Datasheet

AD7767
Rev. C | 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 for any possible aliasing
frequency 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. This occurs at MCLK minus the filter stop band
(MCLK − 0.547 × ODR), as shown in Figure 42.
06859-231
DIGITAL FILTER 100dB
ANTIALIAS PROTECTION
f
MCLK
BAND OF INTEREST
f
MCLK
– (0.547 × ODR)
FIRST IMAGE POINT
DIGITAL FILTER
IMAGE AT
f
MCLK
Figure 42. AD7767/AD7767-1/AD7767 Spectrum
Table 9 shows the attenuation achieved by various orders of
front-end antialias filters prior to the signal entering the AD7767/
AD7767-1/AD7767-2 at the image of the digital filter stop band,
which is 1.024 MHz − 0.547 × ODR.
Table 9. Antialias Filter Order,
Attenuation at at First Image Point
Model Filter Order
Attenuation at
1.024 MHz – 0.547 × ODR
AD7767 First 27 dB
Second 50 dB
Third 70 dB
AD7767-1 First 33 dB
Second 62 dB
Third 89 dB
AD7767-2 First 38 dB
Second 74 dB
Third 110 dB
The AD7764 and AD7765 ∑-∆ devices are available to
customers that require extra antialias protection. These devices
sample the signal internally at a rate of 20 MHz to achieve up to
a maximum 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 is 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 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 results in 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
0
0 1000k900k800k700k600k500k400k300k200k100k
CURRENT (mA)
FREQUENCY (Hz)
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I
REF
AI
DD
DI
DD
Figure 43. AD7767 Current vs. MCLK Frequency
2.5
2.0
1.5
1.0
0.5
0
0 1000k900k800k700k600k500k400k300k200k100k
CURRENT (mA)
FREQUENCY (Hz)
06859-227
I
REF
AI
DD
DI
DD
Figure 44. AD7767-1 Current vs. MCLK Frequency
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0 1000k900k800k700k600k500k400k300k200k100k
CURRENT (mA)
FREQUENCY (Hz)
06859-228
I
REF
AI
DD
DI
DD
Figure 45. AD7767-2 Current vs. MCLK Frequency