### Abstract

This article describes how three external resistors on the DS1841 logarithmic resistor are used to adjust the output range of an APD bias circuit. A spreadsheet is supplied that makes the adjustment process easy.

### APD Bias Circuit

The DS1841 temperature-controlled, NV, I^{2}C, logarithmic resistor contains one 7-bit logarithmic variable resistor. Used in conjunction with a step-up DC-DC converter, the DS1841 adjusts the bias voltage applied to an avalanche photodiode (APD). Three external resistors (R_{SER}, R_{TOP}, and R_{PAR}) are used to adjust the output range (Figure 1).

### Adjusting the APD Bias Range

A spreadsheet, DS1841 APD Bias Range Adjustment (xls), makes it easy to adjust the APD bias range. The spreadsheet has four input variables: R_{TOP}, R_{SER}, R_{PAR}, and V_{FB}. After inputting these resistor values, the spreadsheet then calculates four outputs: V_{APD} (max), V_{APD} (min), STEP (max), STEP (avg). It also generates two graphs: APD Bias vs. DAC Code, Volts Per Step vs. DAC Code. The interface in Figure 2 shows the four variables and the graphics generated from the values input there. Table 1 defines the terms used in the spreadsheet.

V_{FB} |
The voltage present at the feedback node of the DC-DC converter. |

V_{APD} (max) |
The maximum voltage to which the APD bias can be set under worst-case conditions. |

V_{APD} (min) |
The minimum voltage to which the APD bias can be set under worst-case conditions. |

STEP (max) | The maximum calculated voltage step that can occur between two adjacent DAC codes. |

STEP (avg) | The average voltage step size that occurs across the full range. |

V_{STEP} +20% |
The voltage step size when the variable resistor is at the maximum of the process range (+20%). |

V_{STEP} -20% |
The voltage step size when the variable resistor is at the minimum of the process range (-20%). |

V_{APD} +20% |
The APD bias voltage when the variable resistor is at the maximum of the process range (+20%). |

V_{APD} -20% |
The APD bias voltage when the variable resistor is at the minimum of the process range (-20%). |