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

Hello,

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,

Fausto

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  • Hello Bruce,

    thanks for your advice on the capacitor part numbers you provided. It would be very helpful.

    I have another question regarding this circuit configuration. After almost 1 week of continuous operation, the inner buffer of the AD8304 turns out to be damaged by no apparent reason. The output of the AD8304 is connected to the input of an ADC LTC2309 which is monitored to verify the behaviour of the amplified VLOG over time. Since this has been happening in the past.

    I measured  on pin 13  a voltage 4.9 Volts which is strange since I should have a voltage similar to VLOG.  As for the output on pin 11, the measured voltage  is around 0.015V.

    Regarding VLOG , It varies in function of the current in the PD. At least the voltage on this pin shows no issues.

    Do you know what could be wrong?. Would you recommend to use a external opam to perform the final amplification instead of using the inner buffer of the AD8304?, I'm planning to use a LM321 opam for this taks, do you think it is a good option or do you recommend another opam from Aanalog Devices? Something robust, low noise and operating at 5V will be ideal.

    Regards,

    Fausto

Reply
  • Hello Bruce,

    thanks for your advice on the capacitor part numbers you provided. It would be very helpful.

    I have another question regarding this circuit configuration. After almost 1 week of continuous operation, the inner buffer of the AD8304 turns out to be damaged by no apparent reason. The output of the AD8304 is connected to the input of an ADC LTC2309 which is monitored to verify the behaviour of the amplified VLOG over time. Since this has been happening in the past.

    I measured  on pin 13  a voltage 4.9 Volts which is strange since I should have a voltage similar to VLOG.  As for the output on pin 11, the measured voltage  is around 0.015V.

    Regarding VLOG , It varies in function of the current in the PD. At least the voltage on this pin shows no issues.

    Do you know what could be wrong?. Would you recommend to use a external opam to perform the final amplification instead of using the inner buffer of the AD8304?, I'm planning to use a LM321 opam for this taks, do you think it is a good option or do you recommend another opam from Aanalog Devices? Something robust, low noise and operating at 5V will be ideal.

    Regards,

    Fausto

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