The circuit described in this document uses the monitor current output, IPDM, of the ADL5317 to interface directly to an Analog Devices translinear logarithmic amplifier, such as the AD8304, AD8305, ADL5306, or ADL5310. Figure 1 shows the basic connections necessary for interfacing the ADL5317 to the AD8305. In this configuration, the designer can use the full current mirror range of the ADL5317 for high accuracy power monitoring.

Measured rms noise voltage at the output of the AD8305 vs. input current is shown in Figure 2 for the AD8305 by itself and in cascade with the ADL5317. The relatively low noise produced by the ADL5317, combined with the additional noise filtering inherent in the frequency response characteristics of the AD8305, result in minimal degradation to the noise performance of the AD8305.

The ADL5317 is primarily designed for wide dynamic range applications simplifying APD bias circuit architecture. Accurate control of the bias voltage across the APD becomes critical to maintain the proper avalanche multiplication factor as the temperature and input power vary. Figure 3 shows how to use the ADL5317 with an external temperature sensor to monitor the ambient temperature of the APD. Using a lookup table and DAC to drive V_SET, it is possible to apply the correct V_APD for the conditions. Note that Pin 9, Pin 10, and Pin 12 to Pin 15 were removed for simplification.

In this application, the ADL5317 is operating in linear mode. The bias voltage to the APD, delivered at Pin VAPD, is controlled by the voltage (V_SET) at Pin VSET. The bias voltage at VAPD is equal to 30 × VSET.

The range of voltages available at VAPD for a given high voltage supply is limited to approximately 33 V (or less, for V_APD < 41 V). This is because the GARD and VAPD pins are clamped to within ~40 V below VPHV, preventing internal device breakdowns.

The input current, IAPD, is divided down by a factor of 5 and precisely mirrored to Pin IPDM. This interface is optimized for use with any of the Analog Devices translinear logarithmic amplifiers (for example, the AD8304 or AD8305) to offer a precise, wide dynamic range measurement of the optical power incident upon the APD.

If a voltage output is preferred at IPDM, a single external resistor to ground is all that is necessary to perform the conversion. Voltage compliance at IPDM is limited to V_PLV or V_APD/3, whichever is lower.

Careful consideration should be made to the layout of the circuit board in this configuration. Leakage current paths in the board itself could lead to measurement errors at the output of the translinear log amp, particularly when measuring the low end of the ADL5317’s dynamic range. It is recommended that when designing such an interface that a guard potential be used to minimize this leakage. This can be done by connecting the translinear log amp’s VSUM pin to the NC pin of the ADL5317, with the VSUM guard trace running on both sides of the IPDM trace, as shown in Figure 1. Additional details on using VSUM can be found in the AD8304 and AD8305 data sheets. The VSET pin of the ADL5317 can be used in a similar fashion to guard the VAPD trace.