Datasheet

AD7765
Rev. A | Page 16 of 32
AD7765 ANTIALIAS PROTECTION
The decimation of the AD7765, along with its counterparts in
the AD776x family, namely the AD7760, AD7762, AD7763, and
AD7764, provides top of the range antialias protection.
The decimation filter of the AD7765 features more than 115 dB
of attenuation across the full stop band, which ranges from the
Nyquist frequency, namely ODR/2, up to ICLK − ODR/2
(where ODR is the output data rate). Starting the stop band at
the Nyquist frequency prevents any signal component above
Nyquist (and up to ICLK − ODR/2) from aliasing into the
desired signal bandwidth.
Figure 26 shows the frequency response of the decimation filter
when the AD7765 is operated with a 40 MHz MCLK in
decimate 128× mode. Note that the first stop-band frequency
occurs at Nyquist. The frequency response of the filter scales
with both the decimation rate chosen and the MCLK frequency
applied. When using low power mode, the modulator sample
rate is MCLK/4.
Taking as an example the AD7765 in normal power and in
decimate 128× mode, the first possible alias frequency is at the
ICLK frequency minus the pass band of the digital filter (see
Figure 27).
NYQUIST = 78kHz ODR = 156kHz
MODULATOR
SAMPLING RATE =
MCLK/2 = 20MHz
FIRST ALIAS POINT
20MHz TO 78kHz
SIMPLFIES ANTIALIAS
FILTER ROLL-OFF REQUIRED
DIGITAL FILTER
RESPONSE
FREQUENCY
(Hz)
DIGITAL FILTER
RESPONSE IMAGE
NOISE SHAPING
AMPLITUDE (dB)
NO ALIASING OF SIGNALS
INTO PASSBAND AROUND
NYQUIST FREQUENCY
06519-300
Figure 27. Antialias Example Using the AD7765 in Normal Mode, Decimate 128× Using MCLK/2 = ICLK = 20 MHz