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AD2S93 View Datasheet(PDF) - Analog Devices

Part NameDescriptionManufacturer
AD2S93 Low Cost LVDT-to-Digital Converter ADI
Analog Devices ADI
AD2S93 Datasheet PDF : 12 Pages
1 2 3 4 5 6 7 8 9 10 Next Last
Calculation of the component values for the bandwidth is de-
tailed below. For more detailed information on component
value selection for the AD2S93, please consult the “Passive
Component Selection and Dynamic Modeling Software for the
AD2S93 LVDT-to-Digital Converter.”
VCO Gain G (1) Mode 1
The available bandwidth with this option is from 0.5 kHz to
1.25 kHz.
FREF > 8 × Fo
C1 = 1/(800 × Fo2)
C2 = 8 × C1
R2 = 45 × Fo
Where FREF is the reference frequency, Fo is the closed-loop
3 dB point.
VCO Gain G (2) Mode 2
The available bandwidth with this option is from 45 Hz
to 500 Hz.
FREF > 8 × Fo
C1 = 1/(2400 × Fo2)
C2 = 8 C1
R2 = 45 × Fo
Where FREF is the reference frequency, Fo is the closed-loop
3 dB point.
INTERFACING TO THE AD2S93 (SEE “TIMING
CHARACTERISTICS”)
The absolute position information is extracted via a three-wire
interface, DATA, CS and SCLK. The DATA output is held in
a high impedance state when CS is high.
Upon the application of logic low to the CS pin, the DATA is
enabled and the current position information is transferred from
the counters to the serial interface. Data is retrieved by applying
an external clock to the SCLK pin. The maximum data rate of
the SCLK is 2 MHz. To ensure secure data retrieval, it is
important to note that SCLK should not be applied until a
minimum period of 600 ns after the application of logic low to
CS. Data is then clocked out on successive positive edges of
SCLK: 16 clock edges are required to extract the entire data
word. Subsequent positive edges greater than the defined reso-
lution of the converter will clock zeros from the data output if
CS remains in a low state. The format of the data read is shown
in Table I.
Table I.
Function
DB0
LOS
DB1
OVR
DB2
UNR
DB3
SIGN
DATA DB4–D15
MSB LSB
MAGNITUDE
If less than the full 16-bit word is required, then the data read
can be terminated by releasing CS after the required number of
bits have been read.
CS can be released a minimum of 100 ns after the last positive
edge. If the user is reading data continuously, CS can be reap-
plied after a minimum of 600 ns after it is released. The mini-
mum repetitive read time of the same converter is given by (16
bits read @ 2 MHz). Min RD Time = [600 + (16 × 500) +
600] = 9.2 µs.
AD2S93
IN-BUILT DIAGNOSTICS
The first three bits read from the serial interface preceding the
sign and magnitude data can be used to determine whether the
data is valid or not. Over and underrange (OVR, UNR) denote
the two extremes of the LVDT stroke where linearity of the
LVDT may degrade. Loss of signal LOS is an open drain out-
put which pulls high (12 kpull up) when one of the following
conditions is satisfied:
1. A and/or B is disconnected.
2. REF is disconnected.
Note: LOS has a response time of 50 ms max to the conditions
stated above, see “Specifications.”
CONNECTING THE CONVERTER
Positive power supply VDD = +5 V dc ± 5% should be con-
nected to Pin 17 and negative power supply VSS = –5 V dc ± 5%
to Pin 16. Reversal of these power supplies will destroy this device.
For LVDT connections to the converter please refer to Figures
5 through 7. On all connections, the maximum input reference
signal VREF = 2.0 V rms ± 10%. To operate within the standard
operating range, A–B should not exceed 1.0 V rms ± 10%. The
AD2S93 AGND point is the point at which all analog signal
grounds should be connected. Ground returns from the LVDT
should be connected to AGND. The AD2S93 DGND pin
should be connected to the AD2S93 AGND pin. Ancillary Digi-
tal circuitry must be connected to the Star Point and not to the
AD2S93 AGND pin.
In all cases, the AD2S93 has been configured with the following
dynamics.
Reference Frequency 5 kHz
3 dB Bandwidth
625 Hz
Vco Gain is set in MODE 1 where VCO GAIN is connected to
VEL.
Using the procedure described in “setting the converter band-
width” the following preferred values (E12 series) were calcu-
lated:
C1 = 3.3 nF
C2 = 27 nF
R2 = 27 k
CALCULATING HF FILTER (C3, C4, R5, R6)
15 kΩ ≤ R5 = R6 56 k
1
C3= C4 =
2π R5 FREF
So, C3 = 1 nF, R5 = R6 = 33 k, C4 = 1 nF and in all cases
R7 = 15 k.
Half-Bridge Type LVDT Connection
In this method of connection, it is necessary to add two addi-
tional bridge completion resistors RC and RC, in order to derive
a reference for the AD2S93. In selecting the bridge completion
resistor, it is important to remember that mismatch between RC1
and RC2 will cause nonzero errors at null. If two LVDTs are be-
ing used for differential measurements, the resistors can be re-
placed by the second LVDT.
REV. A
–7–
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