AD698 Analog Devices, AD698 Datasheet - Page 9

• Manufacturer Part Number: AD698
• Description: Universal LVDT Signal Conditioner
• Manufacturer: Analog Devices

REV. B
Figure 15. Gain and Phase Characteristics vs. Frequency
(0 kHz–50 kHz)
Figure 16. Gain and Phase Characteristics vs. Frequency
(0 kHz–10 kHz)
–120
–180
–240
–300
–360
–420
–120
–180
–240
–300
–360
–10
–20
–30
–40
–50
–60
–70
–60
–10
–20
–30
–40
–50
–60
–70
–60
10
10
0
0
0
0
0
0
R2 = 81k
f
R2 = 81k
f
R2 = 81k
f
EXC
EXC
EXC
R2 = 81k
f
EXC
= 10kHz
= 10kHz
= 10kHz
= 10kHz
100
100
FREQUENCY – Hz
FREQUENCY – Hz
0.1µF
0.1µF
0.1µF
1k
0.033µF
0.033µF
0.01µF
1k
10k
0.1µF
0.033µF
0.033µF
0.01µF
0.01µF
0.01µF
100k
10k
–9–
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 with a
Schaevitz E100 LVDT.
Figure 17. Output Voltage Ripple vs. Filter Capacitance
Figure 18. Output Voltage Ripple vs. Filter Capacitance
100
100
0.1
0.1
1k
10
1k
10
1
1
0.001
0.01
0.01
C2, C3, C4; C2 = C3 = C4 – F
C2, C3, C4; C2 = C3 = C4 – F
0.1
0.1
2.5kHz, C
1
10kHz, C
10kHz, C
2.5kHz, C
1
SHUNT
SHUNT
SHUNT
SHUNT
AD698
10nF
10nF
1nF
1nF
10
10