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TLV170: Butterworth filter

Kungyeh Chen
Prodigy 80 points
Part Number: TLV170
Other Parts Discussed in Thread: TLV9002, OPA2990

Hi,

I want to use a "Bandpass Filter-4th order Butterworth" circuit in audio, connected between the microphone and codec.

Please refer to the attachment for the Simulation Export.

.filter-design-report-20.pdf

I have selected TLV170 and set the supply voltage to Vcc=3.3V and Vee=0V (GND).

Is there anything that needs to be paid attention to?

Thanks.

  • +1
    Guru 48395 points

    Well Kngyeh

    This circuit will have problems on single supply as you have biased the input at Ground. Out of range for any output swing. 

    You might fix this by splitting that V+ R to ground into a 1/2 voltage divider off of the Vcc supply by doubling the value shown into a divider. You would then also need to add a blocking cap in series with the gain resistor to hold the output pin at midsupply DC. 

  • +1 in reply to Michael Steffes
    TI__Mastermind 26310 points

    Hi Kungyeh,

    Michael is correct. You need to make sure to adjust the op amps common mode voltage or you will be distorting the signal. 

    Another potential issue with this circuit is the TLV170's limited common mode voltage range can't go past 2V below the positive supply as shown in the datasheet: 

    This will cause distortion in your circuit depending on your input signal. 

    Also, If you are only using a 3.3V single supply, why are you looking at a high voltage device like the TLV170? I would recommend taking a look at the TLV9002 if you only plan on needing a low voltage device (less than 5V). If you do want a high voltage op amp, then I would recommend the OPA2990. It's the next generation of the TLV170, but with true rail to rail input and output! 

    Best Regards, 

    Robert Clifton 

  • +1
    Guru 140200 points

    Hi Kungyeh,

    you could do it this way:

    kung_tlv9002.TSC

    If the 1.65VDC pseudo-ground potential at the output of last stage is unwanted, add a high pass filter to provide AC coupling.

    Kai

  • +1 in reply to Robert Clifton56
    Guru 48395 points

    Hey Robert, I suspect the TLV170 came out of the Filter design tool as a suggestion, I have not looked for awhile, but the part selection flow there was very primitive. That is always a very difficult task intrinsically, and usually not linked to new product offerings very well. 

  • 0 in reply to Michael Steffes
    Guru 140200 points

    Hi,

    I think it's a bit illusionary to assume that there's something like an optimum filter with optimally chosen components and with an optimum chosen OPAmp. The optimum OPAmp depends on so many factors that's impossible to suggest an "optimum" OPAmp. Think only of noise, offset voltage, input bias currents, input voltage range, and so many many other parameters. In a circuit with little demands on precision it would not make any sense to choose an expensive OPAmp and -vice versa- in an precision appliction it would not make sense to choose a cheap OPAmp. And in a +3V single supplied circuit another OPAmp has to be chosen than in a +/-15V application. So, the recommended OPAmp in such a filter tool can only be a very rough suggestion. No more.

    Another issue is the resistor values. It's a difference whether you have a high speed OPAmp which needs low valued resistors or a micropower or even nanopower application where you can only choose high valued resistors. Even in the above example, R1_S2 = 301R is way too low ohmic for a standard OPAmp like the TLV9002. So, in most cases the resistors and caps would need to be properly scaled for the actual application anyway.

    So, from my point-of-view, it's impossible to recommend the one and only optimum filter circuit. I think what counts in such a filter tool is mainly the filter circuit topology and the proper time constants. So, if I would fabricate such a filter tool, I would use an ideal OPAmp and would recommend a "suited" OPAmp in a second step, if at all. And I would give some examples on how to scale the filter components (R x 10 in combination with C / 10, e.g.).

    Kai

  • 0 in reply to kai klaas69
    Guru 48395 points

    Well of course Kai, 

    I had set up an automated screening flow for the Intersil tool and "optimum" was never in my thinking - my like functional and plausible. At minimum

    I/O ranges satisfied, 

    Min GBP satisfied

    SR satisfied

    Overshoot does not clip

    etc. 

    But this intended up with a ranked order of minimal to more design margin selections where from there things like offset, supply current, price could be traded off.