CAV-1202_15 Datasheet, PDF (3/4 Pages) Analog Devices – 12-Bit Eurocard Analog to Digit Calonverter

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3.5
(88.94)
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
6.5891167.361
6.31160)
..! 10.06211.571-1 i-
-IMT 3.5":)~ ~
1(88.94)
3.93
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l¹iT ^0.218~o----
(5.54)~
0.191
14.851
5.7 (145.0)
THEORY OF OPERATION
O_{Refer to the block diagram of the CAV-1202.}
B Analog input signals to be digitized are applied to a track-and-hold
(T/H) amplifier which is normally operating as a buffer amplifier
S _{in the "track" mode, following all changes in analog input as}
O ^{they occur. The user of the CAV-1202 determines the point at}
which the analog signal is to be digitized by applying an Encode
Command.
LE _{The transition from digital "0" to digital "1" of the TTL-com-}
patible encode command causes the track-and-hold to switch
TE momentarily to the "hold" mode of operation. This "freezes"
--0 ^I
0.517
(13.13)
Expressed in its simplest terms, the digital correction logic
circuits use the information iri the 8-bit signal to determine
what modifications of Bits 1-5 may be necessary. The value of
the MSB in the 8-bit byte establishes whether the 5-bit data are
passed "as is" or whether they are increased by a value of
binary "1". The remaining bits (2-8) of the 8-bit byte become
Bits 6-12 of the CAV-I202 digital output.
Digitally-corrected subranging (DCS), the innovative technique
described here, helps compensate for a wide range of potential
errors which could otherwise be avoided only if the CAV-I202
design included expensive, high precision components.
the analog input signal long enough to begin the digitizing process.
The instant this switching action occurs is affected by one of the OFFSET AND GAIN ADJUSTMENTS
parameters of the CAV-1202, called out as Effective Aperture
The design and manufacture of the CAV-1202 AID Converter
Delay Time in the Specifications Table.
are innovative and precise, and have resulted in a high-performance
Basically, effective aperture delay time is a measure of the dif-
ference between the converter's digital and analog delays (1:ct-tJ
and can assume a zero, positive, or negative value depending on
the comparative lengths of the two delays. In the CAV-I202,
converter which is virtually adjustment-free. This elimination of
variable controls helps make the unit less susceptible to per-
formance degradation caused by vibration, shock, or inad~i:rtent
and/or incorrect adjustment.
the analog delay (tJ is longer than the digital delay (td)' and
Only two control settings are available to the user; factory ad-
causes effective aperture delay to be typically -I9ns.
justments use selected fIXed resistors to assure optimum per-
The "held" value of analog signal at the output of the T/H is
formance without a need for "tweaking" by the user.
applied to a 5-bit encoder. It is also applied as one input to a
OFFSET and GAIN controls are available, but even these are
fast-settling operational amplifier.
sealed before shipment. In those rare instances where they may
The output of the encoder is applied as a 5-bit input to a 12-bit
require readjustment, the procedure below is the one to use.
D/A converter. Via registers, the same digital signal is directed
Refer to Figure 1, the CAV-I202 Adjustment Controls.
to the digital correction logic circuits and a second set of registers.
These data will represent Bits 1-5 of the 12-bit digital output of
the CAV-I202.
This reconstructed output of the D/A converter becomes the
OFFSET
second input to the operational amplifier mentioned earlier. The
output of the wideband, fast-settling op amp represents the
GAIN
residue which remains after a 5-bit digital representation of the
analog input has been subtracted from that input.
ANALOG INPUT
This residue, or error, signal is encoded by a second converter
and is applied as 8-bit digital information to the digital correction
logic circuits which contain Bits 1-5.
The correction circuits combine the 5-bit and 8-bit bytes of
data to compensate for possible nonlinearities and other errors
to assure the fmal12-bit output of the CAV-I202 is 12-bit
accurate.
Figure 1. Offset and Gain Controls
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