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AD7707 Datasheet(PDF) 29 Page  Analog Devices 

AD7707 Datasheet(HTML) 29 Page  Analog Devices 
29 / 53 page AD7707 Rev. B  Page 28 of 52 DIGITAL FILTERING The AD7707 contains an onchip lowpass digital filter that processes the output of the part’s ΣΔ modulator. Therefore, the part not only provides the analogtodigital conversion function but also provides a level of filtering. There are a number of system differences when the filtering function is provided in the digital domain rather than the analog domain and the user should be aware of these. First, because digital filtering occurs after the ADC process, it can remove noise injected during the conversion process. Analog filtering cannot do this. Also, the digital filter can be made programmable far more readily than an analog filter. Depending on the digital filter design, this gives the user the capability of programming cutoff frequency and output update rate. On the other hand, analog filtering can remove noise superim posed on the analog signal before it reaches the ADC. Digital filtering cannot do this and noise peaks riding on signals near full scale have the potential to saturate the analog modulator and digital filter, even though the average value of the signal is within limits. To alleviate this problem, the AD7707 has over range headroom built into the ΣΔ modulator and digital filter, which allows overrange excursions of 5% above the analog input range. If noise signals are larger than this, consideration should be given to analog input filtering, or to reducing the input channel voltage so that its full scale is half that of the analog input channel full scale. This provides an overrange capability greater than 100% at the expense of reducing the dynamic range by one bit (50%). In addition, the digital filter does not provide any rejection at integer multiples of the digital filter’s sample frequency. However, the input sampling on the part provides attenuation at multiples of the digital filter’s sampling frequency so that the unattenuated bands actually occur around multiples of the sampling frequency, fS (as defined in Table 27). Thus, the unattenuated bands occur at n × fS (where n = 1, 2, 3…). At these frequencies, there are frequency bands of ±f3 dB width (f3 dB is the cutoff frequency of the digital filter) where noise passes unattenuated to the output. FILTER CHARACTERISTICS The AD7707’s digital filter is a lowpass filter with a (sinx/x)3 response (also called sinc3). The transfer function for this filter is described in the z domain by: 3 1 1 1 1 ) ( − − − − × = Z Z N z H N and in the frequency domain by: 3 / ( ) / ( 1 ) ( S S f f SIN f f N SIN N f H × π × π × × − where N is the ratio of the modulator rate to the output rate. Phase response: Rad f f N H S / ) 2 ( 3 × − π − = ∠ Figure 16 shows the filter frequency response for a cutoff frequency of 2.62 Hz, which corresponds to a first filter notch frequency of 10 Hz. The plot is shown from dc to 65 Hz. This response is repeated at either side of the digital filter’s sample frequency and at either side of multiples of the filter’s sample frequency. The response of the filter is similar to that of an averaging filter, but with a sharper rolloff. The output rate for the digital filter corresponds with the positioning of the first notch of the filter’s frequency response. Thus, for the plot of Figure 16 where the output rate is 10 Hz, the first notch of the filter is at 10 Hz. The notches of this (sinx/x)3 filter are repeated at multiples of the first notch. The filter provides attenuation of better than 100 dB at these notches. FREQUENCY (Hz) 0 –140 –240 0 10 20 30 40 50 60 –20 –160 –180 –60 –100 –40 –80 –200 –220 –120 Figure 16. Frequency Response of AD7707 Filter Simultaneous 50 Hz and 60 Hz rejection is obtained by placing the first notch at 10 Hz. Operating with an update rate of 10 Hz places notches at both 50 Hz and 60 Hz giving better than 100 dB rejection at these frequencies. The cutoff frequency of the digital filter is determined by the value loaded to Bit FS0 to Bit FS2 in the clock register. Programming a different cutoff frequency via FS0, FS1, and FS2 does not alter the profile of the filter response; it changes the frequency of the notches. The output update of the part and the frequency of the first notch correspond. Because the AD7707 contains this onchip, lowpass filtering, a settling time is associated with step function inputs and data on the output will be invalid after a step change until the settling time has elapsed. The settling time depends upon the output rate chosen for the filter. The settling time of the filter to a fullscale step input can be up to four times the output data period. For a synchronized step input (using the FSYNC function), the settling time is three times the output data period. 
