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AD2S93BP View Datasheet(PDF) - Analog Devices

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Description
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AD2S93BP
ADI
Analog Devices ADI
AD2S93BP Datasheet PDF : 12 Pages
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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.
IN
+
VEL OUT
G1 (s)
G2 (s)
OUT
Figure 9. AD2S93 Transfer Function
Note: The AD2S93 has been configured with the following dy-
namics.
Reference Frequency
3 dB Bandwidth
10 kHz
1250 Hz
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:
C1 = 820 pF
C2 = 6.8 nF
R2 = 56 k
C3 = C4 = 470 pF, R7 = 15 k, R5 = R6 = 33 k, C4 =
470 pF
The open-loop transfer function is given by:
G1(s) = K1
s
1 + st1
1 + st2
G2(s) =
K2
s
where:
t2
=
R2

C1
C1
×
+
C2
C2

t1 = R2 C2
and:
K1
=
4 ×103
25 ×103
= 160
× 109
×
1
C1 + C2
= 21
K2
=
RV
4
× CV
Note A2 has two values depending on which mode is being used
K2 (MODE1) = 640 × 103
K2 (MODE2) = 160 × 103
The AD2S93 acceleration constant is given by:
Ka = K1 × K2
Therefore in the example given,
Ka = K1 × K2 = 21 × 640 × 103 = 13.44 × 106 s–2
The AD2S93’s design has been optimized with a critically
damped response. The closed-loop transfer function is given
by:
θOUT
θ IN
=
1 + st1
1 + st1
+
s2
K1K 2
+
s3t2
K1K 2
θOUT
θ IN
= K1K 2
s2
(1 + st1 )
1 + st2
The normalized gain and phase diagrams are given in Figures 10
and 11 with a bandwidth of 1.25 kHz.
5
0
–5
–10
–15
–20
–25
–30
–35
–40
–45
1
10
100
1k
10k
FREQUENCY – Hz
Figure 10. AD2S93 Gain Plot
0
–20
–40
–60
–80
–100
–120
–140
–160
–180
1
10
100
1k
10k
FREQUENCY – Hz
Figure 11. AD2S93 Phase Plot
–10–
REV. A
 

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