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 0.1

µF ceramic bypass capacitor should

be placed as close to the ADS8321 package as possible. In

addition, a 1

µF 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 0.1

µF

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 VCC, as the reference input is

connected directly to the power supply. The 5

Ω resistor and

1

µF 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

+

+

5

Ω

1

µF to

10

µF

1

µF to 10µF

0.1

µF

0V to 5V

+2.5V

Reference

Microcontroller

5V