AD7846
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.
ASIN ω t
LVDT
x ASIN ω t
V
5
6
OUT
V
7
REF+
R
IN
AD7846*
8
V
REF–
DGND
20
–(1–x) ASIN ω t
DB15 DB0
10
SIGNAL
GROUND
3
PROCESSOR DATA BUS
9
16
R1
100kΩ
10
13
AD630*
TO
C1
1µF
PROCESSOR PORT
*ADDITIONAL PINS OMITTED FOR CLARITY
Figure 25. AD7846 in Position Measurement Application
Rev. G | Page 15 of 24
ADI [ ADI ]
