Datasheet

ADS8321
12
SBAS123B
www.ti.com
power devices, to name a few. This particular source of error
can be very difficult to track down if the glitch is almost
synchronous to the converter’s DCLOCK signal—as the
phase difference between the two changes with time and
temperature, causing sporadic misoperation.
With this in mind, power to the ADS8321 should be clean
and well bypassed. A 0.1µF ceramic bypass capacitor should
be placed as close to the ADS8321 package as possible. In
addition, a 1µF to 10µF capacitor and a 5Ω or 10Ω series
resistor may be used to lowpass filter a noisy supply.
The reference should be similarly bypassed with a 0.1µF
capacitor. Again, a series resistor and large capacitor can be
used to lowpass filter the reference voltage. If the reference
voltage originates from an op amp, be careful that the op
amp can drive the bypass capacitor without oscillation (the
series resistor can help in this case). Keep in mind that while
the ADS8321 draws very little current from the reference on
average, there are still instantaneous current demands placed
on the external input and reference circuitry.
Texas Instruments OPA627 op amp provides optimum per-
formance for buffering both the signal and reference inputs.
For low cost, low voltage, single-supply applications, the
OPA2350 or OPA2340 dual op amps are recommended.
Also, keep in mind that the ADS8321 offers no inherent
rejection of noise or voltage variation in regards to the
FIGURE 11. Basic Data Acquisition System.
reference input. This is of particular concern when the
reference input is tied to the power supply. Any noise and
ripple from the supply will appear directly in the digital
results. While high frequency noise can be filtered out as
described in the previous paragraph, voltage variation due to
the line frequency (50Hz or 60Hz), can be difficult to
remove.
The GND pin on the ADS8321 should be placed on a clean
ground point. In many cases, this will be the “analog”
ground. Avoid connecting the GND pin too close to the
grounding point for a microprocessor, microcontroller, or
digital signal processor. If needed, run a ground trace di-
rectly from the converter to the power supply connection
point. The ideal layout will include an analog ground plane
for the converter and associated analog circuitry.
APPLICATION CIRCUITS
Figure 11 shows a basic data acquisition system. The
ADS8321 input range is 0V to V
CC
, as the reference input is
connected directly to the power supply. The 5Ω resistor and
1µF to 10µF capacitor filter the microcontroller “noise” on
the supply, as well as any high-frequency noise from the
supply itself. The exact values should be picked such that the
filter provides adequate rejection of the noise.
ADS8321
V
CC
CS
D
OUT
DCLOCK
V
REF
+In
–In
GND
+
+
5Ω
1µF to
10µF
1µF to 10µF
0.1µF
0V to 5V
+2.5V
Reference
Microcontroller
5V