AD698

REV. B

–9–

FREQUENCY – Hz

0

100k

100

1k

10k

10

0

–30

–60

–70

0

–10

–20

–50

–40

–360

–60

–240

–300

–420

–180

–120

0.1µF

0.033µF

0.01µF

R2 = 81k

Ω

0.1µF

0.033µF

0.01µF

R2 = 81k

Ω

Figure 15. Gain and Phase Characteristics vs. Frequency

(0 kHz–50 kHz)

FREQUENCY – Hz

0

100

1k

10k

10

0

–30

–60

–70

–10

–20

–50

–40

0

–360

–60

–240

–300

–180

–120

0.1µF

0.033µF

0.01µF

R2 = 81k

Ω

0.1µF

0.033µF

0.01µF

R2 = 81k

Ω

Figure 16. Gain and Phase Characteristics vs. Frequency

(0 kHz–10 kHz)

Figure 16 shows a more limited frequency range with enhanced

accuracy. The figures are transfer functions with the input to be

considered as a sinusoidally varying mechanical position and the

output as the voltage from the AD698; 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 fig-

ures. The response is approximately that of two real poles.

However, there is appreciable excess phase at higher frequen-

cies. An additional pole of filtering can be introduced with a

shunt capacitor across R2, Figure 7; this will also increase 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 17 and 18 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, Figure 7,

is shown as a parameter. The value of R2 used was 81 k

Ω with a

Schaevitz E100 LVDT.

C2, C3, C4; C2 = C3 = C4 –

µF

1k

100

0.1

0.01

0.1

10

1

10

1

2.5kHz, C

SHUNT

1nF

2.5kHz, C

SHUNT

10nF

Figure 17. Output Voltage Ripple vs. Filter Capacitance

C2, C3, C4; C2 = C3 = C4 –

µF

1k

100

0.1

0.001

0.01

10

0.1

10

1

10kHz, C

SHUNT

1nF

10kHz, C

SHUNT

10nF

1

Figure 18. Output Voltage Ripple vs. Filter Capacitance