AD7771 datasheet(56/99 Pages) AD

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Data Sheet

AD7771

Rev. 0 | Page 55 of 98

SRC Bandwidth

The sinc3 and sinc5 filters architecture allows the user to select

a noninteger value as the decimation range This versatility

means that the filter notches must be adjusted dynamically:

two notches (sinc3) or four notches (sinc5) at the variable

frequency, and one fixed notch to remove the PGA chopping

tone. Consequently, the traditional formula for the −0.1 dB and

−3 dB bandwidth must be adjusted depending on the selected

decimation rate.

The bandwidth transfer function is not linear but can be

approximated by using a linear function.

Figure 126 to Figure 129 show the correction factor for the

−0.1 dB and −3 dB bandwidth, respectively. In low power mode,

the offset must be divided by 4. For example, for sinc5 when the

ODR = 1000 SPS, the −0.1 dB point is

BW = 0.0377 × 1000 +

355

= 50.03 Hz

100

ODR (kHz)

y = 0.049x + 120.41

Figure 126. −0.1 dB Correction Factor, Sinc3 Filter Enabled

100

ODR (kHz)

y = 0.0377x + 49.355

Figure 127. −0.1 dB Correction Factor, Sinc5 Filter Enabled

100

ODR (kHz)

y = 0.2653x + 634.03

Figure 128. −3 dB Correction Factor, Sinc3 Filter Enabled

100

ODR (kHz)

y = 0.2053x + 263.94

Figure 129. −3 dB Correction Factor, Sinc5 Filter Enabled

SRC Group Delay

The SRC group delay depends on the selected ODR and is

defined by the following equation:

SRC Group Delay =

ODR

SRC

where:

PM is a constant equal to 8.

SRC_N is the integer value of the programmed ODR.

ODR is the programmed output data rate.

When using the sinc5 filter, the equation that defines the group

delay is

SRC Group Delay =

ODR

SRC

The latency is the contribution of the group delay and the

calibration time.

Latency = Group Delay + tCAL

In high resolution mode, the calibration delay is defined as 62 ×

tMCLK, with a maximum error of 2 × tMCLK. In low power mode,

the calibration delay is defined as 121 × tMCLK, with a maximum

error of 4 × tMCLK. tMCLK is the modulator period and is 488 ns in

high resolution mode and 1.9 µs in low power mode.

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