AD7730BRUZ Datasheet AD7730BRUZ, AD7730BNZ, AD7730BRZ | P40-P42

AD7730/AD7730L

–40–

APPLICATIONS

The on-chip PGA allows the AD7730 to handle analog input

voltage ranges as low as 10 mV full scale. This allows the user to

connect a transducer directly to the input of the AD7730. The

AD7730 is primarily targeted for weigh-scale and load-cell

applications. The majority of the applications have a strain-

gage transducer whose resistance changes when subjected to

mechanical stress. Normally, the gages are configured in a

Wheatstone bridge arrangement. The strain gage is a passive

device and requires an excitation voltage (or in some cases a

current) to derive a voltage output. Two types of voltage excita-

tion can be provided for the bridge: dc excitation or ac excita-

tion. These are discussed in the following sections. While the

desire in most applications is to provide a single supply solution

(something that is aided by the AD7730’s single supply capabil-

ity), some applications provide a bipolar excitation voltage in

order to increase the output voltage from the bridge. In such

cases, the input voltage applied to the AD7730 can be slightly

negative with respect to ground. Figure 23 shows how to config-

ure the AD7730 to handle this type of input signal.

DC Excitation of Bridge

In dc-excitation applications, the excitation voltage provided for

the bridge is a fixed dc voltage. Connections between the AD7730

and the bridge are very straightforward in this type of applica-

tion as illustrated in Figure 23. The bridge configuration shown

is a six-lead configuration with separate return leads for the

reference lines. This allows a force/sense effect on the load cell

excitation voltage, eliminating voltage drops caused by the exci-

tation current flowing through the lead resistances. In applica-

tions where the lead lengths are short, a four-wire configuration

can be used with the excitation voltage and analog ground con-

nected local to the AD7730’s REF IN(+) and REF IN(–) termi-

nals. Illustrating a major advantage of the AD7730, the 5 V

excitation voltage for the bridge can be used directly as the refer-

ence voltage for the AD7730, eliminating the need for precision

matched resistors in generating a scaled-down reference.

The application is a ratiometric one with variations in the exci-

tation voltage being reflected in variations in the analog input

voltage and reference voltage of the AD7730. Because the

AD7730 is a truly ratiometric part, with the reference voltage

and excitation voltages equal, it is possible to evaluate its total

excitation voltage rejection. This is unlike other converters

which give a separate indication of the rejection of reference,

analog inputs and power supply. The combined (total) rejection

for the AD7730 when moving the excitation voltage (which was

also the power supply voltage) was better than 115 dB when

evaluated with a load cell simulator.

Drift considerations are a primary concern for load cell applica-

tions. It is recommended for these applications that the AD7730

is operated in CHOP mode to accrue the benefits of the excel-

lent drift performance of the part in CHOP mode. A common

source of unwanted drift effects are parasitic thermocouples.

Thermocouple effects are generated every time there is a junc-

tion of two dissimilar metals. All components in the signal path

should be chosen to minimize thermocouple effects. IC sockets

and link options should be avoided as much as possible. While

it is impossible to remove all thermocouple effects, attempts should

be made to equalize the thermocouples on each leg of the differen-

tial input to minimize the differential voltage generated.

Figure 23. Typical Connections for DC-Excited Bridge Application

SIGMA-

DELTA

MODULATOR

AD7730

6-BIT

DAC

SERIAL INTERFACE

AND CONTROL LOGIC

CLOCK

GENERATION

PROGRAMMABLE

DIGITAL

FILTER

SIGMA-DELTA A/D CONVERTER

BUFFER

PGA

AIN2(+)/D1

AIN2(–)/D0

ACX

STANDBY

SYNC

MCLK IN

MCLK OUT

SCLK

DIN

DOUT

RESET

RDY

POL

DGNDAGND

MUX

EXCITATION

CLOCK

CALIBRATION

MICROCONTROLLER

IN+

OUT–

IN–

OUT+

REF IN(+)

REF IN(–)

AIN1(+)

AIN1(–)

EXCITATION VOLTAGE = +5V

+/–

REV. B

AD7730/AD7730L

–41–

Long lead lengths from the bridge to the AD7730 facilitate the

pickup of mains frequency on the analog input, the reference

input and the power supply. The analog inputs to the AD7730

are buffered, which allows the user to connect whatever noise

reduction capacitors are necessary in the application. The AD7730

boasts excellent common-mode and normal- mode rejection of

mains frequency on both the analog and reference inputs. In

CHOP mode, care must be taken in choosing the output update

rate so it does not result in reducing line frequency rejection

(see DIGITAL FILTERING section). The input offset current

on the AD7730 is 10 nA maximum which results in a maxi-

mum, dc offset voltage of 1.75 mV in a 350 Ω bridge applica-

tion. Care should taken with inserting large source impedances

on the reference input pins as these inputs are not buffered and

the source impedances can result in gain errors.

