AD7709 datasheet(25/32 Pages) AD

25 / 32 page

REV. A

AD7709

–25–

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 m

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

architecture, it makes the part more immune to noisy

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

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 VREF = 1.25 V. The

differential inputs of the part allow this analog input range to

have an absolute value anywhere between GND + 100 mV and

VDD – 100 mV. It allows the user to connect the transducer

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 k

W and 6 kW as per Figure 18 gives a 1.875 V

reference voltage for the AD7709 when the excitation voltage is 5 V.

AD7709

IN+

OUT+

OUT–

IN–

10k

EXCITATION VOLTAGE = 5V

VDD

AIN1

AIN2

REFIN1(+)

PWRGND

GND

REFIN2(–)

Figure 18. Pressure Measurement Using the AD7709

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.

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