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UAF42: Problem with UAF42AP band pass filter generating unwanted signal at its center frequency

Prodigy 90 points

Replies: 29

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Part Number: UAF42

Hello,

I have designed a band pass filter using part UAF42AP at f = 500 HZ (See my circuit schematics at 1.tiff). The filter is working well and neutralizing unwanted signals at other frequencies. However, while using the spectrum analyzer to evaluate its performance, I noticed that a signal (although weak) is generated by the IC at its center frequency. For example, you can notice this when inputs such as 1 Volts sin 50 Hz (see 3.tiff) or 1 Volts sin 5 KHz (see 2.tiff) were used. In other words, a self generated signal is added/mixed to the signal I am trying to collect at the center frequency, which is problematic. I also found an application data describing how to use UAF42AP to make a sine generator (see sbfa002.pdf). Therefore, I am suspecting that for some reason, maybe me not connecting some of the pins right, this chip is partially performing as an oscillator/sine generator. In addition, the self generated signal at the center frequency is in direct relation with the gain of the filter.

Please advise if any experts can help me resolve this problem. 

On a separate, any suggestions on how to reduce the noise when using UAF42AP would be appreciated as well.

P.S. I am using breadboard for this test. The green signal is the output and yellow is the input.

Best Regards,

sbfa002.pdf1.tiff2.tif

  • Hi again,

    I just noticed I did not upload "3.tiff" in my previous post. I upload it in this one. 

    My apologies,3.tif

  • Hi Kamran,

    It appears my first response to your UAF42 inquiry didn't post. Sorry about that and the delay it caused. I assume you used the Burr-Brown/TI APPLICATION BULLETIN, "FILTER DESIGN PROGRAM FOR THE UAF42 UNIVERSAL ACTIVE FILTER," as a guideline? If so, can you tell me which POLE-PAIR (PP) SUBCIRCUIT you used for the band-pass filter design?

    Additionally, I see your frequency domain measurements. Would it be possible for you to connect a DSO at each of the HP, LP, BP and Vout filter points and see if you can capture an image of the oscillation wherever it may be appearing? It would be helpful to see what it looks like in the time domain.

    Do you have power supply bypass capacitors on the supply pins to ground in your circuit? They need to be in place.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • In reply to Thomas Kuehl:

    Hi Kamran,

    I just found one problem with your UAF42 BP filter schematic and it may just be the way it is drawn. The 10 kilohm input resistor in the output amplifier stage is connected to the summing junction (pin 14) of the low-pass integrator stage. That isn't correct; the resistor should connect to the Band-pass Out at pin 7.

    I don't know if that will correct the 5 kHz oscillation problem, but it does fix one issue.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • In reply to Thomas Kuehl:

    input 1mV at 500 Hz real time output BP pin 6.tifinput 1mV at 500 Hz real time output HP pin 13.tifinput 1mV at 500 Hz real time output LP pin 1.tifinput 1V at 5 KHz real time output BP pin 6.tifinput 1V at 5 KHz real time output HP pin 13.tifinput 1V at 5 KHz real time output LP pin 1.tifinput 1V at 50 Hz real time output BP pin 6.tifinput 1V at 50 Hz real time output HP pin 13.tifinput 1V at 50 Hz real time output LP pin 1.tifHi Thomas,

    Thank you or your response.

    -I used the inverting pole-pair

    - You can find the real-time response of the filter with different inputs and LP,HP, and BP in the attached pictures. Please pay attention to scale of every signal in the bottom left of the pictures. As you will note, the unwanted 500 HZ signal shows itself in real-time as well, specifically for the case of 1 Volts sin 50 Hz.

    - Yes, I have used two 100 uf electrolytic capacitors for the + and - supply pins of the filter.

    Best Regards,

  • In reply to Thomas Kuehl:

    Hi Thomas,

    You are correct. I drew the sketch wrong. the 10 Kohm resistor is connected between pin 6 and pin7.

    Best Regards,

  • In reply to kamran keramatnejad:

    Hello Kamran,

    Then, I don't find anything incorrect about your UAF42 band-pass filter circuit from a schematic standpoint. Usually when that is the case there is something about the circuit construction that is leading to an unexpected oscillation. Do you have an image of your circuit construction you can share? Also, try adding some 100 nF bypass capacitors to the power supply pins. A 47 uF capacitor is a low frequency bypass and may be ineffective at higher frequencies. And you mentioned that you are driving another op amp with the UAF42 output op amp. Can you provide more details about the load?

    Have you tested more than one UAF42 device in your circuit? If so, did you still have the oscillation? I have to ask, did you obtain the UAF42 device you are using from TI directly, or a TI authorized distributor, or from somewhere else? Devices of unknown origin can have problems.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • In reply to Thomas Kuehl:

    Hi Thomas

    I added the 100 nf ceramic capacitors as you suggested and still no improvement was observed. You can find the picture of the setup I am studying right now. It consists of a LMC7660 voltage regulator in order generate the negative supply and the UAF42. No additional external op amp is being used at this point.

