LT6650 input/output oscillations

Hi to everyone,

I'm sorry if the question could seems trivial to someone, I'm an electronic engineering student and I'm trying to use the LT6650 as voltage reference (>450mV) for a comparator input.

I've tried the basic configuration shown in Figure. 8 at page 9th of the datasheet and it works very well, then after a deep reading I've tried to modify the input's RC filter and the output's load capacitance to better suite a noisy environment as explained at page 8th. 

First try:

1) Input RC filter changed as shown in Figure. 2, output capacitance at 1 uF as reference:

The result is that during the output's settling time the OUT pin of the IC sinks a current with a maximum of about 9 mA resulting in an output's voltage undershoot that is recovered in about 3 ms.

2) RC input's filter as 1) and output capacitance changed to 10 uF:

The current in the output capacitance behave as shown in the figure.

3) Input filter as in reference Figure 8 (1 k - 1 uF) and output capacitance increased to 20 uF:

The current in the output capacitance oscillate.  The problem could be resolved increasing the feedback capacitor, it's sufficient 2 nF to make the oscillation disappear.

Is there anyone who can explain me why the circuit behave like that? Is it wrong changing the feedback capacitor? Is it possible to avoid the strange output capacitor's current behaviour when increasing the RC time constant of the input filter? (Case 2) Thank you to everyone.

PS: the circuit topology is as follow:

  • 0
    •  Analog Employees 
    on Aug 14, 2019 8:21 AM over 1 year ago
    Hi DarioLaezza
    Simply put the one you are reading is the noise/AC signal that the output Capacitor is bypassing through ground.
    On what application are you going to use the component? Do you have any specific kind of load?
    For example, try assuming that you have a 50Kohm load and do the reading at this load. The current that will be passing through the load is the current that your actual load will be having. The noisy current that you are reading right now will be brought to ground by the bypass capacitor.
    Best Regards,
    Mon
  • Hi mon23,

    Thank you for the reply. I would use this output as a reference voltage for a comparator, I've tried with the comparator attached to the output but without a load in between. Do you have any advice for a that kind of application? Thank you so much.

    Dario Laezza

  • 0
    •  Analog Employees 
    on Aug 15, 2019 6:44 AM over 1 year ago in reply to DarioLaezza

    Hi Dario Laezza,

    What I mean by the load is the one you will be driving by the output of the Voltage Reference, in this case the Comparator.

    So basically the 50kohm resistor that I said in my first comment will be replaced by your comparator. Comparator has an input resistance indicated in the data sheet. So when looking from the output of the voltage reference in a quick analysis you can just treat the comparator as a resistor.

    I would recommend to just use the first circuit that you said, the one with the input filter as shown in Figure 2, with you resistor combination with 1nF cap at the output and use the 1uF capacitor load. Just connect the circuit at the comparator. and see to it that 1uF capacitor is as near as possible from the output of the reference and input of the comparator.

    Just want to ask, what supply do you plan to use in the voltage reference that cause you to use the input filter? 

    Best Regards,

    Mon

  • Hi mon,

    Thank you for your reply. I'm evaluating an input filter because the circuit will be deployed in an automotive environment. The power supply should be the one from the Low Voltage battery. Do you have any advice in this sense? Thank you

    Best regards,

    Dario Laezza

  • 0
    •  Analog Employees 
    on Aug 19, 2019 5:52 AM over 1 year ago in reply to DarioLaezza

    Hi Dario Laezza,

    Other than what is indicated in the datasheet, just keep the input capacitor near the input and the output capacitor near the output of the Vref. And also just to make sure that no noise is induced at the input of your comparator, add a bypass capacitor near the input of it.

    Best Regards,

    Mon