AD698

REV. B

–5–

THEORY OF OPERATION

A block diagram of the AD698 along with an LVDT (linear

variable differential transformer) connected to its input is shown

in Figure 5 below. The LVDT is an electromechanical trans-

ducer—its input is the mechanical displacement of a core, and

its output is an ac voltage proportional to core position. Two

popular types of LVDTs are the half-bridge type and the series

opposed or four-wire LVDT. In both types the moveable core

couples flux between the windings. The series-opposed con-

nected LVDT transducer consists of a primary winding ener-

gized by an external sine wave reference source and two

The output voltage across the series secondary increases as the core

is moved from the center. The direction of movement is detected

by measuring the phase of the output. Half-bridge LVDTs have a

single coil with a center tap and work like an autotransformer. The

excitation voltage is applied across the coil; the voltage at the center

tap is proportional to position. The device behaves similarly to a

resistive voltage divider.

A

B

AMP

OSCILLATOR

VOLTAGE

REFERENCE

A

B

FILTER

AMP

AD698

Figure 5. Functional Block Diagram

The AD698 energizes the LVDT coil, senses the LVDT output

voltages and produces a dc output voltage proportional to core

position. The AD698 has a sine wave oscillator and power am-

plifier to drive the LVDT. Two synchronous demodulation

stages are available for decoding the primary and secondary

voltages. A decoder determines the ratio of the output signal

voltage to the input drive voltage (A/B). A filter stage and out-

put amplifier are used to scale the resulting output.

The oscillator comprises a multivibrator that produces a triwave

output. The triwave drives a sine shaper that produces a low dis-

tortion sine wave. Frequency and amplitude are determined by a

single resistor and capacitor. Output frequency can range from

20 Hz to 20 kHz and amplitude from 2 V to 24 V rms. Total har-

monic distortion is typically –50 dB.

The AD698 decodes LVDTs by synchronously demodulating

the amplitude modulated input (secondaries), A, and a fixed in-

put reference (primary or sum of secondaries or fixed input), B.

A common problem with earlier solutions was that any drift in

the amplitude of the drive oscillator corresponded directly to a

calculating the ratio of the LVDT output to its input excitation in

order to cancel out any drift effects. This device differs from the

AD598 LVDT signal conditioner in that it implements a different

circuit transfer function and does not require the sum of the LVDT

secondaries (A + B) to be constant with stroke length.

The AD698 block diagram is shown below. The inputs consist

of two independent synchronous demodulation channels. The B

channel is designed to monitor the drive excitation to the LVDT.

The full wave rectified output is filtered by C2 and sent to the

computational circuit. Channel A is identical except that the

comparator is pinned out separately. Since the A channel may

reach 0 V output at LVDT null, the A channel demodulator is

usually triggered by the primary voltage (B Channel). In addi-

tion, a phase compensation network may be required to add a

phase lead or lag to the A Channel to compensate for the LVDT

primary to secondary phase shift. For half-bridge circuits the

phase shift is noncritical, and the A channel voltage is large

enough to trigger the demodulator.

AD698

COMP

±1

FILTER

B

CHANNEL

–BIN

+BIN

DUTY CYCLE

DIVIDER

A/B = 1 = 100%

DUTY

±1

–ACOMP

+ACOMP

–AIN

+AIN

FILTER

DEMODULATOR

A

CHANNEL

A

B

OFF 2

OFF 1

BFILT1

BFILT2

C2

IREF

500µA

V

OUT

FILTER

C4

FB

R2

C5

AFILT2

AFILT1

C3

V/I

COMP

V/I

Figure 6. AD698 Block Diagram

Once both channels are demodulated and filtered a division cir-

cuit, implemented with a duty cycle multiplier, is used to calcu-

late the ratio A/B. The output of the divider is a duty cycle.

When A/B is equal to 1, the duty cycle will be equal to 100%.

(This signal can be used as is if a pulse width modulated output

is required.) The duty cycle drives a circuit that modulates and

filters a reference current proportional to the duty cycle. The

output amplifier scales the 500

µA reference current converting

it to a voltage. The output transfer function is thus: