AD7791 datasheet(19/20 Pages) AD

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AD7791

Rev. 0 | Page 19 of 20

Reference voltage sources like those recommended above

(e.g., ADR391) will typically have low output impedances and

are, therefore, tolerant to having decoupling capacitors on

REFIN(+) without introducing gain errors in the system.

Deriving the reference input voltage across an external resistor

will mean that the reference input sees a significant external

source impedance. External decoupling on the REFIN pins

would not be recommended in this type of circuit

configuration.

VDD MONITOR

Along with converting external voltages, the analog input chan-

nel can be used to monitor the voltage on the VDD pin. When the

CH1 and CH0 bits in the communications register are set to 1,

the voltage on the VDD pin is internally attenuated by 5 and the

resultant voltage is applied to the ∑-∆ modulator using an inter-

nal 1.17 V reference for analog to digital conversion. This is

useful because variations in the power supply voltage can be

monitored.

GROUNDING AND LAYOUT

Since the analog inputs and reference inputs of the ADC are

differential, most of the voltages in the analog modulator are

common-mode voltages. The excellent common-mode rejec-

tion of the part will remove common-mode noise on these

inputs. The digital filter will provide rejection of broadband

noise on the power supply, except at integer multiples of the

modulator sampling frequency. The digital filter also removes

noise from the analog and reference inputs, provided that these

noise sources do not saturate the analog modulator. As a result,

the AD7791 is more immune to noise interference than a con-

ventional high resolution converter. However, because the

resolution of the AD7791 is so high, and the noise levels from

the AD7791 are so low, care must be taken with regard to

grounding and layout.

The printed circuit board that houses the AD7791 should be

designed such that the analog and digital sections are separated

and confined to certain areas of the board. A minimum etch

technique is generally best for ground planes because it gives

the best shielding.

It is recommended that the AD7791’s GND pin be tied to the

AGND plane of the system. In any layout, it is important that

the user keep in mind the flow of currents in the system, ensur-

ing that the return paths for all currents are as close as possible

to the paths the currents took to reach their destinations. Avoid

forcing digital currents to flow through the AGND sections of

the layout.

The AD7791’s ground plane should be allowed to run under the

AD7791 to prevent noise coupling. The power supply lines to

the AD7791 should use as wide a trace as possible to provide

low impedance paths and reduce the effects of glitches on the

power supply line. Fast switching signals such as clocks should

be shielded with digital ground to avoid radiating noise to other

sections of the board, and clock signals should never be run

near the analog inputs. Avoid crossover of digital and analog

signals. Traces on opposite sides of the board should run at

right angles to each other. This will reduce the effects of

feedthrough through the board. A microstrip technique is by far

the best, but it is not always possible with a double-sided board.

In this technique, the component side of the board is dedicated

to ground planes, while signals are placed on the solder side.

Good decoupling is important when using high resolution

ADCs. VDD should be decoupled with 10 µF tantalum in parallel

with 0.1 µF capacitors to GND. To achieve the best from these

decoupling components, they should be placed as close as

possible to the device, ideally right up against the device. All

logic chips should be decoupled with 0.1 µF ceramic capacitors

to DGND.

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