AD7193 datasheet(52/57 Pages) AD

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Data Sheet

AD7193

Rev. E | Page 51 of 56

Figure 65 shows the filter response when FS[9:0] is set to 5 and

the post filter averages by 16. In this case, the output data rate is

equal to 53.33 Hz when the first filter notch is placed at 60 Hz.

The rejection at 60 Hz ± 0.5 Hz is equal to 40 dB minimum.

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FREQUENCY (Hz)

Figure 65. Filter Response for Average + Decimate Filter

(Sinc3 Filter, FS[9:0] = 5, Average by 16)

Simultaneous 50 Hz/60 Hz rejection is achieved when FS[9:0] is

set to 30 and the postfilter averages by 16. The output data rate

is equal to 8.9 Hz whereas the rejection at 50 Hz ± 0.5 Hz and

60 Hz ± 0.5 Hz is typically 42 dB.

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FREQUENCY (Hz)

Figure 66. Filter Response for Average + Decimate Filter

(Sinc3 Filter, FS[9:0] = 30, Average by 16)

Simultaneous 50 Hz and 60 Hz rejection is also achieved by using

an FS word of 96 and averaging by 16, which places a notch at

50 Hz. Setting the REJ60 bit to 1 places a notch at 60 Hz (see

Figure 67). The output data rate is reduced to 2.78 Hz with this

configuration, but the rejection is improved to 94 dB typically at

50 Hz ± 1 Hz and 60 Hz ± 1 Hz.

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Figure 67. Filter Response for Average + Decimate Filter

(Sinc3 Filter, FS[9:0] = 96, Average by 16)

FAST SETTLING MODE (CHOP ENABLED)

Chop can be enabled in the fast settling mode. With chop

enabled, the ADC offset and offset drift are minimized. The

analog input pins are continuously swapped. With the analog

input pins connected in one direction, the settling time of the

sinc filter is allowed and a conversion is recorded. The analog

input pins are then inverted, and another settled conversion is

obtained. Subsequent conversions are averaged so that the offset

is minimized. This continuous swapping of the analog input

pins and the averaging of subsequent conversions means that

the offset drift is also minimized.

Chopping does not change the output data rate. However, the

settling time equals

tSETTLE = 2/fADC

Consequently, if chop is enabled, the sinc4 filter is selected, FS[9:0]

is set to 6, averaging by 16 is enabled, and the output data rate is

equal to 42.1 Hz. Therefore, the conversion time equals 1/42.10 Hz

or 23.75 ms, and the settling time is equal to 47.5 ms.

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