AD7880
–12–
REV. 0
as a function of time for such operation. The total conversion
time for each cycle is 11 × 15 µs (where 15 µs is the time taken
for a single conversion) corresponding to 1.65 × 10
–4
secs.
Hence:
Average Power = Power
CONVERTING
+ Power
POWER-DOWN
= {20 mW × (1.65 × 10
–4
)/(10)}
+ {2 mW × (9.9998)/(10)}
= 2.029 mW
AD7880 DATA ACQUISITION LAYOUT
Figure 24 shows the AD7880 in a data acquisition circuit. The
corresponding printed circuit board (PCB) layout and
silkscreen are shown in Figures 25 to 27.
The only additional component required for a full data acquisi-
tion system is an antialiasing filter. There is a component grid
provided near the analog input on the PCB which may be used
for such a filter or any other input conditioning circuitry. To fa-
cilitate this option there is a shorting link (labeled LK1 on the
PCB) on the analog input track. With LK1 in place, the analog
input connects to the buffer amplifier driving the AD7880.
With LK1 removed, a wire link is needed to connect the analog
input to the PCB component grid.
INTERFACE CONNECTIONS
The data acquisition board contains a parallel connection port
labeled SKT4. This is a 26-contact IDC Connector and pro-
vides for direct microprocessor connection to the board. This
connector, the pinout of which is shown in Figure 23, contains
all data, control and status signals of the AD7880 (with the ex-
ception of the
CONVST and the CLKIN inputs both of which
are provided via SKT2 and SKT3 respectively). It also contains
decoded R/
W and STRB inputs which are necessary for inter-
facing to many microprocessors including the TMS320C25 and
the Motorola 68000 series. Link LK7 selects
RD directly or al-
ternatively, the decoded version. Note that the AD7880
CS in-
put must be decoded prior to the AD7880 evaluation board.
SKT1, SKT2 and SKT3 are three sub-miniature connectors
(SMC) which provide input connections for the analog input,
the
CONVST input and the CLKIN input. Three different in-
put ranges can be accepted by the AD7880 each of which is
configured by selecting shorting plug options A, B or C of LK4.
Position A corresponds to the 0 V to 5 V unipolar configuration
of Figure 5, position B corresponds to the bipolar ±5 V configu-
ration of Figure 7 and position C allows for a 0 V to +10 V uni-
polar range as shown in Figure 6.
POWER SUPPLY CONNECTIONS
The PCB requires a single +5 V power supply (labeled V
DD
).
Good decoupling allows this supply to drive the AD7880 V
DD
which also drives the V
REF
input as well as the op amp power
supply. In circumstances where bipolar ±5 V or a unipolar 0 V
to 10 V input ranges are required, provision has been allowed
for the connection of separate op amp power supplies (±15 V,
±12 V, ±5 V, etc.) to V+ and V–. LK2 and LK3 shorting links
allow for the selection of user defined op amp power supplies or
the on-board single +5 V supply.
LINK OPTIONS
There are seven link options which must be set before using the
board. These are outlined below:
LK1 Connects the analog input to a buffer amplifier. The
analog input may also be connected to a component
grid for signal conditioning.
LK2, LK3 Allows for various op amp power supplies to be
used to drive the input buffer of the AD7880. Ex-
ternal supplies may be connected to V+ and V–.
Alternatively, the AD7880’s +5 V system supply
and AGND can be selected to drive a single supply
op amp.
LK4 Configures the various analog input ranges, 0 V to
5 V, 0 V to 10 V or ±5 V.
LK5 Selects reference input to V
REF
of AD7880. Nor-
mally connected to V
DD
. An external reference
could also be wired in.
LK6 Selects power-down or sleep mode. The shorting
plug is connected to V
DD
for normal operation.
LK7 Connects the AD7880
RD input directly to the RD
input of SKT4 or to a decoded
STRB and R/W
input. This shorting plug setting depends on the
microprocessor, e.g., the TMS320C25 requires a
decoded
RD signal.
DB10
DB8
DB6
DB4
DB2
DB0
GND
R/W
RD
CS
1
3
5
7
9
13
15
17
19
25
23
21
11
N/C
2
4
6
8
10
14
16
18
20
26
24
22
12 DB11
DB9
DB7
DB5
DB3
DB1
GND
STRB
BUSY
N/C
N/C
N/C
5V
+
5V
+
BUSY
Figure 23. SKT4, IDC Connector Pinout
COMPONENT LIST
IC1 Op Amp*
IC2 AD7880 Analog-to-Digital Converter
IC3 74HC00 Quad NAND Gate
C1, C3, C5 10 µF Capacitors
C2, C4, C6, C7 0.1 µF Capacitors
R1, R2 10 kΩ Pull-up Resistors
LK1, LK2, LK3 Shorting Links
LK4, LK5, LK6
LK7
SKT1, SKT2, SKT3 Sub-Miniature Connectors
Vendor No: Sealectro 50-051-0000 (Socket)
Vendor No: Sealectro 50-007-0000 (Plug)
SKT4 26-Contact (2 Row) IDC Connector
NOTE
*See ANALOG INPUT BUFFERING section.
.
