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AD8304's internal buffer and VPDB not working as expected?


I want to integrate the AD8304 into my design. My plan is to build an optical power meter within the range of 1uW to 1mW.

According to the datasheet, a recommended configuration is shown in figure 7 which I have implemented. 

Although this configuration seems to work in the long term it only works for a couple of days. Puzzled by this behavior I took note of some observations over several samples during the last 2 months. Hopefully I can get some support or comments on this issue so I can move on. Otherwise I will appreciate recommendations for a replacement that can be easily implemented.

The observations are the following:

1.- When No buffer enabled, VP = 5 V and no PD current . VLOG varies from 0.9 V to 0.55V after a couple of days. After that period of time, VLOG is stable and VPDB is 0.6V.

2. When Buffer is enabled, VP = 5V and PD current flowing. The inner buffer was configured to provide a gain of 3. For instance a VLOG of 1V, VOUT is 3 V. After one day of operation, VOUT goes up 4.5Volts without no reason. Exploring the why of this situation, I found that VLOG remains stable but VPDB, which went up from 0.6 to 1.5V. Unfortunately, I can no longer use this configuration. This is has been a common behavior over several samples.

My hypothesis is that probably the power supply is not adequate for this application. It features large ripple  and it is not well regulated. It could happen that the AD8304 tries to compensate this power variations internally and then breaks out.

Could you confirm this possibility or advice what other thing could be the cause?.

best regards,