AD7851ARS datasheet(32/36 Pages) AD

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AD7851

–32–

REV. A

Evaluating the AD7851 Performance

The recommended layout for the AD7851 is outlined in the

evaluation board for the AD7851. The evaluation board pack-

age includes a fully assembled and tested evaluation board,

documentation, and software for controlling the board from the

PC via the EVAL-CONTROL BOARD. The EVAL-CON-

TROL BOARD can be used in conjunction with the AD7851

Evaluation board, as well as many other Analog Devices evalua-

tion boards ending in the CB designator, to demonstrate/evalu-

ate the ac and dc performance of the AD7851.

The software allows the user to perform ac (fast Fourier trans-

form) and dc (histogram of codes) tests on the AD7851. It also

gives full access to all the AD7851 on-chip registers allowing for

various calibration and power-down options to be programmed.

AD785x Family

All parts are 12 bits, 200 kSPS, 3.0 V to 5.5 V.

AD7853 – Single Channel Serial

AD7854 – Single Channel Parallel

AD7858 – Eight Channel Serial

AD7859 – Eight Channel Parallel

APPLICATION HINTS

Grounding and Layout

The analog and digital supplies to the AD7851 are independent

and separately pinned out to minimize coupling between the

analog and digital sections of the device. The part has very good

immunity to noise on the power supplies as can be seen by the

PSRR versus Frequency graph. However, care should still be

taken with regard to grounding and layout.

The printed circuit board that houses the AD7851 should be de-

signed such that the analog and digital sections are separated

and confined to certain areas of the board. This facilitates the

use of ground planes that can be separated easily. A minimum

etch technique is generally best for ground planes as it gives the

best shielding. Digital and analog ground planes should only be

joined in one place. If the AD7851 is the only device requiring

an AGND to DGND connection, then the ground planes should

be connected at the AGND and DGND pins of the AD7851. If

the AD7851 is in a system where multiple devices require

AGND to DGND connections, the connection should still be

made at one point only, a star ground point which should be

established as close as possible to the AD7851.

Avoid running digital lines under the device as these will couple

noise onto the die. The analog ground plane should be allowed

to run under the AD7851 to avoid noise coupling. The power

supply lines to the AD7851 should use as large a trace as pos-

sible to provide low impedance paths and reduce the effects of

glitches on the power supply line. Fast switching signals like

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 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 also important. All analog supplies should

be decoupled with 10

µF tantalum in parallel with 0.1 µF ca-

pacitors to AGND. All digital supplies should have a 0.1

µF

disc ceramic capacitor to AGND. To achieve the best from these

decoupling components, they must be placed as close as possible

to the device, ideally right up against the device. In systems

where a common supply voltage is used to drive both the AVDD

and DVDD of the AD7851, it is recommended that the system’s

AVDD supply is used. In this case there should be a 10

Ω resistor

between the AVDD pin and DVDD pin. This supply should have

the recommended analog supply decoupling capacitors between

the AVDD pin of the AD7851 and AGND and the recommended

digital supply decoupling capacitor between the DVDD pin of the

AD7851 and DGND.

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