In many load-cell applications, a portion of the dynamic range

of the bridge output is consumed by a pan weight or tare weight.

In such applications, the 6-bit TARE DAC of the AD7730 can

be used to adjust out this tare weight as outlined previously.

AC Excitation of Bridge

AC excitation of the bridge addresses many of the concerns with

thermocouple, offset and drift effects encountered in dc-excited

applications. In ac-excitation, the polarity of the excitation volt-

age to the bridge is reversed on alternate cycles. The result is the

elimination of dc errors at the expense of a more complex sys-

tem design. Figure 24 outlines the connections for an ac-excited

bridge application based on the AD7730.

The excitation voltage to the bridge must be switched on

alternate cycles. Transistors T1 to T4 in Figure 24 perform

the switching of the excitation voltage. These transistors can be

discrete matched bipolar or MOS transistors, or a dedicated

bridge driver chip such as the 4427 from Micrel can be used to

perform the task.

Since the analog input voltage and the reference voltage are

reversed on alternate cycles, the AD7730 must be synchronized

with this reversing of the excitation voltage. To allow the

AD7730 to synchronize itself with this switching, it provides the

logic control signals for the switching of the excitation voltage.

These signals are the nonoverlapping CMOS outputs ACX

and ACX.

One of the problems encountered with ac-excitation is the set-

tling time associated with the analog input signals after the

excitation voltage is switched. This is particularly true in appli-

cations where there are long lead lengths from the bridge to the

AD7730. It means that the converter could encounter errors

because it is processing signals which are not fully settled. The

AD7730 addresses this problem by allowing the user to program

a delay of up to 48.75 μs between the switching of the ACX

signals and the processing of data at the analog inputs. This is

achieved using the DL bits of the Filter Register.

The AD7730 also scales the ACX switching frequency in accor-

dance with the output update rate. This avoids situations where

the bridge is switched at an unnecessarily faster rate than the

system requires.

The fact that the AD7730 can handle reference voltages which

are the same as the excitation voltages is particularly useful in

ac-excitation where resistor divider arrangements on the

reference input add to the settling time associated with the

switching.

Figure 24. Typical Connections for AC-Excited Bridge Application

SIGMA-

DELTA

MODULATOR

AD7730

6-BIT

DAC

SERIAL INTERFACE

AND CONTROL LOGIC

CLOCK

GENERATION

PROGRAMMABLE

DIGITAL

FILTER

SIGMA-DELTA A/D CONVERTER

BUFFER

PGA

AIN2(+)/D1

AIN2(–)/D0

ACX

STANDBY

SYNC

MCLK IN

MCLK OUT

SCLK

DIN

DOUT

RESET

RDY

POL

DGNDAGND

MUX

EXCITATION

CLOCK

CALIBRATION

MICROCONTROLLER

IN+

OUT–

IN–

OUT+

REF IN(+)

REF IN(–)

AIN1(+)

AIN1(–)

EXCITATION VOLTAGE = +5V

T4T3

T2T1

+/–

REV. B

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

and DV

can be at separate potentials and AGND and

DGND can also be at separate potentials. The only stipulation

is that AV

or DV

must not exceed the AGND by 5.5 V.

In Figure 25, the DV

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 AV

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 AV

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 DV

voltage. For example, if R1 = 3 kΩ,

R2 = 10 kΩ and R3 = 7 kΩ, the AV

and AGND voltages

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

to be used with a +3.6 V DV

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 AV

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 AV

and that the

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

+5V

–5V

SIGMA-

DELTA

MODULATOR

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(+)

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

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21 P22-P24 P25-P27 P28-P30 P31-P33 P34-P36 P37-P39 P40-P42 P43-P45 P46-P48 P49-P51 P52-P53

AD7730BRUZ

Mfr. #:

Buy AD7730BRUZ

Manufacturer:

Analog Devices Inc.

Description:

Analog to Digital Converters - ADC CMOS 24-Bit Bridge Transducer

Lifecycle:

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AD7730BRUZ

AD7730BRUZ

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