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

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

• Alvin,

Do you have supply voltage(s) chosen? Is it 5V? TL082 won't work well with that.
5V and 4.7M is 1.06uA ; t = CV/I; t = 100uF * 5V / 1.06uA = 472s = 7.9 minutes

Regards,
Ronald Michallick
Linear Applications

TI assumes no liability for applications assistance or customer product design. Customer is fully responsible for all design decisions and engineering with regard to its products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning Customer's design. If Customer desires engineering services, the Customer should rely on its retained employees and consultants and/or procure engineering services from a licensed professional engineer (LPE).

• In reply to Ron Michallick:

Alvin,

For 5V, I recommend TLV9002

Regards,
Ronald Michallick
Linear Applications

TI assumes no liability for applications assistance or customer product design. Customer is fully responsible for all design decisions and engineering with regard to its products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning Customer's design. If Customer desires engineering services, the Customer should rely on its retained employees and consultants and/or procure engineering services from a licensed professional engineer (LPE).

• In reply to Ron Michallick:

Alvin,

For 5V, I suggest TLV9002

Regards,
Ronald Michallick
Linear Applications

TI assumes no liability for applications assistance or customer product design. Customer is fully responsible for all design decisions and engineering with regard to its products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning Customer's design. If Customer desires engineering services, the Customer should rely on its retained employees and consultants and/or procure engineering services from a licensed professional engineer (LPE).

• In reply to Ron Michallick:

What was supplied was an AC integrator not a DC one.

Alvin P. Schmitt

APSTRON LLC

P.O. Box 201

Newport VA 24128-0201

• In reply to Alvin P. Schmitt:

Alvin,

Can you provide an example of an integrator that only works with DC?

Regards,
Ronald Michallick
Linear Applications

TI assumes no liability for applications assistance or customer product design. Customer is fully responsible for all design decisions and engineering with regard to its products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning Customer's design. If Customer desires engineering services, the Customer should rely on its retained employees and consultants and/or procure engineering services from a licensed professional engineer (LPE).

• In reply to Ron Michallick:

Texas Instruments app note in the early 70's has a jfet with r1 connected to first gate c1 connected to first gate and first drain. first source connected to ground. first drain connected to second source second gate connected to r r in series dropping the bias to one half VCC second fet used as a current source. It is amazing no one at TI knew how to solve this.

Alvin P. Schmitt

APSTRON LLC

P.O. Box 201

Newport VA 24128-0201

• In reply to Ron Michallick:

Texas Instruments app note in the early 70's has a jfet with r1 connected to first gate c1 connected to first gate and first drain. first source connected to ground. first drain connected to second source second gate connected to r3 r4 in series dropping the bias to one half VCC second fet used as a current source. It is amazing no one at TI knew how to solve this. Probably because you were not alive in the 1970 when discretes were used to solve problems not integrated circuits.

Alvin P. Schmitt

APSTRON LLC

P.O. Box 201

Newport VA 24128-0201

• In reply to Alvin P. Schmitt:

Hi Alvin,

:-) you are a funny guy! Of course, we know how to do it, but we hoped that you would give some more details.

It's not so simple to find a 100µF cap which isn't plagued by considerable leakage current and bias voltage caused capacitance drop.

Apart from that, we are not here to do your design work, but only to help to get a chip work and fix issues in your design.

By the way, many of the "good old" discrete circuits didn't work very well, you remember? And it's just the JFET which comes with huge manufacturing tolerances which can make problems here.

Why not using a tiny microcontroller for all this?

Kai
• In reply to kai klaas69:

Alvin

Like Kai said, we are waiting for more details especially VCC voltage so this design only needs to be completed one time.

Max input is 5V (fastest output ramp) , what is minimum input (no output ramp)?

In the 1970's I was repairing televisions with tubes.

Anyway, I used your text to draw this. It seems incomplete.

Regards,
Ronald Michallick
Linear Applications

TI assumes no liability for applications assistance or customer product design. Customer is fully responsible for all design decisions and engineering with regard to its products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning Customer's design. If Customer desires engineering services, the Customer should rely on its retained employees and consultants and/or procure engineering services from a licensed professional engineer (LPE).

• In reply to Ron Michallick:

second transistor drain is vcc. first transistor drain is output. top of resistor divider is vcc other side of r1 is dc voltage input.

Alvin P. Schmitt

APSTRON LLC

P.O. Box 201

Newport VA 24128-0201