AD7705_15 Datasheet, PDF (28/44 Pages) Analog Devices – 3 V/5 V, 1 mW, 2-/3-Channel

AD7705/AD7706
THEORY OF OPERATION
CLOCKING AND OSCILLATOR CIRCUIT
The AD7705/AD7706 each require 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 17. In this case, the clock
circuit functions as an oscillator, providing the clock source for
the part. The input sampling frequency, modulator sampling
frequency, –3 dB frequency, output update rate, and calibration
time are directly related to the master clock frequency, fCLKIN.
Reducing the master clock frequency by a factor of two halves
the above frequencies and update rate and doubles the
calibration time. The current drawn from the VDD power supply
is also related to fCLKIN. Reducing fCLKIN by a factor of two halves
the digital part of the total VDD current, but does not affect the
current drawn by the analog circuitry.
CRYSTAL OR
CERAMIC
RESONATOR
C1
MCLK IN
AD7705/AD7706
MCLK OUT
C2
Figure 17. Crystal/Resonator Connection for the AD7705/AD7706
Using the part with a crystal or ceramic resonator between the
MCLK IN pin and MCLK OUT pin generally causes more
current to be drawn from VDD than does clocking the part 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
AD7705/AD7706 is achieved with an externally applied clock at
the MCLK IN pin with MCLK OUT unconnected, unloaded, and
disabled.
When operating with a clock frequency of 2.4576 MHz, there is
a 50 μA difference in the current between an externally applied
clock and a crystal resonator operated with a VDD of 3 V. With
VDD = 5 V and fCLKIN = 2.4576 MHz, the typical current increases
by 250 μA for a crystal- or 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 VDD = 3 V, and 200 μA with VDD = 5 V. When using
a crystal with an ESR of 3 kΩ, the increase in the typical current
over an externally applied clock is 100 μA with VDD = 3 V, but
400 μA with VDD = 5 V.
There is a start-up time before the on-chip oscillator circuit
oscillates at its correct frequency and voltage levels. Typical start-
up times with VDD = 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 a 3 V
power supply is used. With 3 V supplies, depending on the loading
capacitances on the MCLK pins, a 1 MΩ feedback resistor might
be required across the crystal or resonator to keep the start-up
times around 20 ms.
The AD7705/AD7706 master clock appears on the MCLK OUT
pin of the device. The maximum recommended load on this pin
is 1 CMOS load. When using a crystal or ceramic resonator to
generate the AD7705/AD7706 clock, it might be desirable to
use this clock as the clock source for the system. In this case, it
is recommended that the MCLK OUT signal be buffered with a
CMOS buffer before being applied to the rest of the circuit.
The amount of additional current taken by the oscillator
depends on a number of factors. For example, the larger the
value of the capacitor (C1 and C2) placed on the MCLK IN and
MCLK OUT pins, the larger the current consumption on the
AD7705/AD7706. To avoid unnecessarily consuming current,
care should be taken not to exceed the capacitor values
recommended by the crystal and ceramic resonator manufac-
turers. Typical values for C1 and C2 are recommended by
crystal or ceramic resonator manufacturers, usually 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 AD7705/
AD7706. As a general rule, the lower the ESR value, the lower
the current taken by the oscillator circuit.
SYSTEM SYNCHRONIZATION
The FSYNC bit of the setup register allows the user to reset the
modulator and digital filter without affecting the setup conditions
on the part. This allows the user to start gathering samples of the
analog input at a known point in time, that is, when the FSYNC
changes 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 does not process input samples. When a 0 is written to the
FSYNC bit, the modulator and filter are taken out of this reset
state, and the part resumes gathering samples on the next
master clock edge.
Rev. C | Page 28 of 44