APPLICATION INFORMATION

LAYOUT

21

Ω

0.1

µF

1

µF

0.1

µF

0.1

µF

1

µF

Analog Input

Circuit

21

Ω

+VA

AGND

REFOUT

REFIN

REFM

+IN

−IN

ADS7882

Analog 5 V

AGND

External Reference from

REF5025 or REF3225

AGND

ADS7882

www.ti.com ........................................................................................................................................................................................... SLAS630 – DECEMBER 2008

For optimum performance, care should be taken with the physical layout of the ADS7882 circuitry.

As the ADS7882 offers single-supply operation, it is often used in close proximity with digital logic,

micro-controllers, microprocessors, and digital signal processors. The more digital logic present in the design and

the higher the switching speed, the more difficult it is to achieve acceptable performance from the converter.

The basic SAR architecture is sensitive to glitches or sudden changes on the power supply, reference, ground

connections, and digital inputs that occur just prior to the end of sampling (within quiet sampling time) and just

prior to latching the output of the analog comparator during the conversion phase. Thus, driving any single

conversion for an n-bit SAR converter, there are n+1 windows in which large external transient voltages can

affect the conversion result. Such glitches might originate from switching power supplies, nearby digital logic, or

high power devices.

The degree of error in the digital output depends on the reference voltage, layout, and the exact timing of the

external event.

On average, the ADS7882 draws very little current from an external reference as the reference voltage is

internally buffered. If the reference voltage is external and originates from an op amp, make sure that it can drive

the bypass capacitor or capacitors without oscillation. A 0.1-

µF bypass capacitor and 1-µF storage capacitor are

recommended from REFIN (pin 1) directly to REFM (pin 48).

The AGND and BDGND pins should be connected to a clean ground point. In all cases, this should be the

analog ground. Avoid connections which are too close to the grounding point of a micro-controller or digital signal

processor. If required, run a ground trace directly from the converter to the power supply entry point. The ideal

layout consists of an analog ground plane dedicated to the converter and associated analog circuitry.

As with the AGND connections, +VA should be connected to a 5-V power supply plane that is separate from the

connection for +VBD and digital logic until they are connected at the power entry point onto the PCB. Power to

the ADS7882 should be clean and well bypassed. A 0.1-

µF ceramic bypass capacitor should be placed as close

to the device as possible. See Table 3 for the placement of capacitor. In addition to a 0.1-

µF capacitor, a 1-µF

capacitor is recommended. In some situations, additional bypassing may be required, such as a 100-

µF

electrolytic capacitor or even a Pi filter made up of inductors and capacitors, all designed to essentially low-pass

filter the 5-V supply, removing the high frequency noise.

Table 3. Power Supply Decoupling Capacitor Placement

POWER SUPPLY PLANE

CONVERTER

CONVERTER ANALOG SIDE

DIGITAL SIDE

SUPPLY PINS

Pairs of pins that require a shortest path to decoupling

(4,5), (9,8), (10,11), (13, 15), (43, 44) (46, 45)

(24, 25), (34, 35)

capacitors

Pins that require no decoupling

14, 12

Figure 33. Using External Reference

Copyright © 2008, Texas Instruments Incorporated

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