REV. B

–6–

AD698

CONNECTING THE AD698

The AD698 can easily be connected for dual or single supply

operation as shown in Figures 7, 8 and 13. The following gen-

eral design procedures demonstrate how external component

values are selected and can be used for any LVDT that meets

AD698 input/output criteria. The connections for the A and B

channels and the A channel comparators will depend on which

transducer is used. In general follow the guidelines below.

Parameters set with external passive components include: exci-

tation frequency and amplitude, AD698 input signal frequency,

and the scale factor (V/inch). Additionally, there are optional

features; offset null adjustment, filtering, and signal integration,

which can be implemented by adding external components.

R1

C1

15nF

C2

C3

R4

R3

13

16

15

14

24

23

22

21

20

19

18

17

12

11

10

9

8

1

2

3

4

7

6

5

AD698

EXC1

EXC2

LEV1

LEV2

FREQ1

BFILT1

BFILT2

–BIN

+BIN

–AIN

FREQ2

SIG REF

OFFSET2

OFFSET1

OUT FILT

FEEDBACK

SIG OUT

–ACOMP

AFILT2

AFILT1

+ACOMP

+AIN

C4

R2

33k

Ω

1000pF

SIGNAL

REFERENCE

100nF

6.8µF

–15V

+15V

100nF

6.8µF

Figure 7. Interconnection Diagram for Half-Bridge LVDT

and Dual Supply Operation

DESIGN PROCEDURE

DUAL SUPPLY OPERATION

Figure 7 shows the connection method for half-bridge LVDTs.

Figure 8 demonstrates the connections for 3- and 4-wire

LVDTs connected in the series opposed configuration. Both ex-

amples use dual

±15 volt power supplies.

A. Determine the Oscillator Frequency

Frequency is often determined by the required BW of the sys-

tem. However, in some systems the frequency is set to match

the LVDT zero phase frequency as recommended by the

manufacturer; in this case skip to Step 4.

1. Determine the mechanical bandwidth required for LVDT

2. Select minimum LVDT excitation frequency approximately

10

× f

3. Select a suitable LVDT that will operate with an excitation

frequency of 2.5 kHz. The Schaevitz E100, for instance, will

operate over a range of 50 Hz to 10 kHz and is an eligible

candidate for this example.

4. Select excitation frequency determining component C1.

C1

= 35 µFHz/f

EXCITATION

R1

C1

C2

C3

R4

R3

13

16

15

14

24

23

22

21

20

19

18

17

12

11

10

9

8

1

2

3

4

7

6

5

AD698

EXC1

EXC2

LEV1

LEV2

FREQ1

BFILT1

BFILT2

–BIN

+BIN

–AIN

FREQ2

SIG REF

OFFSET2

OFFSET1

OUT FILT

FEEDBACK

SIG OUT

–ACOMP

AFILT2

AFILT1

+ACOMP

+AIN

C4

R2

1000pF

SIGNAL

REFERENCE

100nF

6.8µF

–15V

+15V

100nF

6.8µF

1M

AB

C

D

PHASE

LAG/LEAD

NETWORK

AB

CD

PHASE LEAD

C

C

A

B

CD

PHASE LAG

C

PHASE LAG = Arc Tan (Hz RC);

PHASE LEAD = Arc Tan 1/(Hz RC)

Figure 8. AD698 Interconnection Diagram for Series

Opposed LVDT and Dual Supply Operation

B. Determine the Oscillator Amplitude

Amplitude is set such that the primary signal is in the 1.0 V to

3.5 V rms range and the secondary signal is in the 0.25 V to

3.5 V rms range when the LVDT is at its mechanical full-scale

position. This optimizes linearity and minimizes noise suscepti-

bility. Since the part is ratiometric, the exact value of the excita-

tion is relatively unimportant.

4-wire LVDT determine the voltage transformation ratio,

VTR, of the LVDT at its mechanical full scale. VTR =

LVDT sensitivity

× Maximum Stroke Length from null.

LVDT sensitivity is listed in the LVDT manufacturer’s cata-

log and has units of volts output per volts input per inch dis-

placement. The E100 has a sensitivity of 2.4 mV/V/mil. In

the event that LVDT sensitivity is not given by the manufac-

turer, it can be computed. See section on determining LVDT

sensitivity.