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AD7715 Datasheet(PDF) 23 Page - Analog Devices |
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AD7715 Datasheet(HTML) 23 Page - Analog Devices |
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23 / 41 page  AD7715 Data Sheet Rev. E | Page 22 of 40 Filter Characteristics The AD7715’s digital filter is a low-pass 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 ) ( × π × π × × = S S f f Sin f f N Sin N f H where N is the ratio of the modulator rate to the output rate and fMOD is the modulator rate. Figure 6 shows the filter frequency response for a cutoff frequency of 15.72 Hz which corresponds to a first filter notch frequency of 60 Hz. The plot is shown from dc to 390 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. FREQUENCY (Hz) 0 –40 –60 –80 –100 –120 –140 –160 –180 –200 –220 –20 –240 360 0 300 180 120 60 240 Figure 6. Frequency Response of AD7715 Filter The response of the filter is similar to that of an averaging filter but with a sharper roll-off. 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 6 where the output rate is 60 Hz, the first notch of the filter is at 60 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. The cutoff frequency of the digital filter is determined by the value loaded to the FS0 to FS1 bits in the setup register. program- ming a different cutoff frequency via FS0 and FS1 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 AD7715 contains this on-chip, low-pass filtering, there is a settling time associated with step function inputs and data on the output is 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 full- scale step input can be up 4 times the output data period. For a synchronized step input (using the FSYNC function), the settling time is 3 times the output data period. Post-Filtering The on-chip modulator provides samples at a 19.2 kHz output rate with fCLKIN at 2.4576 MHz. The on-chip digital filter decimates these samples to provide data at an output rate that corresponds to the programmed output rate of the filter. Because the output data rate is higher than the Nyquist criterion, the output rate for a given bandwidth satisfys most application requirements. However, there may be some applications that require a higher data rate for a given bandwidth and noise performance. Applications that need this higher data rate do require some post-filtering following the digital filter of the AD7715. For example, if the required bandwidth is 7.86 Hz but the required update rate is 100 Hz, the data can be taken from the AD7715 at the 100 Hz rate giving a −3 dB bandwidth of 26.2 Hz. Post-filtering can be applied to this to reduce the bandwidth and output noise, to the 7.86 Hz bandwidth level, while maintaining an output rate of 100 Hz. Post-filtering can also be used to reduce the output noise from the device for bandwidths below 13.1 Hz. At a gain of 128 and a bandwidth of 13.1 Hz, the output rms noise is 520 nV. This is essentially device noise or white noise and because the input is chopped, the noise has a primarily flat frequency response. By reducing the bandwidth below 13.1 Hz, the noise in the resultant pass-band can be reduced. A reduction in bandwidth by a factor of 2 results in a reduction of approximately 1.25 in the output rms noise. This additional filtering results in a longer settling time. |
Similar Part No. - AD7715_17 |
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Similar Description - AD7715_17 |
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