REV. C

AD624

–13–

WEIGH SCALE

Figure 44 shows an example of how an AD624 can be used to

condition the differential output voltage from a load cell. The

10% reference voltage adjustment range is required to accom-

modate the 10% transducer sensitivity tolerance. The high

linearity and low noise of the AD624 make it ideal for use in

applications of this type particularly where it is desirable to

measure small changes in weight as opposed to the absolute

value. The addition of an autogain/autotare cycle will enable the

system to remove offsets, gain errors, and drifts making possible

true 14-bit performance.

G100

G200

G500

AD624

+INPUT

–INPUT

R5

3M

R6

100k

ZERO ADJUST

(COARSE)

A/D

CONVERTER

+10V FULL

SCALE

OUTPUT

REFERENCE

SENSE

GAIN = 500

R4

10k

ZERO

ADJUST

(FINE)

100

R3

10

+15V

R1

30k

NOTE 2

10V

10%

R2

20k

R3

10k

SCALE

ERROR

ADJUST

AD584

+10V

+5V

+2.5V

VBG

TRANSDUCER

SEE NOTE 1

NOTES

1. LOAD CELL TEDEA MODEL 1010 10kG. OUTPUT 2mV/V 10%.

2. R1, R2 AND R3 SELECTED FOR AD584. OUTPUT 10V

10%.

+15V

AD707

2N2219

R7

100k

OUT

Figure 44. AD624 Weigh Scale Application

AC BRIDGE

Bridge circuits which use dc excitation are often plagued by

errors caused by thermocouple effects, l/f noise, dc drifts in the

electronics, and line noise pickup. One way to get around these

problems is to excite the bridge with an ac waveform, amplify

the bridge output with an ac amplifier, and synchronously

demodulate the resulting signal. The ac phase and amplitude

information from the bridge is recovered as a dc signal at the

output of the synchronous demodulator. The low frequency

system noise, dc drifts, and demodulator noise all get mixed to

the carrier frequency and can be removed by means of a low-

pass filter. Dynamic response of the bridge must be traded off

against the amount of attenuation required to adequately sup-

press these residual carrier components in the selection of the

filter.

Figure 45 is an example of an ac bridge system with the AD630

used as a synchronous demodulator. The oscilloscope photo-

graph shows the results of a 0.05% bridge imbalance caused by

the 1 Meg resistor in parallel with one leg of the bridge. The top

trace represents the bridge excitation, the upper middle trace is

the amplified bridge output, the lower-middle trace is the out-

put of the synchronous demodulator and the bottom trace is the

filtered dc system output.

This system can easily resolve a 0.5 ppm change in bridge

impedance. Such a change will produce a 6.3 mV change in the

low-pass filtered dc output, well above the RTO drifts and noise.

The AC-CMRR of the AD624 decreases with the frequency of

the input signal. This is due mainly to the package-pin capaci-

tance associated with the AD624’s internal gain resistors. If

AC-CMRR is not sufficient for a given application, it can be

trimmed by using a variable capacitor connected to the amplifier’s

AD624C

G = 1000

10k

1kHz

BRIDGE

EXCITATION

1M

1k

1k

1k

1k

4–49pF

CERAMIC ac

BALANCE

CAPACITOR

–V

10k

B

10k

5k

2.5k

PHASE

SHIFTER

AD630

MODULATED

OUTPUT

SIGNAL

MODULATION

INPUT

CARRIER

INPUT

2.5k

B

A

COMP

Figure 45. AC Bridge

0V

0V

0V

0V

BRIDGE EXCITATION

(20V/div) (A)

AMPLIFIED BRIDGE

OUTPUT (5V/div) (B)

DEMODULATED BRIDGE

OUTPUT (5V/div) (C)

FILTER OUTPUT

2V/div) (D)

2V

Figure 46. AC Bridge Waveforms