LTC2498_15 Datasheet, PDF (16/38 Pages) Linear Technology – 24-Bit 8-/16-Channel ADC with Easy Drive Input Current Cancellation

LTC2498
Applications Information
fier is used, the LTC2498 automatically calibrates both the
offset and drift of this circuit and the Easy Drive sampling
scheme enables a wide variety of amplifiers to be used.
the conversion is complete, if CS is brought LOW, EOC
will be driven LOW indicating the conversion is complete
and the result is ready to be shifted out of the device.
In order to achieve optimum performance, if an external
amplifier is not used, short these pins directly together
(ADCINP to MUXOUTP and ADCINN to MUXOUTN) and
minimize their capacitance to ground.
Chip Select (CS)
The active low CS pin is used to test the conversion status,
enable I/O data transfer, initiate a new conversion, control
the duration of the sleep state, and set the SCK mode.
Serial Interface Pins
The LTC2498 transmits the conversion result, reads the
input configuration, and receives a start of conversion
command through a synchronous 3- or 4-wire interface.
During the conversion and sleep states, this interface
can be used to access the converter status. During the
data output state, it is used to read the conversion result,
program the input channel, rejection frequency, speed
multiplier, and select the temperature sensor.
Serial Clock Input/Output (SCK)
At the conclusion of a conversion cycle, while CS is HIGH,
the device remains in a low power sleep state where the
supply current is reduced several orders of magnitude. In
order to exit the sleep state and enter the data output state,
CS must be pulled LOW. Data is now shifted out the SDO
pin under control of the SCK pin as described previously.
A new conversion cycle is initiated either at the conclusion
of the data output cycle (all 32 data bits read) or by pulling
CS HIGH any time between the first and 32nd rising edges
of the serial clock (SCK). In this case, the data output is
aborted and a new conversion begins.
The serial clock pin (SCK) is used to synchronize the data
input/output transfer. Each bit is shifted out of the SDO
pin on the falling edge of SCK and data is shifted into the
SDI pin on the rising edge of SCK.
The serial clock pin (SCK) can be configured as either a
master (SCK is an output generated internally) or a slave
(SCK is an input and applied externally). Master mode
(internal SCK) is selected by simply floating the SCK pin.
Slave mode (external SCK) is selected by driving SCK LOW
during power-up and each falling edge of CS. Specific
details of these SCK modes are described in the Serial
Interface Timing Modes section.
Serial Data Output (SDO)
The serial data output pin (SDO) provides the result of the
last conversion as a serial bit stream (MSB first) during
the data output state. In addition, the SDO pin is used as
an end of conversion indicator during the conversion and
sleep states.
When CS is HIGH, the SDO driver is switched to a high
impedance state in order to share the data output line with
other devices. If CS is brought LOW during the conversion
phase, the EOC bit (SDO pin) will be driven HIGH. Once
Serial Data Input (SDI)
The serial data input (SDI) is used to select the input
channel, rejection frequency, speed multiplier and to ac-
cess the integrated temperature sensor. Data is shifted
into the device during the data output/input state on the
rising edge of SCK while CS is LOW.
OUTPUT DATA FORMAT
The LTC2498 serial output stream is 32 bits long. The
first bit indicates the conversion status, the second bit is
always zero, and the third bit conveys sign information.
The next 24 bits are the conversion result, MSB first. The
remaining 5 bits are sub LSBs beyond the 24-bit level
that may be included in averaging or discarded without
loss of resolution.
Bit 31 (first output bit) is the end of conversion (EOC)
indicator. This bit is available on the SDO pin during the
conversion and sleep states whenever CS is LOW. This
bit is HIGH during the conversion cycle, goes LOW once
the conversion is complete, and is Hi-Z when CS is HIGH.
Bit 30 (second output bit) is a dummy bit (DMY) and is
always LOW.
2498fg
16
For more information www.linear.com/LTC2498