AD598

REV. A

–8–

tions, make sure the voltage drop across R5 is met as

defined in Step 12.

As a final check on the power supply voltages, verify that the

14. C5 is a bypass capacitor in the range of 0.1

µF to 1 µF.

EXC 1

EXC 2

LEV 1

LEV 2

FREQ 1

FREQ 2

B1 FILT

B2 FILT

OFFSET 1

OFFSET 2

SIG REF

SIG OUT

FEEDBACK

OUT FILT

A1 FILT

A2 FILT

R1

C2

AD598

C3

R2

C4

LVDT

SCHAEVITZ E100

R3

R4

0.1

µF

6.8

µF

SIGNAL

REFERENCE

30V

+

R

L

1

2

3

4

5

6

7

8

9

10

11

12

13

14

16

15

17

18

19

20

R5

R6

C5

C1

15nF

33k

V

B

V

B

Figure 12. Interconnection Diagram for Single

Supply Operation

Gain Phase Characteristics

To use an LVDT in a closed loop mechanical servo application,

it is necessary to know the dynamic characteristics of the trans-

ducer and interface elements. The transducer itself is very quick

to respond once the core is moved. The dynamics arise prima-

rily from the interface electronics. Figures 13, 14 and 15 show

the frequency response of the AD598 LVDT Signal Condi-

tioner. Note that Figures 14 and 15 are basically the same; the

difference is frequency range covered. Figure 14 shows a wider

range of mechanical input frequencies at the expense of accu-

racy. Figure 15 shows a more limited frequency range with en-

hanced accuracy. The figures are transfer functions with the

input to be considered as a sinusoidally varying mechanical posi-

tion and the output as the voltage from the AD598; the units of

the transfer function are volts per inch. The value of C2, C3 and

C4, from Figure 7, are all equal and designated as a parameter

in the figures. The response is approximately that of two real

poles. However, there is appreciable excess phase at higher fre-

quencies. An additional pole of filtering can be introduced with

a shunt capacitor across R2, (see Figure 7); this will also in-

crease phase lag.

When selecting values of C2, C3 and C4 to set the bandwidth of

the system, a trade-off is involved. There is ripple on the “dc”

position output voltage, and the magnitude is determined by the

filter capacitors. Generally, smaller capacitors will give higher

system bandwidth and larger ripple. Figures 16 and 17 show the

magnitude of ripple as a function of C2, C3 and C4, again all

equal in value. Note also a shunt capacitor across R2 shown as a

parameter (see Figure 7). The value of R2 used was 81 k

Ω with

a Schaevitz E100 LVDT.

Figure 13. Gain and Phase Characteristics vs. Frequency

(0 kHz–10 kHz)

Figure 14. Gain and Phase Characteristics vs. Frequency

(0 kHz–50 kHz)