AD7705 datasheet(29/32 Pages) AD

29 / 32 page

AD7705/AD7706

–29–

REV. A

APPLICATIONS

The AD7705 provides a dual channel, low cost, high resolution

analog-to-digital function. Because the analog-to-digital func-

tion is provided by a sigma-delta architecture, it makes the part

more immune to noisy environments thus making the part ideal

for use in industrial and process control applications. It also

provides a programmable gain amplifier, a digital filter and

calibration options. Thus, it provides far more system level

functionality than off-the-shelf integrating ADCs without the

disadvantage of having to supply a high quality integrating ca-

pacitor. In addition, using the AD7705 in a system allows the

system designer to achieve a much higher level of resolution

because noise performance of the AD7705 is better than that of

the integrating ADCs.

The on-chip PGA allows the AD7705 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 and VDD

when the part is operated in unbuffered mode. It allows the user

to connect the transducer directly to the input of the AD7705.

The programmable gain front end on the AD7705 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.

AD7705

VDD

+5V

AIN1(+)

AIN1(–)

EXCITATION VOLTAGE = +5V

OUT(–)

IN–

OUT(+)

IN+

AIN2(+)

AIN2(–)

24k

THERMOCOUPLE

JUNCTION

15k

REF IN(+)

REF IN(–)

GND

DOUT DIN

CS SCLK

MCLK IN

MCLK OUT

RESET

DRDY

Figure 22. Pressure Measurement Using the AD7705

Pressure Measurement

One typical application of the AD7705 is pressure measure-

ment. Figure 22 shows the AD7705 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 pressure (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 is also used to generate the reference voltage for the

AD7705. Therefore, variations in the excitation voltage do

not introduce errors in the system. Choosing resistor values

of 24 k

Ω and 15 kΩ, as per Figure 22, gives a 1.92 V reference

voltage for the AD7705 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 AD7705 being 15 mV, which corre-

sponds with the output span from the transducer. The second

channel on the AD7705 can be used as an auxiliary channel to

measure a secondary variable such as temperature as shown in

Figure 22. This secondary channel can be used as a means of

adjusting the output of the primary channel, thus removing

temperature effects in the system.

AD7705

VDD

+5V

AIN1(+)

AIN1(–)

THERMOCOUPLE

JUNCTION

REF IN(+)

REF IN(–)

GND

MCLK IN

MCLK OUT

RESET

DRDY

REF192

GND

OUTPUT

DOUT DIN

SCLK

+5V

Figure 23. Temperature Measurement Using the AD7705

Temperature Measurement

Another application area for the AD7705 is in temperature

measurement. Figure 23 outlines a connection from a thermo-

couple to the AD7705. In this application, the AD7705 is oper-

ated in its buffered mode to allow large decoupling capacitors

on the front end to eliminate any noise pickup that may have

been in the thermocouple leads. When the AD7705 is operated

in buffered mode, it has a reduced common-mode range. In

order to place the differential voltage from the thermocouple on

a suitable common-mode voltage, the AIN1(–) input of the

AD7705 is biased up at the reference voltage, +2.5 V.

Figure 23 shows another temperature measurement application

for the AD7705. In this case, the transducer is an RTD (Re-

sistive Temperature 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 as the input current to the AD7705 is very low.

The lead resistances present a small source impedance so it

would not generally be necessary to turn on the buffer on the

AD7705. If the buffer is required, the common-mode voltage

should be set accordingly by inserting a small resistance be-

tween the bottom end of the RTD and GND of the AD7705.

In the application shown, an external 400

µA current source

provides the excitation current for the PT100 and also generates

the reference voltage for the AD7705 via the 6.25 k

Ω resistor.

Variations in the excitation current do not affect the circuit as

both the input voltage and the reference voltage vary radiometri-

cally with the excitation current. However, the 6.25 k

Ω resistor

must have a low temperature coefficient to avoid errors in the

reference voltage over temperature.

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