AD7709ARUZ Datasheet AD7709BRUZ, AD7709ARUZ-REEL7, AD7709BRUZ-REEL7 | P25-P27

REV. A

AD7709

–24–

When the ADC is configured for bipolar operation, the coding

is offset binary with a negative full-scale voltage resulting in a

code of 000 . . . 000, a zero differential voltage resulting in a

code of 100 . . . 000, and a positive full-scale voltage resulting in

a code of 111 . . . 111. The output code from the ADC for any

analog input voltage can be represented as follows:

Code AIN GAIN V

REF

=¥ ¥ ¥

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21024 1

where:

AIN is the analog input voltage.

GAIN in the PGA gain, i.e., 1 on the ± 2.56 V range and 128

on the ± 20 mV range.

N = 16.

Excitation Currents

The AD7709 also contains three software configurable constant

current sources. IEXC1 and IEXC2 provide 200 mA of current

while IEXC3 provides 25 mA of current. All source current

from V

is directed to either the IOUT1 or IOUT2 pins of the

device. These current sources are controlled via bits in the

Configuration Register. The configuration bits enable the current

sources, and they can be configured to source current individually

to both pins or a combination of currents, i.e., 400 mA, 225 mA, or

425 mA to either of the selected output pins. These current sources

can be used to excite external resistive bridge or RTD sensors.

Crystal Oscillator

The AD7709 is intended for use with a 32.768 kHz watch crys-

tal. A PLL internally locks onto a multiple of this frequency to

provide a stable 4.194304 MHz clock for the ADC. The modu-

lator sample rate is the same as the crystal oscillator frequency.

The start-up time associated with 32.768 kHz crystals is typically

300 ms. The OSCPD bit in the Communications Register can

be used to prevent the oscillator from powering down when the

AD7709 is placed in power-down mode. This avoids having to

wait 300 ms after exiting power-down to start a conversion at the

expense of raising the power-down current.

Reference Input

The AD7709 has a fully differential reference input capability

for the channel. On the channel, the reference inputs can be

REFIN1(+) and REFIN1(–) or REFIN2(+) and REFIN2(–).

They provide a differential reference input capability. The

common-mode range for these differential inputs is from GND

to V

. The reference input is unbuffered and therefore

excessive R-C source impedances will introduce gain errors.

The nominal reference voltage, V

REF

, ((REFIN1(+)

– REFIN1(–) or (REFIN2(+) – REFIN2(–)), for specified

operation is 2.5 V, but the AD7709 is functional with reference

voltages from 1 V to V

. In applications where the excitation

(voltage or current) for the transducer on the analog input also

drives the reference voltage for the part, the effect of the low

frequency noise in the excitation source will be removed

because the application is ratiometric. If the AD7709 is used

in a nonratiometric application, a low noise reference should be

used. Recommended reference voltage sources for the AD7709

include the AD780, REF43, and REF192. It should also be noted

that the reference inputs provide a high impedance, dynamic load.

Because the input impedance of each reference input is dynamic,

resistor/capacitor combinations on these inputs can cause dc gain

errors, depending on the

output impedance of the source that is

driving the reference inputs.

Reference voltage sources like those

recommended above (e.g.,

AD780) will typically have low output

impedances and are therefore

tolerant to having decoupling capaci-

tors on the REFIN(+)

without

introducing gain errors in the system.

Deriving the reference input

voltage across an external resistor, as

shown in Figure 18, will mean

that the reference input sees a

significant external source impedance.

External decoupling on the

REFIN pins would not be recommended

in this type of circuit

configuration.

Reset Input

The RESET input on the AD7709 resets all the logic, the digital

filter, and the analog modulator while all on-chip registers are reset

to their default state. RDY is driven high and the AD7709 ignores

all communications to any of its registers while the RESET

input is low. When the RESET input returns high, the AD7709

operates with its default setup conditions and it is necessary to

set up all registers after a RESET command.

Power-Down Mode

Loading 0 to the STBY bit in the ADC Communications Register

places the AD7709 in device power-down mode. The AD7709

retains the contents of all its on-chip registers (including the data

The device power-down mode does not affect the digital interface,

but it does affect the status of the RDY pin. Putting the AD7709

into power-down mode will reset the RDY line high. Placing the

part in power-down mode reduces the total current to 26 mA

typical when the part is operated at 5 V with the oscillator running

during power-down mode. With the oscillator shut down, the total

is 1.5 mA typical at 3 V and 6.5 mA typical at 5 V.

