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LTC2408I 数据表(PDF) 29 Page - Linear Technology |
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LTC2408I 数据表(HTML) 29 Page - Linear Technology |
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29 / 36 page  29 LTC2404/LTC2408 noise bandwidth of the system to 6Hz. The noise band- width of the LTC2408 without any input bandwidth lim- iting is approximately 150Hz. A roll-off at 1500Hz eliminates all higher order images of the base bandwidth of 6Hz. In the example shown, the optional bandwidth- limiting filter has a – 3dB point at 1450Hz. This filter can be inserted after the multiplexer provided that higher source impedance prior to the multiplexer does not reduce the – 3dB frequency, extending settling time, and resulting in charge sharing between samples. The settling time of this filter to 20+ bits of accuracy is less than 2ms. In the pres- ence of external wideband noise, this filter reduces the apparent noise by a factor of 5. Note that the noise band- width for noise developed in the amplifier is 150Hz. In the example shown, the gain of the amplifier is set to 40, the point at which amplifier noise gain dominates the LTC2408 noise. Input voltage range as shown is then 0V to 125mV DC. The recommended capacitor at C2 for a gain of 40 would be 560pF. Interfacing the LTC2404/LTC2408 to the 68HC11 Microcontroller The listing in Figure 28 is a simple assembler routine for the 68HC11 microcontroller. It uses PORT D, configuring it for SPI data transfer between the controller and the LTC2408. The program shows how to select and enable a MUX channel and retrieve conversion data. Figure 27 shows the simple 4-wire SPI connection. APPLICATIONS INFORMATION LTC2408 CLK SCK SD0 CSADC CSMUX DIN 19 25 24 23 20 21 68HC11 24048 F27 SCK (PD4) MISO (PD2) SS (PD5) MOSI (PD3) Figure 27. Connecting the LTC2408 to a 68HC11 MCU Using the SPI Serial Interface The code begins by declaring variables and allocating four memory locations to store the 32-bit conversion result and a fifth location to store the MUX channel address. This is followed by initializing PORT D’s SPI configuration. The program then enters the main sequence. It begins by sending the MUX channel data. It then activates the LTC2408’s serial interface by setting the SS output low, sending a logic low to CSADC/CSMUX. This also activates the selected MUX channel. It next waits in a loop for a logic low on the data line, signifying end-of-conversion. After the loop is satisfied, four SPI transfers are completed, retrieving the conversion. The main sequence ends by setting SS high. This places the LTC2408’s serial interface in a high impedance state and initiates another conver- sion. The program in Figure 30 modifies the MUX channel selection routine in Figure 28’s listing for selection of 16 channels. Figure 29 shows the connections between the LTC1391, LTC2408 and the 68HC11 controller. ********************************************************** ** * This example program loads multiplexer channels selection data into * * the LTC2408’s internal MUX and then transfers the LTC2408’s 32-bit * * output conversion result to four consecutive 8-bit memory locations. * ** ********************************************************** * *************************************** * 68HC11 register definitions * *************************************** * PORTD EQU $1008 Port D data register * “ - , - , SS* ,CSK ;MOSI,MISO,TxD ,RxD “ DDRD EQU $1009 Port D data direction register SPCR EQU $1028 SPI control register * “SPIE,SPE ,DWOM,MSTR;SPOL,CPHA,SPR1,SPR0” SPSR EQU $1029 SPI status register * “SPIF,WCOL, - ,MODF; - , - , - , - “ SPDR EQU $102A SPI data register; Read-Buffer; Write-Shifter * * RAM variables to hold the LTC2408’s 32 conversion result * |
类似零件编号 - LTC2408I |
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类似说明 - LTC2408I |
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