LINEAR TECHNOLOGY LTC1967 Technical data

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Precision LVDT Signal Conditioning Using Direct RMS to DC
Conversion –

Design Note 362

Cheng-Wei Pei

Introduction

Linear variable differential transformers (LVDTs) are theo­retically infinite-resolution displacement measurement
devices. An LVDT operates by comparing the magnetic
flux coupled into two transformer secondary windings to
determine the displacement of a moving transformer
core. A low distortion sine wave acts as the input. The
amplitude and phase of the output signal across the two
secondary windings determine the distance and polarity
of the LVDT core with respect to the center.

Proper signal conditioning circuitry allows extremely pre­cise measurements in demanding applications, such as in
production manufacturing, fluid level measurements and
structural/strain testing. One common application is to
use an LVDT at the end of a Bourdon tube to measure
minute changes in system or barometric pressure.

The most common method of LVDT signal conditioning
is demodulation (i.e., full-wave rectification) and simple
lowpass filtering of the rectified sine wave. However, the

precision of the demodulation method depends on the
accuracy of the phase adjustment. In addition, there are
losses associated with demodulation, which usually
involve switches and the associated charge injection and
timing jitter. It is difficult to achieve 12-bit precision
under these circumstances.

A better approach to LVDT signal conditioning is shown in
Figure 1. The LTC

®

1967 true RMS to DC converter can
directly convert an LVDT output sine wave into a precise
DC voltage with 0.15% linearity error and 0.3% gain error.
The LTC1967’s performance is independent of input phase,
and maintains exceptional performance over tempera­ture. A separate circuit determines the phase of the LVDT
output, which can be used to determine the polarity of the
LVDT core position. True precision performance is achiev­able with this simple circuit and a minimal amount of
calibration.

, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.

04/05/362

PHASEPHASE LAG

SINE

INPUT

MOVABLE

TRANSFORMER

CORE

YELLOW-BLK

RED-YELLOW

•

SCHAEVITZ

E-100
LVDT

•

•

RED

BLUE

GREEN

BLACK

100k

4700pF

+
–

50Ω

1/2 LT1807

1µF

1k

1k

1k

2

3

5V

+
–

+
–

5V

7

LTC1967

1

1/2 LT1720

1/2 LT1720

1/2 LT1807

5V

+

–

5

6

74AC86

0.012µF

5V

2.2µF

10k

10k

6.65k

BIAS

AMPLITUDE

DISPLACEMENT
(AMPLITUDE)

DN362 F01

DIRECTION
(PHASE)

Figure 1. A New Approach to LVDT Signal Conditioning Which Combines an LTC1967 RMS to DC
Converter with a Separate Phase Detector Circuit

LVDT Operation

LVDTs are driven by a low distortion sine wave in the
primary winding of the transformer. In a 12-bit system,
the input sine wave needs less than –74dB distortion and
better than 0.02% amplitude stability. In the null (center)
position, the two secondary windings receive the same
amount of magnetic coupling, but the differential voltage
across them is not zero due to the flux leakage of the LVDT
(see Figure 2). When the LVDT core moves in one
direction or the other, the differential voltage amplitude
increases. The phase of the differential output changes
depending on which side of center the LVDT core sits.

Circuit Description

The LVDT shown in Figure 1, a Schaevitz E-100 with
±2.5mm of linear range, is driven by a low distortion,
amplitude-stable 3V

(the manufacturer’s recom-

RMS

mended amplitude) sine wave on the primary windings of
the transformer. The frequency, 10kHz, is the maximum
recommended for the E-100, though LVDTs exist that
work well up to hundreds of kilohertz. Use of higher
excitation frequencies with the higher-frequency LTC1968
RMS to DC converter would result in faster settling times
and reduced audio-frequency signals that could cause
interference.

To facilitate single supply operation, one-half of an LT

®

1807
amplifier biases the output sine wave DC level to fall within
the common mode range of the measurement circuit
(approximately 2V on a 5V supply). The other half of the
LT1807 buffers the LVDT output for good signal fidelity.
The LT1807 was chosen for its high open-loop gain at
10kHz, extremely low distortion and high common mode
rejection to maintain the accuracy of the LVDT amplitude
signal. The LT1807’s buffered output signal is converted
by an LTC1967 true RMS to DC converter to a DC signal
that is linearly proportional to the displacement of the
transformer core.

The phase detector portion of the circuit consists of a
phase adjustment network (which provides phase lead or
lag, depending on the specific LVDT and excitation fre­quency), an exclusive-OR (XOR) logic gate and an RC
lowpass filter network. The circuit output is high when the
LVDT core is on one side of center and low when it moves
to the other side. Two LT1720 comparators sense the
zero-crossing points of the phase-adjusted input and

output sine waves. The XOR gate has a low output when
the inputs agree and a high output when the inputs
disagree. The RC networks limit the bandwidth of the
phase network to 1.3kHz in order to limit the effect of
comparator output spikes due to slight phase mismatch.
It is recommended that the phase detector band-limiting
network be lower in frequency than the LVDT excitation
frequency, to minimize the phase output ripple.

Circuit Calibration

To calibrate the signal conditioning circuit, first move the
LVDT into the null (center) position. The center position is
where the amplitude output is at its minimum. Note the
output voltage of the phase detector; adjust the phase lead/
lag network until the output reaches approximately 2V. Note
the amplitude outputs at the extremes which may vary
between LVDTs.

Conclusion

Figure 2 shows the amplitude and phase outputs of the
circuit in Figure 1. A novel approach to LVDT signal
conditioning yields stable, precise performance and a low
IC count. Unlike a synchronous demodulation scheme,
the accuracy of the circuit does not hinge on a manual
phase adjustment—only on the built-in high precision of
the LTC1967 (or LTC1968) true RMS to DC converter. The
circuit is robust enough for many industrial and instru­mentation applications—it maintains good precision over
temperature and it is immune to input signal phase. A
separate, easily-calibrated phase detector determines
which side of center the LVDT core occupies.

700

600

)

500

RMS

400

300

AMPLITUDE (V

200

100

0

–3

–2 –1

AMPLITUDE

PHASE

13

02

DISTANCE (mm)

Figure 2. Amplitude and Phase Output Versus Position of
the LVDT Circuit. The Non-Zero Center Amplitude is Due to
Flux Leakage in the LVDT and is Not Caused by the
Measurement Circuit

DN362 F02

5.0

4.5

4.0

PHASE OUTPUT (V)

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

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