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LM12L458 Datasheet(PDF) 18 Page - National Semiconductor (TI) |
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LM12L458 Datasheet(HTML) 18 Page - National Semiconductor (TI) |
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18 / 36 page  Application Information 1.0 Functional Description The LM12L458 is a multi-functional Data Acquisition System that include a fully differential 12-bit-plus-sign self-calibrating analog-to-digital converter (ADC) with a two’s-complement output format, an 8-channel analog multiplexer, a first-in-first-out (FIFO) register that can store 32 conversion results, and an Instruction RAM that can store as many as eight instructions to be sequentially executed. All of this cir- cuitry operates on only a single +3.3V power supply. The LM12L458 has three modes of operation: 12-bit + sign with correction 8-bit + sign without correction 8-bit + sign comparison mode (“watchdog” mode) The fully differential 12-bit-plus-sign ADC uses a charge re- distribution topology that includes calibration capabilities. Charge re-distribution ADCs use a capacitor ladder in place of a resistor ladder to form an internal DAC. The DAC is used by a successive approximation register to generate interme- diate voltages between the voltages applied to V REF− and V REF+. These intermediate voltages are compared against the sampled analog input voltage as each bit is generated. The number of intermediate voltages and comparisons equals the ADC’s resolution. The correction of each bit’s ac- curacy is accomplished by calibrating the capacitor ladder used in the ADC. Two different calibration modes are available; one compen- sates 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, aver- aged, and a correction coefficient is created. After comple- tion of either calibration mode, the offset correction coeffi- cient is stored in an internal offset correction register. The LM12L458’s overall linearity correction is achieved by correcting the internal DAC’s capacitor mismatch. Each ca- pacitor 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 ma- chine, 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 lin- earity error, in the background, during the 12-bit + sign con- version. The 8-bit + sign conversion and comparison modes use only the offset coefficient. The 8-bit + sign mode per- forms a conversion in less than half the time used by the 12-bit + sign conversion mode. The LM12L458’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 com- bination of single-ended or fully differential operation. Each input is referenced to ground when a multiplexer channel op- erates in the single-ended mode. Fully differential analog in- put channels are formed by pairing any two channels to- gether. The LM12L458’s internal S/H is designed to operate at its minimum acquisition time (1.5 µs, 12 bits) when the source impedance, R S,is ≤ 80Ω (fCLK ≤ 6 MHz). When 80Ω < RS ≤ 5.56 k Ω, the internal S/H’s acquisition time can be increased to a maximum of 6.5 µs (12 bits, f CLK = 6 MHz). See Section 2.1 (Instruction RAM “00”) Bits 12–15 for more information. Microprocessor overhead is reduced through the use of the internal conversion FIFO. Thirty-two consecutive conver- sions 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 LM12L458 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 LM12L458’s operation. This mode internally connects the voltages present at the V REF+,VREF−, and GND pins to the internal V IN+ 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. 2.0 Internal User-Programmable Registers 2.1 INSTRUCTION RAM The instruction RAM holds up to eight sequentially execut- able instructions. Each 48-bit long instruction is divided into three 16-bit sections. READ and WRITE operations can be issued to each 16-bit section using the instruction’s address and the 2-bit “RAM pointer” in the Configuration register. The eight instructions are located at addresses 0000 through 0111 (A4–A1, BW = 0) when using a 16-bit wide data bus or at addresses 00000 through 01111 (A4–A0, BW = 1) when using an 8-bit wide data bus. They can be accessed and pro- grammed in random order. Any Instruction RAM READ or WRITE can affect the se- quencer’s operation: The Sequencer should be stopped by setting the RESET bit to a “1” or by resetting the START bit in the Configuration Register and waiting for the current instruction to finish ex- ecution before any Instruction RAM READ or WRITE is initi- ated. A soft RESET should be issued by writing a “1” to the Con- figuration Register’s RESET bit after any READ or WRITE to the Instruction RAM. The three sections in the Instruction RAM are selected by the Configuration Register’s 2-bit “RAM Pointer”, bits D8 and D9. The first 16-bit Instruction RAM section is selected with the RAM Pointer equal to “00”. This section provides multi- plexer channel selection, as well as resolution, acquisition time, etc. The second 16-bit section holds “watchdog” limit #1, its sign, and an indicator that shows that an interrupt can be generated if the input signal is greater or less than the programmed limit. The third 16-bit section holds “watchdog” www.national.com 18 |
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