AD598
1000
100
RIPPLE – mV rms
10
10kHz , C
SHUNT
= 0nF
1
10kHz , C
SHUNT
= 1nF
10kHz , C
SHUNT
= 10nF
0.1
0.001
0.01
0.1
1
10
C2, C3, C4; C2 = C3 = C4 –
µF
Figure 17. Output Voltage Ripple vs. Filter Capacitance
Determining LVDT Sensitivity
LVDT sensitivity can be determined by measuring the LVDT
secondary voltages as a function of primary drive and core posi-
tion, and performing a simple computation.
Energize the LVDT at its recommended primary drive level,
V
PRI
(3 V rms for the E100). Set the core to midpoint where
V
A
= V
B
. Set the core displacement to its mechanical full-scale
position and measure secondary voltages V
A
and V
B
.
Figure 15. Gain and Phase Characteristics vs. Frequency
(0 kHz–10 kHz)
1000
Sensitivity
=
From Figure 18,
V
A
(at
Full Scale
) –
V
B
(at
Full Scale
)
V
PRI
×
d
1.71 – 0.99
=
2.4
mV/V/mil
3
×
100
mils
V
A
100
RIPPLE – mV rms
Sensitivity
=
V
SEC
WHEN V
PRI
= 3V rms
10
1.71V rms
2.5kHz, C
SHUNT
= 0nF
1
2.5kHz, C
SHUNT
= 1nF
2.5kHz, C
SHUNT
=10nF
0.1
0.01
0.1
1
10
d = –100 mils
d=0
0.99V rms
V
B
d = +100 mils
Figure 18. LVDT Secondary Voltage vs. Core Displacement
Thermal Shutdown and Loading Considerations
C2, C3, C4; C2 = C3 = C4 –
µF
Figure 16. Output Voltage Ripple vs. Filter Capacitance
The AD598 is protected by a thermal overload circuit. If the die
temperature reaches 165°C, the sine wave excitation amplitude
gradually reduces, thereby lowering the internal power dissipa-
tion and temperature.
Due to the ratiometric operation of the decoder circuit, only
small errors result from the reduction of the excitation ampli-
tude. Under these conditions the signal-processing section of
the AD598 continues to meet its output specifications.
The thermal load depends upon the voltage and current deliv-
ered to the load as well as the power supply potentials. An
LVDT Primary will present an inductive load to the sine wave
excitation. The phase angle between the excitation voltage and
current must also be considered, further complicating thermal
calculations.
REV. A
–9–
AD [ ANALOG DEVICES ]
