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LM12454 Datasheet(PDF) 31 Page - Texas Instruments

Part # LM12454
Description  LM12454/LM12458/LM12H458 12-Bit Sign Data Acquisition System with Self-Calibration
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Manufacturer  TI1 [Texas Instruments]
Direct Link  http://www.ti.com
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LM12454 Datasheet(HTML) 31 Page - Texas Instruments

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LM12454, LM12458, LM12H458
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SNAS079A – MAY 2004 – REVISED FEBRUARY 2006
Bit 0 is the START/STOP bit. Reading Bit 0 returns an indication of the Sequencer's status. A “0” indicates that
the Sequencer is stopped and waiting to execute the next instruction. A “1” shows that the Sequencer is running.
Writing a “0” halts the Sequencer when the current instruction has finished execution. The next instruction to be
executed is pointed to by the instruction pointer found in the status register. A “1” restarts the Sequencer with the
instruction currently pointed to by the instruction pointer. (See Bits 8–10 in the Interrupt Status register.)
Bit 1 is the LM12(H)454/8's system RESET bit. Writing a “1” to Bit 1 stops the Sequencer (resetting the
Configuration register's START/STOP bit), resets the Instruction pointer to “000” (found in the Interrupt Status
register), clears the Conversion FIFO, and resets all interrupt flags. The RESET bit will return to “0” after two
clock cycles unless it is forced high by writing a “1” into the Configuration register's Standby bit. A reset signal is
internally generated when power is first applied to the part. No operation should be started until the RESET bit is
“0”.
Writing a “1” to Bit 2 initiates an auto-zero offset voltage calibration. Unlike the eight-sample auto-zero calibration
performed during the full calibration procedure, Bit 2 initiates a “short” auto-zero by sampling the offset once and
creating a correction coefficient (full calibration averages eight samples of the converter offset voltage when
creating a correction coefficient). If the Sequencer is running when Bit 2 is set to “1”, an auto-zero starts
immediately after the conclusion of the currently running instruction. Bit 2 is reset automatically to a “0” and an
interrupt flag (Bit 3, in the Interrupt Status register) is set at the end of the auto-zero (76 clock cycles). After
completion of an auto-zero calibration, the Sequencer fetches the next instruction as pointed to by the Instruction
RAM's pointer and resumes execution. If the Sequencer is stopped, an auto-zero is performed immediately at the
time requested.
Writing a “1” to Bit 3 initiates a complete calibration process that includes a “long” auto-zero offset voltage
correction (this calibration averages eight samples of the comparator offset voltage when creating a correction
coefficient) followed by an ADC linearity calibration. This complete calibration is started after the currently
running instruction is completed if the Sequencer is running when Bit 3 is set to “1”. Bit 3 is reset automatically to
a “0” and an interrupt flag (Bit 4, in the Interrupt Status register) will be generated at the end of the calibration
procedure (4944 clock cycles). After completion of a full auto-zero and linearity calibration, the Sequencer
fetches the next instruction as pointed to by the Instruction RAM's pointer and resumes execution. If the
Sequencer is stopped, a full calibration is performed immediately at the time requested.
Bit 4 is the Standby bit. Writing a “1” to Bit 4 immediately places the LM12(H)454/8 in Standby mode. Normal
operation returns when Bit 4 is reset to a “0”. The Standby command (“1”) disconnects the external clock from
the internal circuitry, decreases the LM12(H)454/8's internal analog circuitry power supply current, and preserves
all internal RAM contents. After writing a “0” to the Standby bit, the LM12(H)454/8 returns to an operating state
identical to that caused by exercising the RESET bit. A Standby completion interrupt is issued after a power-up
completion delay that allows the analog circuitry to settle. The Sequencer should be restarted only after the
Standby completion is issued. The Instruction RAM can still be accessed through read and write operations while
the LM12(H)454/8 are in Standby Mode.
Bit 5 is the Channel Address Mask. If Bit 5 is set to a “1”, Bits 13–15 in the conversion FIFO will be equal to the
sign bit (Bit 12) of the conversion data. Resetting Bit 5 to a “0” causes conversion data Bits 13 through 15 to hold
the instruction pointer value of the instruction to which the conversion data belongs.
Bit 6 is used to select a “short” auto-zero correction for every conversion. The Sequencer automatically inserts
an auto-zero before every conversion or “watchdog” comparison if Bit 6 is set to “1”. No automatic correction will
be performed if Bit 6 is reset to “0”.
The LM12(H)454/8's offset voltage, after calibration, has a typical drift of 0.1 LSB over a temperature range of
−40°C to +85°C. This small drift is less than the variability of the change in offset that can occur when using the
auto-zero correction with each conversion. This variability is the result of using only one sample of the offset
voltage to create a correction value. This variability decreases when using the full calibration mode because eight
samples of the offset voltage are taken, averaged, and used to create a correction value.
Bit 7 is used to program the SYNC pin (29) to operate as either an input or an output. The SYNC pin becomes
an output when Bit 7 is a “1” and an input when Bit 7 is a “0”. With SYNC programmed as an input, the rising
edge of any logic signal applied to pin 29 will start a conversion or “watchdog” comparison. Programmed as an
output, the logic level at pin 29 will go high at the start of a conversion or “watchdog” comparison and remain
high until either have finished. See Instruction RAM “00”, Bit 8.
Bits 8 and 9 form the RAM Pointer that is used to select each of a 48-bit instruction's three 16-bit sections during
read or write actions. A “00” selects Instruction RAM section one, “01” selects section two, and “10” selects
section three.
Copyright © 2004–2006, Texas Instruments Incorporated
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