AD7191 datasheet(19/20 Pages) AD

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AD7191

Rev. A | Page 19 of 20

APPLICATIONS INFORMATION

The AD7191 provides a low-cost, high resolution analog-to-

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

provided by a ∑-Δ architecture, the part is more immune to

noisy environments, making it ideal for use in sensor

measurement and industrial and process control applications.

WEIGH SCALES

Figure 24 shows the AD7191 being used in a weigh scale

application. The load cell is arranged in a bridge network and

gives a differential output voltage between its OUT+ and OUT–

terminals. Assuming a 5 V excitation voltage, the full-scale

output range from the transducer is 10 mV when the sensitivity

is 2 mV/V. The excitation voltage for the bridge can be used to

directly provide the reference for the ADC because the reference

input range includes the supply voltage.

A second advantage of using the AD7191 in transducer-based

applications is that the bridge power-down switch can be fully

utilized in low power applications. The bridge power-down

switch is connected in series with the low side of the bridge. In

normal operation, the switch is closed and measurements can

be taken. In applications where power is of concern, the

AD7191 can be placed in power-down mode, thus significantly

reducing the power consumed in the application. In addition,

the bridge power-down switch is opened while in power-down

mode, thus avoiding unnecessary power consumption by the

front-end transducer. When the part is taken out of power-

down mode and the bridge power-down switch is closed, the

user should ensure that the front-end circuitry is fully settled

before attempting a read from the AD7191.

The load cell has an offset or TARE associated with it. This

TARE is the main component of the system offset (load cell plus

ADC) and is of a magnitude similar to the full-scale signal from

the load cell. For this reason, a system calibration that calibrates

the offset and full-scale error of the ADC plus the load cell is

required. A microprocessor can be used to perform the

calibrations. The offset (the conversion result from the AD7191

when no load is applied to the load cell) and the full-scale error

(the conversion result from the ADC when the maximum load

is applied to the load cell) must be determined. Subsequent

conversions from the AD7191 are then corrected using the

offset and full-scale coefficients calculated from the above

calibrations.

EMI RECOMMENDATIONS

For simplicity, the EMI filters are not included in Figure 24.

However, an R-C antialias filter should be included on each

analog input. This filter is needed because the on-chip digital

filter does not provide any rejection around MCLK/8 or

multiples of this frequency. Suitable values are a 100 Ω resistor in

series with each analog input, a 0.1 μF capacitor from AIN(+) to

AIN(−), and 0.01 μF capacitors from AIN(+)/AIN(−) to AGND

(see Figure 23).

0.01µF

100

Ω

0.1µF

AD7191

100

Ω

AIN(+): AIN1 OR AIN3

AIN(–): AIN2 OR AIN4

0.01µF

Figure 23. External Filtering Connections

MUX

AD7191

AIN1

DOUT/RDY

SCLK

PDOWN

CHAN

CLKSEL

PGA2

REFERENCE

DETECT

PGA1

AGND

IN+

OUT+

IN–

OUT–

REFIN(+)

REFIN(–)

BPDSW

AVDD

DVDD

DGND

AIN2

AIN3

AIN4

MCLK1

MCLK2

ODR2 ODR1 TEMP

TEMPERATURE

SENSOR

CLOCK

CIRCUITRY

PGA

SERIAL

INTERFACE

AND CONTROL

LOGIC

Σ-Δ

ADC

Figure 24. Typical Application (Weigh Scale)

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