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AD1674 Datasheet(PDF) 10 Page - Analog Devices
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VALUE OF A
1µs DELAY-HOLD SETTLING
NYBBLE B = 0
Figure 10. Equivalent Internal Logic Circuitry
Chip Enable (CE), Chip Select (CS) and Read/ Convert (R/C)
are used to control Convert or Read modes of operation. Either
CE or CS may be used to initiate a conversion. The state of R/C
when CE and CS are both asserted determines whether a data
Read (R/C = 1) or a Convert (R/C = 0) is in progress. R/C
should be LOW before both CE and CS are asserted; if R/C is
HIGH, a Read operation will momentarily occur, possibly
resulting in system bus contention.
The AD1674 can be used in a “stand-alone” mode, which is
useful in systems with dedicated input ports available and thus
not requiring full bus interface capability. Stand-alone mode
applications are generally able to issue conversion start com-
mands more precisely than full-control mode. This improves ac
performance by reducing the amount of control-induced aper-
In stand-alone mode, the control interface for the AD1674 and
AD674A are identical. CE and 12/8 are wired HIGH, CS and
are wired LOW, and conversion is controlled by R/C. The
three-state buffers are enabled when R/C is HIGH and a con-
version starts when R/C goes LOW. This gives rise to two pos-
sible control signals—a high pulse or a low pulse. Operation
with a low pulse is shown in Figure 4a. In this case, the outputs
are forced into the high impedance state in response to the fall-
ing edge of R/C and return to valid logic levels after the conver-
sion cycle is completed. The STS line goes HIGH 200 ns after
R/C goes LOW and returns low 1
µs after data is valid.
If conversion is initiated by a high pulse as shown in Figure 4b,
the data lines are enabled during the time when R/C is HIGH.
The falling edge of R/C starts the next conversion and the data
lines return to three-state (and remain three-state) until the next
high pulse of R/C.
Once a conversion is started, the STS line goes HIGH. Convert
start commands will be ignored until the conversion cycle is
complete. The output data buffers will be enabled a minimum
µs prior to STS going LOW. The STS line will return
LOW at the end of the conversion cycle.
The register control inputs, A
and 12/8, control conversion
length and data format. If a conversion is started with A
a full 12-bit conversion cycle is initiated. If A
is HIGH during a
convert start, a shorter 8-bit conversion cycle results.
During data read operations, A
determines whether the three-
state buffers containing the 8 MSBs of the conversion result (A
= 0) or the 4 LSBs (A
= 1) are enabled. The 12/8 pin deter-
mines whether the output data is to be organized as two 8-bit
words (12/8 tied LOW) or a single 12-bit word (12/8 tied
HIGH). In the 8-bit mode, the byte addressed when A
contains the 4 LSBs from the conversion followed by four trail-
ing zeroes. This organization allows the data lines to be over-
lapped for direct interface to 8-bit buses without the need for
external three-state buffers.
INPUT CONNECTIONS AND CALIBRATION
The 10 V p-p and 20 V p-p full-scale input ranges of the
AD1674 accept the majority of signal voltages without the need
for external voltage divider networks which could deteriorate the
accuracy of the ADC.
The AD1674 is factory trimmed to minimize offset, linearity,
and full-scale errors. In many applications, no calibration trim-
ming will be required and the AD1674 will exhibit the accuracy
limits listed in the specification tables.
In some applications, offset and full-scale errors need to be
trimmed out completely. The following sections describe the
correct procedure for these various situations.
UNIPOLAR RANGE INPUTS
Figure 11 illustrates the external connections for the AD1674 in
unipolar-input mode. The first output-code transition (from
0000 0000 0000 to 0000 0000 0001) should nominally occur
for an input level of +1/2 LSB (1.22 mV above ground for a 10 V
range; 2.44 mV for a 20 V range). To trim unipolar offset to this
nominal value, apply a +1/2 LSB signal between Pin 13 and
ground (10 V range) or Pin 14 and ground (20 V range) and ad-
just R1 until the first transition is located. If the offset trim is
not required, Pin 12 can be connected directly to Pin 9; the two
resistors and trimmer for Pin 12 are then not needed.
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