Datasheet

Data Sheet AD7763
Rev. B | Page 21 of 32
Sampling Switch SS1 and Sampling Switch SS3 are driven by ICLK,
whereas Sampling Switch SS2 and Sampling Switch SS4 are driven
by
ICLK
. When ICLK is high, the analog input voltage is connected
to CS1. On the falling edge of ICLK, the SS1 and SS3 switches open,
and the analog input is sampled on CS1. Similarly, when ICLK
is low, the analog input voltage is connected to CS2. On the
rising edge of ICLK, the SS2 and SS4 switches open, and the
analog input is sampled on CS2.
Capacitor CPA, Capacitor CPB1, and Capacitor CPB2 represent
parasitic capacitances that include the junction capacitances
associated with the MOS switches.
Table 11. Equivalent Component Values
Mode CS1 CS2 CPA CPB1/CPB2
Normal 51 pF 51 pF 12 pF 20 pF
Low Power 13 pF 13 pF 12 pF 5 pF
USING THE AD7763
Following is the recommended sequence for powering up and
using the AD7763.
1. Apply power.
2. Start clock oscillator, applying MCLK.
3. Take
RESET
low for a minimum of 1 MCLK cycle.
4. Wait a minimum of 2 MCLK cycles after
RESET
has been
released.
5. Write to Control Register 2 to power up the ADC and the
differential amplifier, as required.
6. Write to Control Register 1 to set up the output data rate.
7. In circumstances where multiple parts are being
synchronized, a
SYNC
pulse must be applied to the parts;
otherwise, no
SYNC
pulse is required.
The following are conditions for applying the
SYNC
pulse:
The issuing of a
SYNC
pulse to the part must not coincide
with a write to the part.
The
SYNC
pulse should be applied a minimum
of 2.5 ICLK cycles after the
FSI
signal for the previous write
to the part has returned to logic high.
Ensure that the
SYNC
pulse is taken low for a minimum of
2.5 ICLK cycles.
Data can now be read from the part using the default filter,
offset, gain, and overrange threshold values. The conversion
data read is not valid, however, until the settling time of the
filter has passed. When this has occurred, the DVALID bit read
is set, indicating that the data is indeed valid.
The user can then download a user-defined filter, if required
(see Downloading a User-Defined Filter). Values for gain, offset,
and overrange threshold registers can also be written or read at
this stage.
BIAS RESISTOR SELECTION
The AD7763 requires a resistor to be connected between the
R
BIAS
pin and AGND. The value for this resistor is dependent on
the reference voltage being applied to the device. The resistor
value should be selected to give a current of 25 µA through the
resistor to ground. For a 2.5 V reference voltage, the correct
resistor value is 100 kΩ; for a 4.096 V reference voltage, the
correct resistor value is 160 kΩ.