Grounding and Layout

Since the analog inputs and reference inputs on the ADC are

differential, most of the voltages in the analog modulator are

common-mode voltages. The excellent common-mode rejection

of the part will remove common-mode noise on these inputs.

The digital filter will provide rejection of broadband noise on

the power supply, except at integer multiples of the modulator

sampling frequency. The digital filter also removes noise from

the analog and reference inputs, provided these noise sources do

not saturate the analog modulator. As a result, the AD7709 is

more immune to noise interference than a conventional high

resolution converter. However, because the resolution of the

AD7709 is so high, and the noise levels from the AD7709 so

low, care must be taken with regard to grounding and layout.

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 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

⌺-⌬ 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 reso

lution 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

REF

= 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

– 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 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.

AD7709

IN+

OUT+

OUT–

IN–

10k

EXCITATION VOLTAGE = 5V

AIN1

AIN2

REFIN1(+)

PWRGND

GND

REFIN2(–)

6k

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.

REV. A

AD7709

–26–

A second key advantage to using the AD7709 in transducer-based

applications is that the on-chip low-side power switch can be fully

utilized in low power applications. The low-side power switch is

connected in series with the cold side of the bridge. In normal

operation, the switch is closed and measurements can be taken

from the bridge. In applications where power is a concern, the

AD7709 can be put into low power mode, substantially reducing

the power burned in the application. In addition to this, the power

switch can be opened while in low power mode, thus avoiding

the unnecessary burning of power in the front end transducer.

When taken back out of power-down, and the power switch is

closed, the user should ensure that the front end circuitry is fully

settled before attempting a read from the AD7709.

The circuit in Figure 19 shows a method that utilizes three

pseudo-differential input channels on the AD7709 to temperature-

compensate a pressure transducer.

OUT(+)

OUT(–)

IN(–)

IN(+)

PRESSURE

BRIDGE

XTAL1

XTAL2

IOUT1

6.25k

REFIN(+)

REFIN(–)

AIN2

AIN1

AIN3

AINCOM

GND

AD7709

250

Figure 19. Temperature-Compensating a Pressure

Transducer

In this application, pseudo-differential input channel AIN1/

AINCOM is used to measure the bridge output while pseuo-

differential channels AIN2/AINCOM and AIN3/AINCOM

measure the voltage across the bridge. The voltage measured

across the bridge will vary proportionally with temperature,

and the delta in this voltage can be used to temperature-

compensate the output of the pressure bridge.

Temperature Measurement

The AD7709 is also useful in temperature measurement appli-

cations. Figure 20 shows an RTD temperature measurement

application.

REFIN(–)

IOUT1

6.25k

AIN2

AIN1

AD7709

REFIN(+)

IOUT2

GND PWRGND

DRDY

SCLK

DIN

DOUT

XTAL1

XTAL2

REF

RL1

RL2

RL3

RL4

RTD

200A

CONTROLLER

Figure 20. 4-Wire RTD Temperature Measurement

Using the AD7709

In this application, the transducer is an RTD (Resistive Tem-

perature Device), a PT100. The arrangement is a 4-lead RTD

configuration. There are voltage drops across the lead resistances

RL1 and RL4, but these simply shift the common-mode voltage.

There is no voltage drop across lead resistances RL2 and RL3

since the input current to the AD7709 is very low, looking into a

high input impedance buffer. R

is included to shift the analog

input voltage to ensure that it lies within the common-mode

range (GND + 100 mV to V

– 100 mV) of the ADC. In the

application shown, the on-chip 200 mA current source provides

the excitation current for the PT100 and also generates the reference

voltage for the AD7709 via the 6.25 kW resistor. Variations in

the excitation current do not affect the circuit since both the

input

voltage and the reference voltage vary ratiometrically with the

excitation

current. However, the 6.25 kW resistor must

have a low

temperature coefficient to avoid errors in the reference

voltage

over temperature.

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21 P22-P24 P25-P27 P28-P30 P31-P33

AD7709ARUZ

Mfr. #:

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Manufacturer:

Analog Devices Inc.

Description:

Analog to Digital Converters - ADC 16-Bit w/ Current Sources

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AD7709ARUZ

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