–5–

REV. A

AD2S93

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.

DEMODIN

CLKOUT

DEMOD OUT

ERROR

AMP

LOS

LATCHES

FREQUENCY

SHAPING

VCO

PHASE

SENSITIVE

DEMODULATOR

C1

C2

R2

VEL

UP-DOWN

COUNTER

REFERENCE

(PRIMARY

EXCITATION)

DIFFERENTIAL

(SECONDARY

VOLTAGE)

REF

A

B

LOS

GAIN

V

DD

OVR

UNR

NULL

CS

DATA

SCLK

ACERROR

AC RATIO

BRIDGE

DIFF

DECODE

LOGIC

C3

AD2S93

INTIN

R4

R3

SERIAL

INTERFACE

DIR

R5

R1

C4

R6

R7

VCO GAIN

Figure 1. Functional Block Diagram

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 ≤ +REF/2, (± is used to de-

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-

±FSR =

( A

− B )