AD2S93BP AD [Analog Devices], AD2S93BP Datasheet - Page 5

• Manufacturer Part Number: AD2S93BP
• Description: Low Cost LVDT-to-Digital Converter
• Manufacturer: AD [Analog Devices]

GLOSSARY OF TERMS
INTEGRAL LINEARITY
Integral linearity deviation as a percent of full scale. A 0.1% de-
viation is equivalent to 8-LSB change on the output.
Gain
The converter gain is the maximum variation in the ratio of
A–B/REF/2 to the maximum digital input.
Output Offset
The output offset is the digital output code when the analog in-
put signal A–B = 0.
Overrange (OVR)
OVR goes high when A–B is in phase with REF and larger than
REF/2.
Underrange (UNR)
UNR goes high when A–B is out of phase with REF and larger
than REF/2.
PRINCIPLE OF OPERATION
The AD2S93 is based on a Type 2 tracking closed-loop prin-
ciple. The output tracks the position of the LVDT without the
need for external convert and wait states. As the transducer
moves through a position equivalent to the least significant bit
weighting, the output is updated by one LSB. On the AD2S93,
CLKOUT updates corresponding to one LSB increment. Fig-
ure 1 illustrates the principle of operation.
REV. A
DIFFERENTIAL
(SECONDARY
V
EXCITATION)
DD
REFERENCE
VOLTAGE)
R4
(PRIMARY
R3
DATA
SCLK
NULL
GAIN
OVR
DIFF
UNR
LOS
REF
CS
Figure 1. Functional Block Diagram
A
B
DECODE
LOS
LOGIC
ACERROR
ERROR
INTERFACE
SERIAL
AMP
AC RATIO
LATCHES
BRIDGE
UP-DOWN
COUNTER
R6
C3
DEMODIN
DEMODULATOR
C4
FREQUENCY
SENSITIVE
SHAPING
PHASE
R1
AD2S93
R5
VCO
CLKOUT
DEMOD OUT
INTIN
VEL
VCO GAIN
DIR
C1
R7
R2
C2
–5–
Because the conversion depends on the ratio of the input signals
(ratiometric ac bridge), the AD2S93 is remarkably tolerant of
input amplitude and frequency. This, combined with the defin-
able Type 2 tracking closed-loop guarantees the AD2S93's re-
peatability for a given input. A phase sensitive detector,
integrator and voltage controlled oscillator (VCO) form a closed
loop system which seeks to null the output of the ACERROR.
When this is accomplished the word state of the up/down
counter equals within the rated accuracy of the converter, the
LVDT position output.
For more information on the operation of the converter, see
“Circuit Dynamics” section.
DATA FORMAT
OPERATING RANGE
The AD2S93 operating range is defined in Figure 2. The lin-
earity and specified operating range of the converter is the cen-
tral two 12-bit quadrants through zero. The corresponding
input relationship is –REF/2 A–B
note phase coherency). The sign bit is low for inputs with A–B
in phase with REF. The two remaining 12-bit quadrants are
used to denote over (OVR) and underrange (UNR). OVR goes
high when A–B is in phase with REF and larger than REF/2.
UNR goes high when A–B is out of phase with REF and larger
than REF/2. LOS is an open drain output which pulls high
when A and/or B are removed or REF is removed (see “Inbuilt
Diagnostics”), or A + B is less than 100 mV.
SCALING THE INPUTS
In order to match the LVDT output to the AD2S93 output, the
inputs to the AD2S93 need to be scaled. The operating range is
illustrated in Figure 2. The AD2S93 operates across 12-bit
range where the remaining 12-bit quadrants are used to denote
overrange and underrange. The output position word is a func-
tion of the ratio between A–B and V
FSR
( A B )
V
REF
REF
/2
+REF/2, ( is used to de-
(see Figure 2) where:
AD2S93