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AD7707 Datasheet(PDF) 30 Page - Analog Devices |
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AD7707 Datasheet(HTML) 30 Page - Analog Devices |
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30 / 53 page  AD7707 Rev. B | Page 29 of 52 POSTFILTERING 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 satisfies most application requirements. There may, however, be some applications that require a higher data rate for a given bandwidth and noise performance. Applications that need this higher data rate require some postfiltering follow- ing the digital filter of the AD7707. 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 AD7707 at the 100 Hz rate, giving a −3 dB bandwidth of 26.2 Hz. Postfiltering 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. Postfiltering can also be used to reduce the output noise from the device for bandwidths below 2.62 Hz. At a gain of 128 and a bandwidth of 2.62 Hz, the output rms noise is 450 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 2.62 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. ANALOG FILTERING The digital filter does not provide any rejection at integer multiples of the modulator sample frequency, as previously outlined. However, due to the AD7707’s high oversampling ratio, these bands occupy only a small fraction of the spectrum and most broadband noise is filtered. This means that the analog filtering requirements in front of the AD7707 are considerably reduced vs. a conven- tional converter with no on-chip filtering. In addition, because the part’s common-mode rejection performance of 100 dB extends out to several kHz, common-mode noise in this frequency range is substantially reduced. Depending on the application, however, it may be necessary to provide attenuation prior to the AD7707 to eliminate unwanted frequencies from these bands, which the digital filter will pass. It may also be necessary in some applications to provide analog filtering in front of the AD7707 to ensure that differential noise signals outside the band of interest do not saturate the analog modulator. If passive components are placed in front of the AD7707 in unbuffered mode, care must be taken to ensure that the source impedance is low enough not to introduce gain errors in the system. This significantly limits the amount of passive antialiasing filtering, which can be provided in front of the AD7707 when it is used in unbuffered mode. However, when the part is used in buffered mode, large source impedances simply result in a small dc offset error (a 10 kΩ source resistance causes an offset error of less than 10 μV). Therefore, if the system requires any cant source impedances to provide passive analog front of the AD7707, it is recommended that the in buffered mode. |
Similar Part No. - AD7707_17 |
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Similar Description - AD7707_17 |
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