Datasheet

19
®
ADS1250
LAYOUT
POWER SUPPLIES
The analog supply should be well regulated and low noise.
For designs requiring very high resolution from the ADS1250,
power supply rejection will be a concern. Avoid running
digital lines under the device as they may couple noise onto
the die. The requirements for the digital supply are not as
strict, however, high frequency noise on V
D
can capacitively
couple into the analog portion of the ADS1250. This noise
can originate from switching power supplies, very fast
microprocessors, or digital signal processors. For either
supply, high frequency noise will alias back into the pass-
band of the digital filter, affecting the conversion result. If
one supply must be used to power the ADS1250, the V
S
supply should be used to power V
D
. This connection can be
made via a 10Ω resistor which, along with the decoupling
capacitors, will provide some filtering between V
D
and V
S
.
In some systems, a direct connection can be made. Experi-
mentation may be the best way to determine the appropriate
connection between V
S
and V
D
.
GROUNDING
The analog and digital sections of the design should be
carefully and cleanly partitioned. Each section should have
its own ground plane with no overlap between them. AGND
+V
D
Digital
Filter
Serial
Output
ADS1250
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
+V
IN
–V
IN
+V
S
AGND
V
REF
DSYNC
10µF 0.1µF
DGND
G1
G0
CS
DRDY
CLK
SCLK
DOUT
∆Σ
Modulator
PGA
+
–
10µF 0.1µF
DGND
FIGURE 25. Connection Diagram.
TYPICAL CONNECTION
should be connected to the analog ground plane as well as all
other analog grounds. DGND should be connected to the
digital ground plane and all digital signals referenced to this
plane. The ADS1250 pinout is such that the converter is
cleanly separated into an analog and digital portion. This
should allow simple layout of the analog and digital sections
of the design. For a single converter system, AGND and
DGND of the ADS1250 should be connected together,
underneath the converter. Do not join the ground planes, but
connect the two with a moderate signal trace. For multiple
converters, connect the two ground planes at one location as
central to all of the converters as possible. In some cases,
experimentation may be required to find the best point to
connect the two planes together. The printed circuit board
can be designed to provide different analog/digital ground
connections via short jumpers. The initial prototype can be
used to establish which connection works best.
DECOUPLING
Good decoupling practices should be used for the ADS1250
and for all components in the design. All decoupling capaci-
tors, but specifically the 0.1µF ceramic capacitors, should be
placed as close as possible to the pin being decoupled. A
1µF to 10µF capacitor, in parallel with a 0.1µF ceramic
capacitor, should be used to decouple V
S
to AGND. At a
minimum, a 0.1µF ceramic capacitor should be used to
decouple V
D
to DGND, as well as for the digital supply on
each digital component.