AD7730LBR datasheet(34/53 Pages) AD

34 / 53 page

AD7730/AD7730L

–34–

POWER SUPPLIES

There is no specific power sequence required for the AD7730,

either the AVDD or the DVDD supply can come up first. While

the latch-up performance of the AD7730 is very good, it is

important that power is applied to the AD7730 before signals at

REF IN, AIN or the logic input pins in order to avoid latch-up

caused by excessive current. If this is not possible, the current

that flows in any of these pins should be limited to less than 30

mA per pin and less than 100 mA cumulative. If separate sup-

plies are used for the AD7730 and the system digital circuitry,

the AD7730 should be powered up first. If it is not possible to

guarantee this, current limiting resistors should be placed in

series with the logic inputs to again limit the current to less than

30 mA per pin and less than 100 mA total.

Grounding and Layout

Since the analog inputs and reference input are differential,

most of the voltages in the analog modulator are common-mode

voltages. The excellent common-mode rejection of the part will

remove common-mode noise on these inputs. The analog and

digital supplies to the AD7730 are independent and separately

pinned out to minimize coupling between the analog and digital

sections of the device. The digital filter will provide rejection of

broadband noise on the power supplies, except at integer mul-

tiples of the modulator sampling frequency or multiples of the

chop frequency in chop mode. The digital filter also removes

noise from the analog and reference inputs provided those noise

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

AD7730 is more immune to noise interference than a conven-

tional high resolution converter. However, because the resolu-

tion of the AD7730 is so high and the noise levels from the

AD7730 so low, care must be taken with regard to grounding

and layout.

The printed circuit board that houses the AD7730 should be

designed so 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 easily separated. 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 AD7730 is the only device requiring

an AGND to DGND connection, the ground planes should

be connected at the AGND and DGND pins of the AD7730. If

the AD7730 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 that should be established as

closely as possible to the AD7730.

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 AD7730 to avoid noise coupling. The power

supply lines to the AD7730 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 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 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. All analog supplies should be decoupled with 10

μF

tantalum in parallel with 0.1

μF ceramic capacitors to AGND.

To achieve the best from these decoupling components, they

have to 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 disc ceramic capacitors to DGND. In systems where a

common supply voltage is used to drive both the AVDD and

DVDD of the AD7730, it is recommended that the system’s

AVDD supply is used. This supply should have the recom-

mended analog supply decoupling capacitors between the AVDD

pin of the AD7730 and AGND and the recommended digital

supply decoupling capacitor between the DVDD pin of the

AD7730 and DGND.

Evaluating the AD7730 Performance

A recommended layout for the AD7730 is outlined in the evalu-

ation board for the AD7730. The evaluation board package

includes a fully assembled and tested evaluation board, docu-

mentation, software for controlling the board over the printer

port of a PC and software for analyzing the AD7730’s perfor-

mance on the PC. The evaluation board order number is

EVAL-AD7730EB.

Noise levels in the signals applied to the AD7730 may also

affect performance of the part. The AD7730 allows two tech-

niques for evaluating the true performance of the part, indepen-

dent of the analog input signal. These schemes should be used

after a calibration has been performed on the part.

The first method is to select the AIN1(–)/AIN1(–) input chan-

nel arrangement. In this case, the differential inputs to the

AD7730 are internally shorted together to provide a zero differ-

ential voltage for the analog modulator. External to the device,

the AIN1(–) input should be connected to a voltage which is

within the allowable common-mode range of the part.

The second scheme is to evaluate the part with a voltage near

input full scale. This can be achieved by again using input pair

AIN1(–), but by adding a differential voltage via the TARE

DAC. This allows the user to evaluate noise performance with a

near full-scale voltage.

The software in the evaluation board package allows the user to

look at the noise performance in terms of counts, bits and nV.

Once the user has established that the noise performance of the

part is satisfactory in this mode, an external input voltage can

then be applied to the device incorporating more of the signal

chain.

REV. B

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