CN0057

Interfacing the ADL5317 High Side Current Mirror to a Translinear Logarithmic Amplifier in an Avalanche Photodiode Power Detector Application
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Circuit Function & Benefits

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This circuit uses the monitor current output, IPDM, of the ADL5317 to interface directly to an Analog Devices, Inc., 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 of an avalanche photodiode (APD).



Figure 1: Typical connection of ADL5317 to the AD8305 Translinear Logarithmic Amplifier


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, results in minimal degradation to the noise performance of the AD8305.


Figure 2: RMS Noise of AD8305 vs. AD8305 Cascaded with ADL5317

Circuit Description

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The ADL5317 is primarily designed for wide dynamic range applications that simplify APD bias circuit architecture. Accurate control of the bias voltage across the APD becomes critical to maintain the proper avalanche multiplication factor when 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 the VSET voltage, it is possible to apply the correct VAPD for the conditions. Note that all connections to the ADL5317 are not shown for clarity.



Figure 3: Typical APD Biasing Application using the ADL5317

In this application, the ADL5317 operates in linear mode. The bias voltage to the APD, delivered at the VAPD pin, is controlled by the voltage (VSET) at the VSET pin. 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 VAPD < 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 by a factor of 5 and precisely mirrored to the IPDM pin. 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 VPLV or VAPD/3, whichever is lower.

Common Variations

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Careful attention should be paid to the layout of the circuit board in this configuration. Leakage current paths in the board itself can 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, 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.


The circuit must be constructed on a multilayer PC board with a large area ground plane. Proper layout, grounding, and decoupling techniques must be used to achieve optimum performance (see MT-031 Tutorial, MT-101 Tutorial, the ADL5317 evaluation board layout, and the AD8305 evaluation board layout).

Sample Products

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Samples

Product

Description

Available Product
Models to Sample

ADL5306 60 dB-RANGE (100 nA-100 µA) Low-Cost Logarithmic Converter

ADL5306ACPZ-REEL7

ADL5317 Avalanche Photodiode Bias Controller and Wide-range (5 nA - 5 mA) Current Monitor

ADL5317ACPZ-REEL7

ADL5310 120 dB RANGE (3 nA TO 3 mA) Dual Logarithmic Converter

ADL5310ACPZ-REEL7

AD8304 160 dB Logarithmic Amplifier with Photo-Diode Interface

AD8304ARUZ

AD8305 100 dB-range (10nA-1mA) Logarithmic Converter

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