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AD7707 Datasheet(PDF) 34 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

AD7707 Datasheet(HTML) 34 Page - Analog Devices

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Rev. B | Page 33 of 52
The AD7707 requires a master clock input, which can be an
external CMOS compatible clock signal applied to the MCLK
IN pin with the MCLK OUT pin left unconnected. Alternatively,
a crystal or ceramic resonator of the correct frequency can be
connected between MCLK IN and MCLK OUT as shown in
Figure 18, in which case the clock circuit functions as an oscilla-
tor, providing the clock source for the part. The input sampling
frequency, the modulator sampling frequency, the −3 dB fre-
quency, output update rate, and calibration time are all directly
related to the master clock frequency, fCLKIN. Reducing the
master clock frequency by a factor of 2 halves these frequencies
and update rate, and doubles the calibration time. The current
drawn from the DVDD power supply is also related to fCLKIN.
Reducing fCLKIN by a factor of 2 halves the DVDD current but
does not affect the current drawn from the AVDD.
Figure 18. Crystal/Resonator Connection for the AD7707
Using the part with a crystal or ceramic resonator between the
MCLK IN and MCLK OUT pins generally causes more current
to be drawn from DVDD than when the part is clocked from a
driven clock signal at the MCLK IN pin. This is because the on-
chip oscillator circuit is active in the case of the crystal or ceramic
resonator. Therefore, the lowest possible current on the AD7707
is achieved with an externally applied clock at the MCLK IN pin
with MCLK OUT unconnected, unloaded, and disabled.
The amount of additional current taken by the oscillator depends
on a number of factors—first, the larger the value of capacitor
(C1 and C2) placed on the MCLK IN and MCLK OUT pins, the
larger the current consumption on the AD7707. Care should be
taken not to exceed the capacitor values recommended by the
crystal and ceramic resonator manufacturers to avoid consuming
unnecessary current. Typical values for C1 and C2 are recom-
mended by crystal or ceramic resonator manufacturers; these
are in the range of 30 pF to 50 pF. If the capacitor values on
MCLK IN and MCLK OUT are kept in this range, they do not
result in any excessive current. Another factor that influences
the current is the effective series resistance (ESR) of the crystal
that appears between the MCLK IN and MCLK OUT pins of
the AD7707. As a general rule, the lower the ESR value is, the
lower the current taken by the oscillator circuit.
When operating with a clock frequency of 2.4576 MHz, there is
50 μA difference in the current between an externally applied
clock and a crystal resonator when operating with a DVDD of
3 V. With DVDD = 5 V and fCLKIN = 2.4576 MHz, the typical
current increases by 250 μA for a crystal/resonator supplied
clock vs. an externally applied clock. The ESR values for crystals
and resonators at this frequency tend to be low and, as a result
there tends to be little difference between different crystal and
resonator types.
When operating with a clock frequency of 1 MHz, the ESR
value for different crystal types varies significantly. As a result,
the current drain varies across crystal types. When using a
crystal with an ESR of 700 Ω or when using a ceramic resonator,
the increase in the typical current over an externally applied
clock is 20 μA with DVDD = 3 V and 200 μA with DVDD = 5 V.
When using a crystal with an ESR of 3 kΩ, the increase in the
typical current over an externally applied clock is again 100 μA
with DVDD = 3 V but 400 μA with DVDD = 5 V.
The on-chip oscillator circuit also has a start-up time associated
with it before it is oscillating at its correct frequency and correct
voltage levels. Typical start-up times with DVDD = 5 V are 6 ms
using a 4.9512 MHz crystal, 16 ms with a 2.4576 MHz crystal
and 20 ms with a 1 MHz crystal oscillator. Start-up times are
typically 20% slower when the power supply voltage is reduced
to 3 V. At 3 V supplies, depending on the loading capacitances
on the MCLK pins, a 1 MΩ feedback resistor may be required
across the crystal or resonator to keep the start-up times around
the 20 ms duration.
The AD7707’s master clock appears on the MCLK OUT pin of
the device. The maximum recommended load on this pin is one
CMOS load. When using a crystal or ceramic resonator to generate
the AD7707’s clock, it may be desirable to use this clock as the
clock source for the system. In this case, it is recommended that
the MCLK OUT signal is buffered with a CMOS buffer before
being applied to the rest of the circuit.
The FSYNC bit of the setup register allows the user to reset the
modulator and digital filter without affecting any of the setup
conditions on the part. This allows the user to start gathering
samples of the analog input from a known point in time, that is,
when the FSYNC
is changed from 1 to 0.
With a 1 in the FSYNC bit of the setup register, the digital filter
and analog modulator are held in a known reset state and the
part is not processing any input samples. When a 0 is then
written to the FSYNC bit, the modulator and filter are taken out
of this reset state and the part starts to gather samples again on
the next master clock edge.
The FSYNC input can also be used as a software start convert
command allowing the AD7707 to be operated in a conventional
converter fashion. In this mode, writing to the FSYNC bit starts
conversion and the falling edge of DRDY indicates when con-
version is complete. The disadvantage of this scheme is that the
settling time of the filter has to be taken into account for every
data register update. This means that the rate at which the data
register is updated is three times slower in this mode.

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