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TL084: CD-Type works nice in Sallen-Key filter but ID-Type doesn't

Lothar B
Prodigy 40 points
Part Number: TL084
Other Parts Discussed in Thread: TL974

Hallo support-crew and readers,


the following lowpass filter was designed with FilterPro and is used for pwm filtering:

We never had any problems while we used TL084CD operational amplifier. The filter response was excellent and never unstable!

But if we put TL084ID in the circuit the second stage becomes unstable and is oscillating with about 4 kHz.

The question is: Why does TL084CD is working very well but TL084ID does not?

Which op parameter does influence or explain this behaviour?
Is there a possibility to bring this circuit to work with both types of TL084?

Your support is very appreciated.

  • 0
    TI__Guru*** 140106 points

    Lothar,

    Is there anything thing else on the 2nd output (other than C2 and inverting input)?

    Perhaps a cable?

    4 kHz is a low frequency. Is the oscillation sawtooth or sine wave shaped?

  • 0 in reply to Ron Michallick
    Prodigy 40 points

    Hi Ron,

    the output of stage 2 drives a resistor voltage divider with about 15K.

    When we power up the circuit we see the output voltage is rising up to about 2.5V (because we use single power supply 5V, the 2.5V is available at the input of stage 1)

    and stage 2 starts oscillating at TL084ID. The oscillation is a slightly distorted sine wave. With TL084CD  there is no oscillation neither at start up nor when filter is in use.

    We know that sallen-key with higher Q tends to become unstable. But why should TL084CD be stable and TL084ID unstable on the same PCB?

    Maybe it is a parasitical effect? When we build up the circuit with TL084CN (PDIP) on a bread board we also get this bad behaviour.

    We changed the design to a butterworth filter and lower Q which looks stable with both circuits. But pwm filter result is not as good as before.

  • 0 in reply to Lothar B
    TI__Guru*** 140106 points

    Lothar,

    Now I see.  The problem is input common mode voltage. In your application it is VCC- + 2.5V. However the data sheet recommended VCC- + 4V as minimum input voltage. Also total VCC is 5V and 10V is recommended (in data sheet).

    When input voltage is too low, the input transistors lose headroom. VIO will get worse which makes a unity gain amplifier look like it has gain. This coupled with reduced AC performance can cause oscillation with the positive feedback.

    At input common mode voltage of 2.5V, the TL084 sample shows a unity gain of 1.17 which is greater than 1.

    There are many 5V op amps that can replace the TL084. 

  • 0 in reply to Ron Michallick
    Prodigy 40 points
    Dear Ron,

    i am afraid you are absolutely right and we have to replace the TL084.

    Do you think TL974 would be a good choice?
    We already ordered some samples and will verify this.

    Many thanks for your well founded help and best regards!
  • 0 in reply to Lothar B
    TI__Guru*** 140106 points
    Lothar,

    TL974 will work better. The common mode range is +1.15V from power rails. So 1.15V to 3.85V is OK at 5V power.
    Output is class A for low distortion.