Datasheet

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 V
REF+
input, the inverting input to the compara-
tor is positive, and if it is in phase with V
REF−
, 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
DB0DB15
R1
100kΩ
C1
1µF
PROCESSOR DATA BUS
SIGNAL
GROUND
TO
PROCESSOR PORT
*ADDITIONAL PINS OMITTED FOR CLARITY
x ASIN ω t
–(1–x) ASIN ω t
A
SIN ω t
AD630*
DGND
V
OUT
R
IN
V
REF+
V
REF–
6
20
7
5
8
10
13
10
9
16
3
08490-027
Figure 25. AD7846 in Position Measurement Application