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  • TI Thinks Resolved

TL082: Need to make a DC integrator to integrate a slow varying DC voltage over time

Intellectual 650 points

Replies: 13

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Part Number: TL082

This is for sensing sunlight for a product in an agriculture environment. I have the transimpedance circuit already for converting the output of a photo diode to a voltage. The photo diode is looking at the sun from a greenhouse. I want to integrate that essentially DC signal over time. For the max voltage into the integrator (say 5 v) if that max voltage is present all the time I want the integrator to reach its max voltage after 5 minutes. I will short the integrator capacitor through a resistor after the integrator reaches its max output voltage to return it to zero. I am using 4.7 meg as the input resistor (R1) and also as the feedback resistor (Rf) and between 94 and 100 uf for the feedback capacitor (Cf). I believe this should give about 5 minutes of time (tau=0.69CfR1). I would use a TL082 (cheap; low offset V; etc) if it was single supply. I guess I could use EAGLE CAD to supply a circuit if needed. Looking for the right circuit and device (op amp) for this.

Alvin P. Schmitt

APSTRON LLC

P.O. Box 201

Newport VA 24128-0201

  • In reply to Alvin P. Schmitt:

    Hi Alvin,

    do you mean this integrator?

    But this is an AC coupled integrator, which can only be used for AC signals. This integrator is mentioned in this appnote:

    alvin.TSC

    Kai

  • In reply to kai klaas69:

    Hi Alvin,

    I would do it with a digital integrator. A VCO could be driven by the TIA's output, like shown below:

    The output signal of TIA (brown curve) and of VCO (green curve) is shown for the photodiode currents 1µA, 100nA and 10nA. The periods of VCO output signals are 1.15ms, 11.4ms and 127ms. This corresponds to frequencies of 870Hz, 87.7Hz and 7.87Hz.

    The advantage of using a VCO is that you can scale the integration time very easily now. Connect a CD4040 binary counter to the VCO output and divide the frequency of VCO signal by a factor of 32, by using the output Q5 of CD4040. Connect this output to the input of a ICM7217. The ICM7217 is a counter with a 4 digit (7 segment LED) driver, which can count up from "0" to "9999". With a photodiode current of 1µA the ICM7217 needs 6.1 minutes to count up to "9999".

    When using the output Q8 of CD4040, on the other hand, the ICM7217 will need 49 minutes to count up to "9999". And when using the output Q11 of CD4040, the ICM7217 will need 6.5 hours to count up to "9999". In all these examples a photodiode current of 1µA is assumed.

    The CD4040 has Schmitt trigger inputs and is very easy to use. The ICM7217 is also easy to use. The whole circuit can be powered by +5V. Of course, any other VCO or frequency divider can be used here.

    Some words on the TIA and VCO: U4 is a TLV9002 because of its low input bias current, low input offset voltage, rail-to-rail capability at input and output and low cost. I have taken it because Ron has recommended it to you. For the VCO the LM358 is good enough. The simulation does not show any advantage, if the TLV9002 or the LMV358 is used instead of LM358. The VCO works down to a photodiode current of 2.5nA, at least in the simulation. Linearity of VCO suffers at very low photodiode currents. But I think one can live with it in this application.

    alvin2.TSC

    Kai

  • In reply to kai klaas69:

    Actually I am going to do it digitally. My input is the output of a transconductance amplifier going to a ten bit AtoD. All I have to do is sample on a regular basis and add up the ten bit outputs. I already know what the upper value is that I want to integrate to so when the accumulator reaches that output then I know to trigger my operation. That time varies do to what the external environment does which is what I am measuring. Can't go into more than tat because it is my IP. I made the circuit I found and am not happy with their current source so I am just doing the analog version for my own education. I would do it under your spice but I do not know how to use it well enough yet. (TIAL-TI?) Anyway doing it digitally will let me more easily adjust the upper bound - or maybe more importantly let the user adjust it. Thanks for your help.

    Alvin P. Schmitt

    APSTRON LLC

    P.O. Box 201

    Newport VA 24128-0201

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