale – 1 LSB
Midscale + 1 LSB
Midscale
Midscle – 1 LSB
–Full Scale + 1 LSB
–Full Scale
<–Full Scale
Output Code
011 . . . 11
011 . . . 11
011 . . . 10
000 . . . 01
000 . . . 00
111 . . . 11
100 . . . 01
100 . . . 00
100 . . . 00
1Q 2
2D 5
CLK
POWER SUPPLIES AND DECOUPLING
AD677
74HC175
1 1CLK
13 2CLK
6 1QD
12 2CLR
2 1CLR
2QC 9
2QD 8
SAMPLE
The AD677 has three power supply input pins. V
CC
and V
EE
provide the supply voltages to operate the analog portions of the
AD677 including the capacitor DAC, input buffers and com-
parator. V
DD
provides the supply voltage which operates the
digital portions of the AD677 including the data output buffers
and the autocalibration controller.
As with most high performance linear circuits, changes in the
power supplies can produce undesired changes in the perfor-
mance of the circuit. Optimally, well regulated power supplies
with less than 1% ripple should be selected. The ac output im-
pedance of a power supply is a complex function of frequency,
and in general will increase with frequency. In other words, high
frequency switching such as that encountered with digital cir-
cuitry requires fast transient currents which most power supplies
cannot adequately provide. This results in voltage spikes on the
supplies. If these spikes exceed the
±
5% tolerance of the
±
12 V
supplies or the
±
10% limits of the +5 V supply, ADC perfor-
mance will degrade. Additionally, spikes at frequencies higher
than 100 kHz will also degrade performance. To compensate for
the finite ac output impedance of the supplies, it is necessary to
store “reserves” of charge in bypass capacitors. These capacitors
can effectively lower the ac impedance presented to the AD677
power inputs which in turn will significantly reduce the magni-
tude of the voltage spikes. For bypassing to be effective, certain
guidelines should be followed. Decoupling capacitors, typically
0.1
µF,
should be placed as closely as possible to each power
supply pin of the AD677. It is essential that these capacitors be
placed physically close to the IC to minimize the inductance of
the PCB trace between the capacitor and the supply pin. The
logic supply (V
DD
) should be decoupled to digital common and
the analog supplies (V
CC
and V
EE
) to analog common. The ref-
erence input is also considered as a power supply pin in this re-
gard and the same decoupling procedures apply. These points
are displayed in Figure 4.
V
DD
DGND
0.1µF
0.1µF
0.1µF
SYSTEM
DIGITAL
COMMON
SYSTEM
ANALOG
COMMON
0.1µF
AGND
74HC393
Figure 3.
Figure 3 also illustrates the use of a counter (74HC393) to de-
rive the AD677 SAMPLE command from the system clock
when a continuous convert mode is desirable. Pin 9 (2QC) pro-
vides a 96 kHz sample rate for the AD677 when used with a
12.288 MHz system clock. Alternately, Pin 8 (2QD) could be
used for a 48 kHz rate.
If a continuous clock is used, then the user must avoid CLK
edges at the instant of disconnecting V
IN
which occurs at the
falling edge of SAMPLE (see t
FCD
specification). The duty cycle
of CLK may vary, but both the HIGH (t
CH
) and LOW (t
CL
)
phases must conform to those shown in the timing specifica-
tions. The internal comparator makes its decisions on the rising
edge of CLK. To avoid a negative edge transition disturbing the
comparator’s settling, t
CL
should be at least half the value of
t
CLK
. It is not recommended that the SAMPLE pin change state
toward the end of a CLK cycle, in order to avoid transitions dis-
turbing the internal comparator’s settling.
During a conversion, internal dc error terms such as comparator
voltage offset are sampled, stored on internal capacitors and
used to correct for their corresponding errors when needed. Be-
cause these voltages are stored on capacitors, they are subject to
leakage decay and so require refreshing. For this reason there is
a maximum conversion time t
C
(1000
µs).
From the time
SAMPLE goes HIGH to the completion of the 17th CLK pulse,
no more than 1000
µs
should elapse for specified performance.
However, there is no restriction to the maximum time between
individual conversions.
Output coding for the AD677 is twos complement as shown in
Table I. The AD677 is designed to limit output coding in the
event of out-of-range input.
+5V
AD677
V
CC
V
EE
V
REF
+12V
–12V
Figure 4. Grounding and Decoupling the AD677
REV. A
–9–
AD [ ANALOG DEVICES ]
