ADS8320 datasheet(13/21 Pages) TI

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ADS8320

SBAS108D

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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 DCLOCK signal—as the phase

difference between the two changes with time and tempera-

ture, causing sporadic misoperation.

With this in mind, power to the ADS8320 should be clean

and well bypassed. A 0.1

µF ceramic bypass capacitor should

be placed as close to the ADS8320 package as possible. In

addition, a 1

µF to 10µF capacitor and a 5Ω or 10Ω series

resistor may be used to low-pass filter a noisy supply.

The reference should be similarly bypassed with a 0.1

µF

capacitor. Again, a series resistor and large capacitor can be

used to low-pass 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.

Texas Instruments' OPA627 op amp provides optimum

performance for buffering both the signal and reference

inputs. For low-cost, low-voltage, single-supply applica-

tions, the OPA2350 or OPA2340 dual op amps are recom-

mended.

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 includes 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 VCC, as the reference input is connected

directly to the power supply. The 5

Ω resistor and 1µF to

µF 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.

FIGURE 8. Basic Data Acquisition System.

ADS8320

OUT

DCLOCK

REF

+In

–In

GND

Ω

µF to

µF

µF to

µF

0.1

µF

Microcontroller

+2.7V to +5.25V

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