AD7796 Analog Devices, AD7796 Datasheet - Page 17

• Manufacturer Part Number: AD7796
• Description: Low Power 16-Bit Sigma-Delta A/D Converter for Bridge Sensors
• Manufacturer: Analog Devices

Specifications of AD7796

• Resolution (bits): 16bit
• # Chan: 1
• Sample Rate: n/a
• Interface: Ser,SPI
• Analog Input Type: Diff-Bip,Diff-Uni
• Ain Range: ± (Vref/128)
• Adc Architecture: Sigma-Delta
• Pkg Type: SOP

DIGITAL INTERFACE
As outlined in the On-Chip Registers section, the AD7796/
AD7797 programmable functions are controlled by a set of on-
chip registers. Data is written to these registers via the part’s
serial interface and read access to the on-chip registers is also
provided by this interface. All communication with the part
must start with a write to the communication register. After
power-on or reset, the device expects a write to its
communication register. The data written to this register
determines whether the next operation is a read or a write
operation, and determines the register where this operation
occurs. Therefore, write access to any of the other registers on
the part begins with a write operation to the communication
register followed by a write to the selected register. A read
operation from any other register (except when continuous read
mode is selected) starts with a write to the communication
register followed by a read operation from the selected register.
The serial interface of the AD7796/AD7797 consists of four
signals: CS , DIN, SCLK, and DOUT/ RDY . The DIN line is used
to transfer data into the on-chip registers, while DOUT/ RDY is
used for accessing from the on-chip registers. SCLK is the serial
clock input for the device, and all data transfers (either on DIN
or DOUT/ RDY ) occur with respect to the SCLK signal. The
DOUT/ RDY pin also operates as a data-ready signal, that is, the
line goes low when a new data-word is available in the output
register. It is reset high when a read operation from the data
register is complete. DOUT/ RDY also goes high prior to the
data register update to indicate when not to read from the
device. This ensures that a data read is not attempted while the
register is being updated. CS is used to select a device. It can be
used to decode the AD7796/AD7797 in systems where several
components are connected to the serial bus.
Figure 3 and Figure 4 show timing diagrams for interfacing to
the AD7796/AD7797 with CS being used to decode the part.
Rev. A | Page 17 of 24
Figure 3 shows the timing for a read operation from the
AD7796/AD7797 output shift register, while Figure 4 shows the
timing for a write operation to the input shift register. It is
possible to read the same word from the data register several
times, even though the DOUT/ RDY line returns high after the
first read operation. However, care must be taken to ensure that
the read operations have been completed before the next output
update occurs. In continuous read mode, the data register can
be read only once.
The serial interface can operate in 3-wire mode by tying CS low.
In this case, the SCLK, DIN, and DOUT/ RDY lines are used
to communicate with the AD7796/AD7797. The end of the
conversion can be monitored using the RDY bit in the status
register. This scheme is suitable for interfacing to micro-
controllers. If CS is required as a decoding signal, it can be
generated from a port pin. For microcontroller interfaces, it is
recommended that SCLK idle high between data transfers.
The AD7796/AD7797 can be operated with CS being used as a
frame synchronization signal. This scheme is useful for DSP
interfaces. In this case, the first bit (MSB) is effectively clocked
out by CS because CS normally occurs after the falling edge of
SCLK in DSPs. The SCLK can continue to run between data
transfers, provided the timing numbers are obeyed.
The serial interface can be reset by writing a series of 1s on the
DIN input. If a Logic 1 is written to the AD7796/AD7797 DIN
line for at least 32 serial clock cycles, the serial interface is reset.
This ensures that the interface can be reset to a known state if
the interface gets lost due to a software error or glitch in the
system. Reset returns the interface to the state where it is
expecting a write to the communication register. This operation
resets the contents of all registers to their power-on values.
Following a reset, the user should allow a period of 500 μs
before addressing the serial interface.
The AD7796/AD7797 can be configured to continuously
convert or to perform a single conversion. See Figure 14
through Figure 16.
AD7796/AD7797