Single Supply RS232 Interface for Bipolar AD converters

The LT®1180 RS232 transceiver includes a charge pump which produces low ripple supplies with suffi - cient surplus current to drive a CMOS A to D converter and precision voltage reference. The circuit in Figure 1 operates from a single 5V supply, and draws a total quiescent current of only 37mA. These features make the circuit ideal for applications which must process bipolar signals with minimal support electronics. The LTC1094 serial A to D converter requires both a low noise supply and reference voltage for accurate operation.1 These design problems are solved with an LT1021 precision reference, which delivers a stable, low noise, 5V signal from the LT1180’s V+ output.
Single Supply RS232 Interface for Bipolar AD converters - schematic

. The chip select signal, CS, is generated from the incoming clock with a peak detector, constructed with a single PNP transistor. R and C are designed to hold the CS pin low for at least one clock period. Assuming the logic threshold in the LTC1094 is 1.4V, two useful rules of thumb for selecting R and C are: Design RC to be at least four times the clock period. And select C as small as possible to start the converter quickly. Minor aberrations in the CS signal are unimportant because the CS pin is level sensitive. The PNP is biased from the clean reference supply so very little noise is coupled into the A to D. Additional buffers are unnecessary because the peak detector drives a CMOS input.

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