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

NO ALIASING OF SIGNALS

INTO PASSBAND AROUND

NYQUIST FREQUENCY

Figure 27. Antialias Example Using the AD7765 in Normal Mode, Decimate 128× Using MCLK/2 = ICLK = 20 MHz