AD7708 Analog Devices, AD7708 Datasheet - Page 32

• Manufacturer Part Number: AD7708
• Description: 16-Bit 8/10-Channel, Low Voltage, Low Power, Sigma Delta ADC
• Manufacturer: Analog Devices

Specifications of AD7708

• Resolution (bits): 16bit
• # Chan: 10
• Sample Rate: n/a
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni,SE-Uni
• Ain Range: (2Vref/PGA Gain) p-p
• Adc Architecture: Sigma-Delta
• Pkg Type: SOIC,SOP

AD7708/AD7718
Configuring the AD7708/AD7718
All user-accessible registers on the AD7708 and AD7718 are
accessed via the serial interface. Communication with any of
these registers is initiated by first writing to the Communica-
tions Register. Figures 16, 17, and 18 show flow diagrams for
initializing the ADC, a sequence for calibrating the ADC chan-
nels, and a routine that cycles through and reads all channels.
Figure 16 shows a flowchart detailing necessary programming
steps required to initialize the ADC. The following are the general
programming steps required:
1. Configure and initialize the microcontroller or microprocessor
2. Initialize the AD7708/AD7718 by configuring the following
All operations consist of a write to the communications register
to specify the next operation as a write to a specified register.
Data is then written to the specified register. When each sequence
is complete, the ADC defaults to waiting for another write to
the communications register to specify the next operation.
serial port.
registers:
a. IOCON to configure the digital I/O port.
b. FILTER to configure the update rate for each channel.
c. ADCCON to select the active input channel, select
d. MODE to configure the operating mode. The mode
the analog input range, and select unipolar or bipolar
operation.
register selects chop or nonchop operation, buffered/
unbuffered operation of the AINCOM input, 8-/10-
channel mode of operation and reference select along
with the selection of conversion, calibration or idle
modes of operation.
WRITE TO COMMUNICATIONS REGISTER SETTING UP
NEXT OPERATION TO BE A WRITE TO THE ADC
CONTROL REGISTER. CONFIGURE ADCCON
WRITE TO COMMUNICATIONS REGISTER SETTING UP
NEXT OPERATION TO BE A WRITE TO THE MODE
REGISTER. CALIBRATE AND SELECT MODE OF
OPERATION
WRITE TO COMMUNICATIONS REGISTER SELECTING
NEXT OPERATION TO BE A WRITE TO THE IOCON
REGISTER AND CONFIGURE THIS REGISTER
WRITE TO COMMUNICATIONS REGISTER SETTING UP
NEXT OPERATION TO BE A WRITE TO THE FILTER
REGISTER. SET SF WORD FOR REQUIRED
UPDATE RATE
CONFIGURE AND INITIALIZE C/ P SERIAL PORT
POWER-ON/RESET FOR AD7708/AD7718
START
Figure 17 shows a flowchart detailing necessary programming
steps required when calibrating the AD7708/AD7718. The
AD7708/AD7718 have dedicated calibration register pairs for
each of the fully-differential input channels. Having a dedicated
register pair per channel allows each channel to be calibrated as
part of the initialization and the ADC picks up the relevant
coefficients for each channel during normal operation. When
operating is pseudo-differential mode channels share calibration
register pairs. Channels that share coefficients should be config-
ured with the same operating conditions to avoid having to
calibrate each time a channel is switched, especially with chop
mode disabled. The AD7708/AD7718 are factory-calibrated
with chop mode enabled and, therefore, if the ADC is operated
at the same conditions as the factory-calibration field calibra-
tions will not be required. Extremely low offset error and offset
and gain drift errors are a by product of the chopping scheme.
When operating with chop mode disabled, the user can achieve
faster throughput times. An offset calibration is required with
chop disabled when a gain or temperature change occurs. The
following are the general programming steps required when
calibrating a channel on the AD7708/AD7718.
WRITE TO ADCCON SELECTING CHANNEL AND CONFIGURING
INPUT RANGE AND UNIPOLAR/BIPOLAR MODE
WRITE TO MODE REGISTER SELECTING
ZERO-SCALE CALIBRATION
WRITE TO MODE REGISTER SELECTING
FULL-SCALE CALIBRATION
YES
NO
NO
CALIBRATION ROUTINE
ANOTHER
CHANNEL
MD BITS
MD BITS
= 001?
= 001?
CAL
END
YES
YES
NO