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AD7707 Datasheet(PDF) 30 Page - Analog Devices

Part No. AD7707
Description  3-Channel 16-Bit, Sigma-Delta ADC
Download  53 Pages
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD7707 Datasheet(HTML) 30 Page - Analog Devices

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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.


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