AD7709 Analog Devices, AD7709 Datasheet - Page 25

• Manufacturer Part Number: AD7709
• Description: 16-Bit Sigma Delta ADC with Current Sources, Switchable Reference Inputs and I/O Port
• Manufacturer: Analog Devices

Specifications of AD7709

• Resolution (bits): 16bit
• # Chan: 4
• 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: SOP

The printed circuit board that houses the AD7709 should be
designed such that the analog and digital sections are separated
and confined to certain areas of the board. A minimum etch
technique is generally best for ground planes as it gives the best
shielding.
It is recommended that the AD7709 GND pin be tied to the
AGND plane of the system. In any layout, it is important that the
user keep in mind the flow of currents in the system ensuring
that the return paths for all currents are as close as possible to
the paths the currents took to reach their destinations. Avoid
forcing digital currents to flow through the AGND sections of
the layout.
The PWRGND pin is tied internally to GND on the AD7709.
The PWRGND pad internally has a resistance of less than 50 mW
to the PWRGND pin, while the resistance back to the GND pad
is less than 3 W. This means that 19.5 mA of the maximum speci-
fied current (20 mA) will flow to PWRGND with the remaining
0.5 mA flowing to GND. PWRGND and GND should be tied
together at the AD7709, and it is important to minimize the
resistance on the ground return lines.
Avoid running digital lines under the device since these will
couple noise onto the die. The analog ground plane should be
allowed to run under the AD7709 to prevent noise coupling.
The power supply lines to the AD7709 should use as wide a trace as
possible to provide low impedance paths and reduce the effects of
glitches on the power supply line. Fast switching signals like
clocks should be shielded with digital ground to avoid radiating
noise to other sections of the board, and clock signals should
never be run near the analog inputs. Avoid crossover of digital
and analog signals. Traces on opposite sides of the board should
run at right angles to each other, which will reduce the effects of
feedthrough through the board. A microstrip technique is by far the
best, but is not always possible with a double-sided board. In
this technique, the component side of the board is dedicated to
ground planes while signals are placed on the solder side.
Good decoupling is important when using high resolution ADCs.
The supply should be decoupled with 10 mF tantalum in parallel
with 0.1 mF capacitors to GND. To achieve the best from these
decoupling components, they have to be placed as close as possible;
chips should be decoupled with 0.1 mF ceramic capacitors to DGND.
APPLICATIONS
The AD7709 provides a low cost, high resolution, analog-to-digital
function. Because the analog-to-digital function is provided by a
environments, making it ideal for use in sensor measurement and
industrial and process control applications. Given the architecture
used in the AD7709, where the signal chain is chopped and the
device is factory-calibrated at final test, field calibration is not
needed due to the extremely low offset and gain drifts exhibited
by this converter. It also provides a programmable gain amplifier
and a digital filter. Thus, it provides far more system-level func-
tionality than off-the-shelf integrating ADCs without the
disadvantage of having to supply a high quality integrating
capacitor. In addition, using the AD7709 in a system allows the
REV. A
- architecture, it makes the part more immune to noisy
–25–
system designer to achieve a much higher level of resolution because
noise performance of the AD7709 is significantly better than that
of integrating ADCs.
The on-chip PGA allows the AD7709 to handle an analog input
voltage range as low as 10 mV full scale with V
differential inputs of the part allow this analog input range to
have an absolute value anywhere between GND + 100 mV and
V
directly to the input of the AD7709. The programmable gain
front end on the AD7709 allows the part to handle unipolar
analog input ranges from 0 mV to 20 mV and 0 V to 2.5 V
and bipolar inputs of ± 20 mV to ± 2.5 V. Because the part oper-
ates from a single supply, these bipolar ranges are with respect
to a biased-up differential input.
Pressure Measurement
One typical application of the AD7709 is pressure measurement.
Figure 18 shows the AD7709 used with a pressure transducer,
the BP01 from Sensym. The pressure transducer is arranged in
a bridge network and gives a differential output voltage between
its OUT(+) and OUT(–) terminals. With rated full-scale pres-
sure (in this case 300 mmHg) on the transducer, the differential
output voltage is 3 mV/V of the input voltage (i.e., the voltage
between its IN(+) and IN(–) terminals).
Assuming a 5 V excitation voltage, the full-scale output range
from the transducer is 15 mV. The excitation voltage for the
bridge can be used to directly provide the reference for the ADC
as the reference input range includes the supply. Alternatively, a
suitable resistor divider can be implemented that allows the full
dynamic range of the input to be utilized in this application.
This implementation is fully ratiometric, so variations in the
excitation voltage do not introduce errors in the system. Choosing
resistor values of 10 kW and 6 kW as per Figure 18 gives a 1.875 V
reference voltage for the AD7709 when the excitation voltage is 5 V.
Using the part with a programmed gain of 128 results in the
full-scale input span of the AD7709 being 15 mV, which corre-
sponds with the output span from the transducer.
DD
Figure 18. Pressure Measurement Using the AD7709
– 100 mV. It allows the user to connect the transducer
OUT–
EXCITATION VOLTAGE = 5V
IN–
IN+
OUT+
10k
6k
P1
AIN1
AIN2
REFIN1(+)
REFIN2(–)
GND
REF
AD7709
V
PWRGND
DD
AD7709
= 1.25 V. The