AD7730LBRZ datasheet(42/53 Pages) AD

42 / 53 page

AD7730/AD7730L

–42–

Figure 25. AD7730 with Bipolar Excitation of the Bridge

Bipolar Excitation of the Bridge

As mentioned previously, some applications will require that the

AD7730 handle inputs from a bridge that is excited by a bipolar

voltage. The number of applications requiring this are limited,

but with the addition of some external components the AD7730

is capable of handling such signals. Figure 25 outlines one ap-

proach to the problem.

The example shown is a dc-excited bridge that is driven from

±5 V supplies. In such a circuit, two issues must be addressed.

The first is how to get the AD7730 to handle input voltages

near or below ground and the second is how to take the 10 V

excitation voltage which appears across the bridge and generate

a suitable reference voltage for the AD7730. The circuit of Figure

25 attempts to address these two issues simultaneously.

The AD7730’s analog and digital supplies can be split such that

AVDD and DVDD can be at separate potentials and AGND and

DGND can also be at separate potentials. The only stipulation

is that AVDD or DVDD must not exceed the AGND by 5.5 V.

In Figure 25, the DVDD is operated at +3 V, which allows the

AGND to go down to –2.5 V with respect to system ground.

This means that all logic signals to the part must not exceed 3 V

with respect to system ground. The AVDD is operated at +2.5 V

with respect to system ground.

The bridge is excited with 10 V across its inputs. The output of

the bridge is biased around the midpoint of the excitation volt-

ages which in this case is system ground or 0 V. In order for the

common-mode voltage of the analog inputs to sit correctly, the

AGND of the AD7730 must be biased below system ground by

a minimum of 1.2 V. The 10 V excitation voltage must be re-

duced to 5 V before being applied as the reference voltage for

the AD7730.

The resistor string R1, R2 and R3, takes the 10 V excitation

voltage and generates differential voltage of nominally 5 V.

Amplifiers A1 and A2 buffer the resistor string voltages and

provide the AVDD and AGND voltages as well as the REF IN(+)

and REF IN(–) voltages for the AD7730. The differential

reference voltage for the part is +5 V. The AD7730 retains its

ratiometric operation with this reference voltage varying in sym-

pathy with the analog input voltage.

The values of the resistors in the resistor string can be changed

to allow a larger DVDD voltage. For example, if R1 = 3 k

Ω,

R2 = 10 k

Ω and R3 = 7 kΩ, the AVDD and AGND voltages

become +3.5 V and –1.5 V respectively. This allows the AD7730

to be used with a +3.6 V DVDD voltage while still allowing the

analog input range to be within the specified common-mode

range.

An alternate scheme to this is to generate the AVDD and AGND

voltages from regulators or Zener diodes driven from the +5 V

and –5 V supplies respectively. The reference voltage for the

part would be generated in the same manner as just outlined but

amplifiers A1 and A2 would not be required to buffer the volt-

ages as they are now only driving the reference pins of the

AD7730. However, care must be taken in this scheme to ensure

that the REF IN(+) voltage does not exceed AVDD and that the

REF IN(–) voltage does not go below AGND.

+5V

–5V

SIGMA-

DELTA

MODULATOR

DVDD

AD7730

6-BIT

DAC

SERIAL INTERFACE

AND CONTROL LOGIC

CLOCK

GENERATION

PROGRAMMABLE

DIGITAL

FILTER

SIGMA-DELTA A/D CONVERTER

BUFFER

PGA

STANDBY

SYNC

MCLK IN

MCLK OUT

SCLK

DIN

DOUT

RESET

RDY

POL

DGND

MUX

CALIBRATION

MICROCONTROLLER

IN+

OUT–

IN–

OUT+

REF IN(+)

AVDD

AIN1(+)

AIN1(–)

SYSTEM

GROUND

+3V

+5V

–5V

+5V

–5V

10k

1/2 OP284

OR 1/2 OP213

1/2 OP284

OR 1/2 OP213

REF IN(–)

AGND

ALL VOLTAGE VALUES ARE WITH

RESPECT TO SYSTEM GROUND.

+/–

REV. B

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