ADS8320E Burr-Brown Corporation, ADS8320E Datasheet - Page 13

• Manufacturer Part Number: ADS8320E
• Description: 16-Bit/ High-Speed/ 2.7V to 5V microPower Sampling ANALOG-TO-DIGITAL CONVERTER
• Manufacturer: Burr-Brown Corporation

The basic SAR architecture is sensitive to spikes on the
power supply, reference, and ground connections that occur
just prior to latching the comparator output. Thus, during
any single conversion for an n-bit SAR converter, there are
n “windows” in which large external transient voltages can
easily affect the conversion result. Such spikes might origi-
nate from switching power supplies, digital logic, and high
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 ADS8320 should be clean
and well bypassed. A 0.1 F ceramic bypass capacitor should
be placed as close to the ADS8320 package as possible. In
addition, a 1 to 10 F capacitor and a 5
resistor may be used to lowpass filter a noisy supply.
The reference should be similarly bypassed with a 0.1 F
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 ADS8320 draws very little current from the reference on
average, there are still instantaneous current demands placed
on the external input and reference circuitry.
Burr-Brown’s OPA627 op amp provides optimum perfor-
mance 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.
FIGURE 8. Basic Data Acquisition System.
0.1 F
V
+In
–In
GND
REF
ADS8320
or 10
DCLOCK
D
V
OUT
CS
CC
series
+
13
1 F to
10 F
Also, keep in mind that the ADS8320 offers no inherent
rejection of noise or voltage variation in regards to the
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 ADS8320 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 8 shows a basic data acquisition system. The ADS8320
input range is 0V to V
directly to the power supply. The 5
10 F 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.
5
+2.7V to +5.25V
Microcontroller
+
1 F to
10 F
CC
, as the reference input is connected
ADS8320
resistor and 1 F to
®