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AD7569/AD7669

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REV. B

TARE FUNCTION FOR WEIGH SCALE—AD7569

The infinite sample-and-hold just outlined can also form the ba-

sis of a circuit to provide a tare function for a weigh scale sys-

tem. Figure 30 shows a circuit for a weigh scale system. It

incorporates a tare function using a simple circuit based on the

AD7569.

The AD587, along with the 2N6285, provides a buffered +10 V

reference to supply the low impedance load cell transducer. The

load cell output is amplified by the AD624 precision instrumen-

tation amplifier with gain adjustment provided by R1. The out-

put of the AD624 is applied to the noninverting input of a unity

gain differential summing amplifier that uses the AD707, a high

precision op amp with low drift. The AD707 feeds a 3 1/2 digit

panel meter module that converts the signal for digital readout.

The input signal to the panel meter is also applied to the analog

input of the AD7569 for the tare function. When the tare switch

(S1) is closed, a tare cycle commences and VIN is sampled and

held infinitely at VOUT until the next tare cycle. VOUT drives the

inverting input of the differential amplifier and forces its output

to zero. Thus, the tare function is used to give a readout of zero

for any undesired weight, such as a box, when only the item

placed in it is to be weighed. The tare function can also be used

in calibrating the system, to cancel out offset errors due to the

load cell, AD624 and differential amplifier.

The AD7569 offers many advantages in the system outlined,

such as: simple, low cost circuit—no need for microprocessor,

software, etc.—and low power consumption.

Figure 30. Weigh Scale System with Tare Function

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