Multiple LTC2315-12 ADCs Share SPI Bus

The data sheet doesn’t always show everything you need to know about using a part. For example, a common application when using multiple serial ADCs is to share the clock and data output lines to simplify the interface to the processor/FPGA. The data sheet for the LTC2315-12 12-bit 5Msps, SPI compatible ADC with an integrated bandgap and reference buffer does not show how to do this. Sharing these lines is possible as long as the ADCs are not converting simultaneously. The SDO output line of the LTC2315 becomes active when the CSL input goes low, starting a conversion. Therefore only one LTC2315 can be converting at any one time so that the SDO output lines do not fight with each other.

Figure 1. Two LTC2315-12s share a common SPI bus

Figure 1. Two LTC2315-12s share a common SPI bus

The circuit of Figure 1 shows two LTC2315s sharing a common SPI bus. The CSL1 and CSL2 input lines control which ADC is converting. Only one CSL line can go low at any time. Figure 2 shows a timing diagram for the two ADCs. Sufficient time should be allowed after the CSL1 line is brought back high before the CSL2 line is brought low so that the SDO output line of U1 returns to a Hi-Z state before the SDO output line of U2 becomes active. This same technique can be used for other ADCs in this family including the LTC2314, LTC2313 and LTC2312 12-bit and 14-bit ADCs, although the exact timing diagram will need to be adjusted to the specifications of the particular ADC.

Figure 2. Timing Diagram shows two ADCs transferring data with shared SCK and SDO lines using the circuit of Figure 1

Figure 2. Timing Diagram shows two ADCs transferring data with shared SCK and SDO lines using the circuit of Figure 1

作者

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Guy Hoover

Guy Hoover是一名拥有超过30年凌力尔特公司工作经验的工程师,曾担任技术员、IC设计工程师和应用工程师。

T他的职业生涯始于LTC,当时的职位是技术员,并向参与多个产品工作的Bob Dobkin、Bob Widlar、Carl Nelson和Tom Redfern学习,涉及产品包括运算放大器、比较器、开关稳压器和ADC。在此期间,他还花了大量时间来编写测试程序以对这些器件的特性进行表征。

他在LTC的下一段职业生涯是学习PSpice和设计SAR ADC。由Guy设计的产品包括LTC1197系列10位ADC和LTC1864系列12位和16位ADC。

Guy目前是混合信号部的应用工程师,专门从事SAR ADC应用支持工作。这包括为SAR ADC演示板设计、编写Verilog代码和测试程序,帮助客户优化包含LTC SAR ADC的产品,并希望编写有用的应用文章,将有关使用这些器件的知识传递给客户。

Guy毕业于德锐技术学院(现为德锐大学),拥有电子工程技术硕士学位。