AD7764 Analog Devices, AD7764 Datasheet - Page 21

• Manufacturer Part Number: AD7764
• Description: 24-Bit, 312 kSPS, 109 dB Sigma Delta ADC with On-Chip Buffers and Serial Interface
• Manufacturer: Analog Devices

Specifications of AD7764

• Resolution (bits): 24bit
• # Chan: 1
• Sample Rate: 40MSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: 6.5 V p-p
• Adc Architecture: Sigma-Delta
• Pkg Type: SOP

AD7764 INTERFACE
READING DATA
The AD7764 uses an SPI-compatible serial interface. The
timing diagram in Figure 2 shows how the AD7764 transmits its
con-version results.
The data read from the AD7764 is clocked out using the serial
clock output (SCO). The SCO frequency is half that of the
MCLK input to the AD7764.
The conversion result output on the serial data output (SDO)
line is framed by the frame synchronization output, FSO
is sent logic low for 32 SCO cycles. Each bit of the new conversion
result is clocked onto the SDO line on the rising SCO edge and
is valid on the falling SCO edge. The 32-bit result consists of the
24 data bits followed by five status bits followed further by three
zeros. The five status bits are listed in Table 9 and described
below the table.
Table 9. Status Bits During Data Read
D7
FILTER-SETTLE
•
•
•
•
Table 10. Decimation Rate Status Bits
Decimate
64×
128×
256×
1
128× mode.
Don’t care. If the DEC_RATE 1 bit is set to 1, AD7764 is in decimate
The FILTER-SETTLE bit indicates whether the data output
from the AD7764 is valid. After resetting the device (using
the RESET pin) or clearing the digital filter (using the
pin), the FILTER-SETTLE bit goes logic low to indicate
that the full settling time of the filter has not yet passed and
that the data is not yet valid. The FILTER-SETTLE bit also
goes to zero when the input to the part has asserted the
overrange alerts.
The OVR (overrange) bit is described in the
Alerts section.
The LPWR bit is set to logic high when the AD7764 is
operating in low power mode. See the Power Modes
section for further details.
The DEC_RATE 1 and DEC_RATE 0 bits indicate the
decimation ratio used. Table 10 is a truth table for the
decimation rate bits.
D6
OVR
DEC_RATE 1
0
1
0
D5
LPWR
D4
DEC_RATE 1
DEC_RATE 0
1
X
0
1
Overrange
D3
DEC_RATE 0
, which
SYNC
Rev. A | Page 21 of 32
READING STATUS AND OTHER REGISTERS
The AD7764 features a gain correction register, an overrange
register, and a read-only status register. To read back the
contents of these registers, the user must first write to the
control register of the device and set the bit that corresponds to
the register to be read. The next read operation outputs the
contents of the selected register (on the SDO pin) instead of a
conversion result.
To ensure that the next read cycle contains the contents of the
register written to, the write operation to that register must be
completed a minimum of 8 × t
which indicates the start of the next read cycle. See Figure 4 for
further details.
The
the relevant bits in the control register.
WRITING TO THE AD7764
A write operation to the AD7764 is shown in Figure 3. The
serial writing operation is synchronous to the SCO signal. The
status of the frame synchronization input, FSI , is checked on the
falling edge of the SCO signal. If the FSI line is low, then the
first data bit on the serial data in (SDI) line is latched in on the
next SCO falling edge.
Set the active edge of the FSI signal to occur at a position when
the SCO signal is high or low to allow setup and hold times
from the SCO falling edge to be met. The width of the
signal can be set to between 1 and 32 SCO periods wide. A
second, or subsequent, falling edge that occurs before 32 SCO
periods have elapsed is ignored.
Figure 3
the AD7764 through the SDI pin. Thirty-two bits are required
for a write operation. The first 16 bits are used to select the
register address that the data being read is intended for. The
second 16 bits contain the data for the selected register.
Writing to the AD7764 is allowed at any time, even while
reading a conversion result. Note that, after writing to the
devices, valid data is not output until after the settling time
for the filter has elapsed. The FILTER-SETTLE status bit is
asserted at this point to indicate that the filter has settled and
that valid data is available at the output.
AD7764 Registers section provides more information on
details the format for the serial data being written to
SCO
before the falling edge of FSO
AD7764
FSI
,