AD7708 Analog Devices, AD7708 Datasheet - Page 41

• 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

Good decoupling is important when using high resolution ADCs.
All analog supplies should be decoupled with 10 µF tantalum in
parallel with 0.1 µF capacitors to AGND. To achieve the best
from these decoupling components, they have to be placed
as close as possible to the device, ideally right up against the
device. All logic chips should be decoupled with 0.1 µF ceramic
capacitors to DGND. In systems where a common supply
voltage is used to drive both the AV
AD7718, it is recommended that the system’s AV
used. This supply should have the recommended analog supply
decoupling capacitors between the AV
AD7718 and AGND and the recommended digital supply
decoupling capacitor between the DV
AD7718 and DGND.
APPLICATIONS
The AD7708/AD7718 provides a low cost, high resolution
analog-to-digital function. The AD7708 offers 16-bit resolution
while the AD7718 offers 24-bit resolution. The AD7708 and
AD7718 are pin and function compatible. The AD7718 allows a
direct upgradable path from a 16-bit to a 24-bit system with
minimal software and no hardware changes. Because the analog-
to-digital function is provided by a sigma-delta architecture, it
makes the part more immune to noisy environments, thus mak-
ing the part ideal for use in sensor measurement and in industrial
and process control applications. There are two modes of operation
associated with the AD7708/AD7718, chop enabled (CHOP = 0)
or chop disabled (CHOP = 1). With chop enabled the signal chain
is chopped and the device is factory-calibrated at final test in
this mode. Field calibration can be avoided due to the extremely
low offset and gain drifts exhibited by the converter in this
mode. While operating in this mode gives optimum performance
in terms of offset error and offset and gain drift performance, it
offers limited throughput when cycling through all channels.
With chopping disabled, the signal chain is not chopped and
therefore the user needs to ensure that the ADC is calibrated on
range changes and if there is a significant temperature change
as the gain and offset drift performance is degraded.
The key advantage in using the AD7708/AD7718 with chopping
disabled is in channel cycling applications where system through-
put is of prime importance. The max conversion rate with chop
disabled is 1.36 kHz compared with 105 Hz with chop enabled.
The AD7708/AD7718 also provides a programmable gain ampli-
fier, a digital filter, and system calibration options. Thus, it
provides far more system level functionality than off-the-shelf
integrating ADCs without the disadvantage of having to sup-
ply a high quality integrating capacitor. In addition, using the
AD7708/AD7718 in a system allows the system designer to
achieve a much higher level of resolution because noise perfor-
mance of the AD7708/AD7718 is significantly better than that
of integrating ADCs.
The on-chip PGA allows the AD7708/AD7718 to handle an
analog input voltage range as low as 10 mV full scale with V
= 1.25 V. The AD7708/AD7718 can be operated in 8-channel
mode with two reference input options or 10-channel mode with
one reference input. Eight-channel mode allows both ratiometric
or absolute measurements to be performed on any channel using
the two reference input options. The differential analog inputs
of the part allow this analog input range to have an absolute
value anywhere between AGND + 100 mV and AV
DD
and DV
DD
DD
pin of the AD7708/
pin of the AD7708/
DD
of the AD7708/
DD
DD
supply is
– 100 mV.
REF
The buffer on the negative analog input can be bypassed allowing
the AD7708/AD7718 be operated as eight or ten single-ended
input channels. The PGA allows the user to connect transducers
directly to the input of the AD7708/AD7718. The program-
mable gain front end on the AD7708/AD7718 allows the part to
handle unipolar analog input ranges from 0 mV to +20 mV to
0 V to +2.5 V and bipolar inputs of ±20 mV to ± 2.5 V. Because
the part operates from a single supply these bipolar ranges are
with respect to a biased-up differential input.
Data Acquisition
The AD7708/AD7718, with its different configuration options
(five fully-differential input or 10 pseudo-differential input
channels with one reference input or four fully-differential input
or eight pseudo-differential input channels with two reference
inputs), is suited to low bandwidth, high resolution data acquisi-
tion systems. In addition, the 3-wire digital interface allows this
data acquisition front end to be isolated with just three opto-
isolators. The entire system can be operated from a single 3 V
or 5 V supply, provided that the input signals to the AD7708/
AD7718’s analog inputs are all of positive polarity.
Programmable Logic Controllers
The AD7708/AD7718 is also suited to programmable logic
controller applications. In such applications, the ADC is required
to handle signals from a variety of different transducers. The
AD7708/AD7718’s programmable gain front end allows the
part to either handle low level signals directly from a transducer
or full-scale signals that have already been conditioned. The faster
throughput rate and settling time of the part when operated with
chopping disabled makes this the optimum mode of operation in
PLC applications as an important feature in these applications is
loop response time. The configuration of the AD7708/AD7718
in PLC applications is similar to that outlined for a data acquisi-
tion system and is shown in Figure 26. In this application the
AD7708/AD7718 is configured in 10-channel mode, (CHCON
= 1) and can be operated as 10 pseudo-differential inputs with
respect to AINCOM or as five fully-differential input channels.
V
5V
IN
AD780
GND
V
OUT
AIN1
AIN2
AIN3
AIN4
AIN5
AIN6
AIN7
AIN8
AIN9
AIN10
AINCOM
REFIN1(+)
REF1IN(–)
AGND
5V
AV
AD7708/
AD7718
DD
DGND
DV
AD7708/AD7718
MCLKOUT
DD
MCLKIN
RESET
DOUT
SCLK
RDY
DIN
CS
CONTROLLER
32kHz
MICRO-