TL084: Large Offset- Buffer Configuration- Not recognising low voltage signal

Siddharth Gupta84

Intellectual 450 points

Part Number: TL084
Other Parts Discussed in Thread: TMS320F28069, LM324, LM2902

Hi,

I am outputing a PWM signal from controller , passing it through LPF and making it work like a DAC.

Step 2: Passing this output through a buffer (TL084)

Step 3: Amplifying the Output by Gain of 3 (TL084)

Problem: Stage 2 output (output of buffer) works well for higher voltage signals. But at signals below 2V it outputs a constant 1.96 volt (Ofset?/).

What should I do to get same output as input for signals less than 2V in Stage 2. (Vcc of opamp is +12V -Single Supply).

over 7 years ago

0 Ron Michallick over 7 years ago

TI__Guru*** 140126 points

Siddharrth,

The lower end on the input common mode range is 4V (relative to VCC- pin).
Typically common mode range goes lower, however some device may have a large JFET threshold voltage, therefore it is best to keep input at 4V greater than ground (device's VCC- pin )

0 Siddharth Gupta28 over 7 years ago in reply to Ron Michallick

Prodigy 130 points

Hi ,

Thanks for the reply.

The lower Input limit for any opamp is -Vs + Vcm(LO) .

(Reference: http://www.analog.com/media/en/training-seminars/tutorials/MT-041.pdf).

-Vs is 0V in my circuit.

+Vcm(LO) cant find this in datasheet of TL084.

( Reference: http://www.ti.com/lit/ds/symlink/tl084.pdf)

According to your response keeping input voltage higher than 4V always is not possible with signals from controller TMS320F28069 (Vcc is 3.3V). I want to use an opamp as a buffer after the ADC for high input impedance. Please also elaborate how to arrive at minimum input voltage that opamp can understand (when Vcm(LO) parameter is not mentioned in datasheet.)

Siddharth Gupta

0 Siddharth Gupta84 over 7 years ago in reply to Ron Michallick

Intellectual 450 points

Hi ,

Thanks for the reply.

The lower Input limit for any opamp is -Vs + Vcm(LO) .

(Reference: http://www.analog.com/media/en/training-seminars/tutorials/MT-041.pdf).

-Vs is 0V in my circuit.

+Vcm(LO) cant find this in datasheet of TL084.

( Reference: http://www.ti.com/lit/ds/symlink/tl084.pdf)

Siddharth Gupta

0 Ron Michallick over 7 years ago in reply to Siddharth Gupta84

TI__Guru*** 140126 points

Siddharth,

Common mode specified in two tables in data sheet.

For a input signal 0V to 3.3V and output of 0V to 9.9V using a 12V supply, I suggest choosing another op-amp.

What are the other requirements needed?

0 Siddharth Gupta84 over 7 years ago in reply to Ron Michallick

Intellectual 450 points

Hi,

My requirement:

1. Opamp (Supply 12V) as buffer for - 0 to 3.3V DC Signal

2. Opamp (Supply 12V) as amplifier (0 to 3.3V) --> (0 to 10V)

Request to suggest a quad opamp package IC, that could cater to these requirement.

Thank you.

0 Ron Michallick over 7 years ago in reply to Siddharth Gupta84

TI__Guru*** 140126 points

Siddharth,

There is LM324 or LM2902, same part with different temperature specs.
Output near ground is possible when load is terminated to ground or sinking current is very low (<12uA).

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TL084: Large Offset- Buffer Configuration- Not recognising low voltage signal