AD7846APZ datasheet(15/24 Pages) AD

15 / 24 page

AD7846

Rev. G | Page 15 of 24

POSITION MEASUREMENT APPLICATION

Figure 25 shows the AD7846 in a position measurement applica-

tion using an linear variable displacement transducer (LVDT),

an AD630 synchronous demodulator and a comparator to make

a 16-bit LVDT-to-digital converter. The LVDT is excited with a

fixed frequency and fixed amplitude sine wave (usually 2.5 kHz,

2 V p-p). The outputs of the secondary coil are in antiphase and

their relative amplitudes depend on the position of the core in the

LVDT. The AD7846 output interpolates between these two inputs

in response to the DAC input code. The AD630 is set up so that

it rectifies the DAC output signal. Thus, if the output of the DAC is

in phase with the VREF+ input, the inverting input to the compara-

tor is positive, and if it is in phase with VREF−, the output is nega-

tive. By turning on each bit of the DAC in succession starting

with the MSB and deciding to leave it on or turn it off based on

the comparator output, a 16-bit measurement of the core position

is obtained.

AD7846*

LVDT

DB0

DB15

100kΩ

1µF

PROCESSOR DATA BUS

SIGNAL

GROUND

PROCESSOR PORT

*ADDITIONAL PINS OMITTED FOR CLARITY

x ASIN ω t

–(1–x) ASIN ω t

ASIN ω t

AD630*

DGND

VOUT

RIN

VREF+

VREF–

Figure 25. AD7846 in Position Measurement Application

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