    As I mentioned I have not used the 47 uf, but 100 uf electrolytic capacitors for the bypass. The most significant issue I am having right now is not the high frequency operation. It is the generation of an unwanted signal at 500 Hz, presumably by the UAF42AP. I have purchased the IC from DIGI-key which is a credit distributor and I have used multiple number of UAF42AP ICs and I always encounter the issue I mentioned.

    Best Regards,

    1V at 5 KHz frequency domain.tif1V at 5 KHz time domain.tif

  • In reply to kamran keramatnejad:

    Hello Kamran,

    Oh, a white proto board! Certainly this is a quick, convenient way to test a circuit but unfortunately about the worst from signal response and spectral purity standpoints. The board and components are exposed to every possibility problem associated with of a circuit layout and external noise egress.

    I realize that you provided the frequency domain response of the UAF42 band-pass circuit, and it showed peaking around 500 Hz which was assumed to be a low-level oscillation. There was a reason I was asking for a DSO image of the band-pass filter output and Vout. I wanted to make sure that the 500 Hz peaking in the frequency domain was indeed an oscillation. So I request again that you capture the 500 Hz oscillation with the DSO and provide the image.

    The reason I need to see the presence of an actual oscillation is that maybe the 500 Hz peaking is not an oscillation, but instead the noise response of the band-pass filter. The UAF42 universal active filter topology is not know to readily oscillate. If the UAF42 circuit is physically subjected to the open environment of a proto board extraneous noise made up of a wide range of frequencies can be getting in to the circuit. You actually may be observing the noise signature of the filter's band-pass response. Since the filter is tuned to 500 Hz, and there is an addition G = -10 V/V amplifier at the output the noise would be greatest at that frequency. Also, you mention the 50 Hz noise. If your line frequency is 50 Hz the wiring and lights in the room will couple into the open circuit. The line frequency nearly always shows up in the frequency sweeps of even the best laid out audio circuits.

    If you have access to an a closable metal box in which the UAF42 proto board circuit can be enclosed you will almost certainly see the 50 Hz and broadband noises decrease. The power supply wires would need to shielded and bypassed upon entry into the box. A better plan is to power the UAF42 circuit from 9 V volt radio batteries kept inside the shielded box for the test. Once everything is shielded from the and noise ingress reduced look to see if an oscillation is present. 

    Lastly, you are powering the UAF42 from the LMC7660 charge pump, DC-DC converter. Make sure that the oscillator frequency isn't anywhere near 500 Hz. That includes any harmonics. If you find that temporarily powering the UAF42 circuit from the 9 V batteries makes a difference in the frequency domain characteristics, then the charge pump may be generating a frequency related to 500 HZ that is showing up in the output.

    A good, clean PC board layout is key to maximizing analog circuit performance. You may find that the UAF42 circuit performs as expected once the circuit is contained on a well laid out PC board.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • In reply to Thomas Kuehl:

    Hi Thomas, 

    Thank you for your tips and suggestions.

    I understand the limitations of the breadboard. However, as far as testing a circuit before designing/order the PCB goes, breadboard is the way to go. Do you have a better suggestion on how to test a circuit?

    By the way, I should mention that I saw the same issue when the whole module was implemented on a PCB in a different laboratory/location, although there may have been some negligence when designing the PCB as well.

    Can you tell me what you refer to as "DSO" and I shall perform the test and share the results for you ASAP?

    Regarding the 50 Hz noise, let me clarify that I did NOT mention I have a 50 Hz noise. I only used a 1 Volt Cos (2*pi*50Hz *t) and a 1 Volt Cos (2*pi*5KHz *t) as the input of the filter to evaluate how good it neutralizes the unwanted signals other than at 500 Hz. And it does a good job. However, it adds an unwanted 500 Hz to the system, which I am trying to get rid of.

    I am pretty sure that the issue is not coming from the DC-DC converter, since when I use it for a simple inverting opamp, I do not see that oscillation at 500 Hz, on the SAME breadboard. I also tried using the 9V radio batteries you suggested. no improvement still.

    Best Regards,

  • In reply to kamran keramatnejad:

    Hi Kamran,

    "However, it adds an unwanted 500 Hz to the system, which I am trying to get rid of." I find it perplexing that the UAF42 filter appears to be functioning correctly, but is adding a 500 Hz signal on its own.

    By DSO I mean a digital storage oscilloscope. I suggest a 10x probe to reduce the loading the probe adds to the circuit. Make sure the UAF42 circuit's 50 k input resistor is grounded for the measurement. I am looking for an image of the oscillation so that I can get an idea about its characteristics.

    Okay, regarding the 50 Hz noise. My misunderstanding. 

    Let's see what the DSO results look like before I make any other suggestions.

    Regards, Thomas

    Precision Amplifiers Applications Engineering