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LM12454 Datasheet(PDF) 26 Page - Texas Instruments |
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LM12454 Datasheet(HTML) 26 Page - Texas Instruments |
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26 / 45 page  LM12454, LM12458, LM12H458 SNAS079A – MAY 2004 – REVISED FEBRUARY 2006 www.ti.com Two different calibration modes are available; one compensates for offset voltage, or zero error, while the other corrects both offset error and the ADC's linearity error. When correcting offset only, the offset error is measured once and a correction coefficient is created. During the full calibration, the offset error is measured eight times, averaged, and a correction coefficient is created. After completion of either calibration mode, the offset correction coefficient is stored in an internal offset correction register. The LM12(H)454/8's overall linearity correction is achieved by correcting the internal DAC's capacitor mismatch. Each capacitor is compared eight times against all remaining smaller value capacitors and any errors are averaged. A correction coefficient is then created and stored in one of the thirteen internal linearity correction registers. An internal state machine, using patterns stored in an internal 16 x 8-bit ROM, executes each calibration algorithm. Once calibrated, an internal arithmetic logic unit (ALU) uses the offset correction coefficient and the 13 linearity correction coefficients to reduce the conversion's offset error and linearity error, in the background, during the 12- bit + sign conversion. The 8-bit + sign conversion and comparison modes use only the offset coefficient. The 8- bit + sign mode performs a conversion in less than half the time used by the 12-bit + sign conversion mode. The LM12(H)454/8's “watchdog” mode is used to monitor a single-ended or differential signal's amplitude. Each sampled signal has two limits. An interrupt can be generated if the input signal is above or below either of the two limits. This allows interrupts to be generated when analog voltage inputs are “inside the window” or, alternatively, “outside the window”. After a “watchdog” mode interrupt, the processor can then request a conversion on the input signal and read the signal's magnitude. The analog input multiplexer can be configured for any combination of single-ended or fully differential operation. Each input is referenced to ground when a multiplexer channel operates in the single-ended mode. Fully differential analog input channels are formed by pairing any two channels together. The LM12454's multiplexer outputs and S/H inputs (MUXOUT+, MUXOUT − and S/H IN+, S/H IN−) provide the option for additional analog signal processing. Fixed-gain amplifiers, programmable-gain amplifiers, filters, and other processing circuits can operate on the signal applied to the selected multiplexer channel(s). If external processing is not used, connect MUXOUT+ to S/H IN+ and MUXOUT − to S/H IN−. The LM12(H)454/8's internal S/H is designed to operate at its minimum acquisition time (1.13 μs, 12 bits) when the source impedance, RS, is ≤ 60Ω (fCLK ≤ 8 MHz). When 60Ω < RS ≤ 4.17 kΩ, the internal S/H's acquisition time can be increased to a maximum of 4.88 μs (12 bits, fCLK = 8 MHz). See Section 2.1 (Instruction RAM “00”) Bits 12–15 for more information. An internal 2.5V bandgap reference output is available at pin 44. This voltage can be used as the ADC reference for ratiometric conversion or as a virtual ground for front-end analog conditioning circuits. The VREFOUT pin should be bypassed to ground with a 100 μF capacitor. Microprocessor overhead is reduced through the use of the internal conversion FIFO. Thirty-two consecutive conversions can be completed and stored in the FIFO without any microprocessor intervention. The microprocessor can, at any time, interrogate the FIFO and retrieve its contents. It can also wait for the LM12(H)454/8 to issue an interrupt when the FIFO is full or after any number ( ≤32) of conversions have been stored. Conversion sequencing, internal timer interval, multiplexer configuration, and many other operations are programmed and set in the Instruction RAM. A diagnostic mode is available that allows verification of the LM12(H)458's operation. The diagnostic mode is disabled in the LM12454. This mode internally connects the voltages present at the VREFOUT, VREF+, VREF−, and GND pins to the internal VIN+ and VIN− S/H inputs. This mode is activated by setting the Diagnostic bit (Bit 11) in the Configuration register to a “1”. More information concerning this mode of operation can be found in Section 2.2. 26 Submit Documentation Feedback Copyright © 2004–2006, Texas Instruments Incorporated Product Folder Links: LM12454 LM12458 LM12H458 |
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