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OPA4140 Ibias

marcel rossewij
Prodigy 30 points
Other Parts Discussed in Thread: OPA4140, OPA4277, OPA4131

When using the OPA4140 as photodiode (silicon sensor QP50-6 TO8S, R>>100Mohm measured with keitley 2100 over all temperatures) amplifier with 1Mohm (//680pF) gain, I measure higher than expected offset.

According to the datasheet

Ibias(max at 25C) =10pA

Offset max =220uV (-40 to +125 Celcius)

Max offset due to bias at 25 Celcius is: 10pA*1Mohm=10 uV.

Assuming doubling every 10 kelvin rise, the offset at Tamb=65Celcius  10pA *2^((65-25)/10)=160pA=> 160 uV. 

I measure offsets larget than 1mV (almost factor 10 higher)

 

Tamb comes from a sensor in a box.

Tcase comes from a sensor taped on the opamp

Does anyone know what could be wrong?

  • 0
    Guru 21975 points

    Marcel,

    I suggest some additional diagnostic measurements:

    Disconnect the photodiode and repeat the measurements. You may be getting unexpected leakage current from the photodiode or its connections. Is the photodiode reverse biased with a voltage? Perhaps the resistance measurement you made was with a very small reverse bias voltage. A larger bias voltage will increase leakage current. Also, consider that leakage of the photodiode increases with temperature similar to the op amp input bias current.

    Consider the polarity of the error as this may provide clues as to a source of leakage. There could be due to board contamination, especially to power supply connections. These leakages can increase dramatically at high temperature.

    Please attach a schematic if you have further questions as this can aid in the discussion. Provide information on power supply and bias voltages, etc.

    Regards, Bruce.

  • 0 in reply to Bruce Trump
    Prodigy 30 points

    Hello Bruce,

    Thanks for the quick reply.

    I measured the photo diode resistance over the temperature range involved and it always remained > 100MOhm

    The OPA4140 photodiode amplifier (U1) was followed by another opamp OPA4131 (U2) for summing the 4 quadrants and give another 10x gain. I replaced U2 - OPA4131 with a OPA4277 to reduce the offset (which worked very well) and power. I redid the temperature mesurement with a supprising result. The OPA4140 Ibias now performs as expected:

     

    Taking Ibais=8pA @25Celcius, and doubling every 10 degrees gives the yellow line which fits very well with the measured blue one. I do not understant how exchangin the subsequent opamp can affect the output at the first opamp (summer with gain x10 uses 4x10k and one 100kohm resistor). Or there was maybe a contamination... (maybe the tape of temp sensor)

    But it is good that it works now.

    Also nice is the measured offset drift -0,4uV/Celcius (at the start of the blue line) is close to the typical of 0,35uV/Celcius.

    Also nice is that the measured Tcase is around 15 degrees higher than Tambient, which fits well with the  50C/W * 8mA * 30V =  12W

     Thanks again for the quick reply

    Regards

    Marcel