AD2S93BP AD [Analog Devices], AD2S93BP Datasheet - Page 10

• Manufacturer Part Number: AD2S93BP
• Description: Low Cost LVDT-to-Digital Converter
• Manufacturer: AD [Analog Devices]

AD2S93
CIRCUIT DYNAMICS/ERROR SOURCES
TRANSFER FUNCTION
The AD2S93 operates as a Type 2 tracking servo loop. An inte-
grator and VCO/counter perform the two integrations inherent
in a Type 2 loop.
The overall system response of the AD2S93 is that of a unity
gain second order low-pass filter, with the position of the LVDT
as the input and the digital position data as the output. Figure 9
illustrates the AD2S93 system diagram.
Note: The AD2S93 has been configured with the following dy-
namics.
VCO Gain is set in MODE 1 where VCOGAIN is connected to
VEL.
Using the procedure described in “SETTING THE CON-
VERTER BANDWIDTH,” the following preferred values (E12
series) were calculated:
C3 = C4 = 470 pF, R7 = 15 k , R5 = R6 = 33 k , C4 =
470 pF
The open-loop transfer function is given by:
where:
Note A
Therefore in the example given,
2
K
The AD2S93 acceleration constant is given by:
has two values depending on which mode is being used
a
Reference Frequency
3 dB Bandwidth
= K
Figure 9. AD2S93 Transfer Function
t
and:
t
K
K
1
2
= R
1
2
1
R
2
25 10
4 10
2
R
K
IN
G1(s)
C
V
2
K
K
C
C
2
= 21 640 10
4
2 (MODE2)
+
2 (MODE1)
1
1
C1 = 820 pF
C2 = 6.8 nF
R2 = 56 k
C
3
3
V
K
C
C
a
2
2
K
= K
s
160 10
G1 (s)
1
= 640 10
= 160
G2(s)
1
1 st
1 st
VEL OUT
K
2
3
10 kHz
1250 Hz
9
= 13.44 10
1
2
10
K
s
3
3
2
C
G2 (s)
1
1
C
2
6
21
s
OUT
–2
–10–
The AD2S93’s design has been optimized with a critically
damped response. The closed-loop transfer function is given
by:
The normalized gain and phase diagrams are given in Figures 10
and 11 with a bandwidth of 1.25 kHz.
OUT
IN
–100
–120
–140
–160
–180
–25
–10
–15
–20
–30
–35
–40
–45
–20
–40
–60
–80
–5
5
0
0
1
1
1 st
Figure 11. AD2S93 Phase Plot
Figure 10. AD2S93 Gain Plot
1
K
1 st
10
10
1
s
2
K
2
1
FREQUENCY – Hz
FREQUENCY – Hz
K
s
3
1
100
K
t
100
2
2
OUT
IN
1k
1k
K
1
s
K
2
2
(1 st
1 st
10k
10k
REV. A
1
2
)