AD7767 datasheet(21/24 Pages) AD

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AD7767

Rev. 0 | Page 21 of 24

ANTIALIASING

The AD7767/AD7767-1/AD7767-2 sample the analog input

at a maximum rate of 1.024 MHz. The on-board digital filter

provides up to 100 dB attenuation of any possible aliasing

frequencies in the range from the beginning of the filter stop

band (0.547 × ODR) to where the image of the digital filter pass

band occurs at MCLK − filter stop band (MCLK − 0.547 × ODR),

which is the first alias point. This is illustrated in Figure 42.

DIGITAL FILTER 100dB

ANTIALIAS PROTECTION

fMCLK

BAND OF INTEREST

fMCLK – (0.547 × ODR)

FIRST ALIAS POINT

DIGITAL FILTER

IMAGE AT

fMCLK

Figure 42. AD7767/AD7767-1/AD7767 Spectrum

Table 8 shows the attenuation achieved by various orders of

front-end antialias filters prior to the AD7767/AD7767-1/

AD7767-2 at the image of the digital filter stop band, which is

1.024 MHz − 0.547 × ODR.

Table 8. Antialias Filter Order Attenuation at First Alias Point

Model

Filter Order

Attenuation at

1.024 MHz – 0.547 × ODR

1st

27 dB

2nd

50 dB

AD7767

3rd

70 dB

1st

33 dB

2nd

62 dB

AD7767-1

3rd

89 dB

1st

38 dB

2nd

74 dB

AD7767-2

3rd

110 dB

The AD7764 and AD7765 sigma-delta devices are available to

customers that require extra antialias protection. These devices

sample internally at a rate of 20 MHz to achieve up to a maxi-

mum of 156 kHz or 312 kHz output data rate. This means that

the first alias point of these devices when run at the maximum

speeds are 19.921 MHz and 19.843 MHz, respectively.

POWER DISSIPATION

The AD7767/AD7767-1/AD7767-2 offer exceptional perform-

ance at ultralow power. Figure 43, Figure 44, and Figure 45

show how the current consumption of the AD7767/AD7767-1/

AD7767-2 scales with the MCLK frequency applied to the

device. Both the digital and analog currents scale as the MCLK

frequency is reduced. The actual throughput of each of the

AD7767/AD7767-1/AD7767-2 equals the MCLK frequency

applied divided by the decimation rate employed by the device

in use. For instance, operating the AD7767 device with an

MCLK of 800 kHz gives an output data rate of 100 kHz due to

the decimate by 8 filtering.

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

1000k

900k

800k

700k

600k

500k

400k

300k

200k

100k

FREQUENCY (Hz)

IREF

AIDD

DIDD

Figure 43. AD7767 Current vs. MCLK Frequency

2.5

2.0

1.5

1.0

0.5

1000k

900k

800k

700k

600k

500k

400k

300k

200k

100k

FREQUENCY (Hz)

IREF

AIDD

DIDD

Figure 44. AD7767-1 Current vs. MCLK Frequency

1.4

1.2

1.0

0.8

0.6

0.4

0.2

1000k

900k

800k

700k

600k

500k

400k

300k

200k

100k

FREQUENCY (Hz)

IREF

AIDD

DIDD

Figure 45. AD7767-2 Current vs. MCLK Frequency

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