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Old 5th March 2009, 09:35 AM   #1
Dxvideo is offline Dxvideo  Turkey
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Default A Simple opamp question?

Hi all,

I know that is not the right place for that..
But I've asked this question in "digital forum" and have no question until now..
Anyway, I am expecting to have an answer in here as usual
The question is simple;
I am designing an USB DAC and using PCM2702.. The digital part is not the matter of here...
For the LPF stage, I thought that an AD8397 will be a good choice because of its output current capability. With this way I expect to have a headphone output (like Amb guys did) also.. So I have designed a simple 2nd order LPF circuit with some gain (2x) for AD8397. This gain is added to drive both 32/64R headphones..
Now the question comes;
I am not sure to use that op-amp as both LPF and headphone driver.. I dont know but it seems "too good to be true".. What do you say about that? Will I have good results with that configuration?
Click the image to open in full size.
PS: LPF R/C values defined by trying on the simulation and the corner frequency is ~85Khz ..
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Old 5th March 2009, 10:45 AM   #2
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Should work fine.
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Old 5th March 2009, 11:18 PM   #3
bichi is offline bichi  United States
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Dxvideo:
- appears to a 2-pole Sallen-Key configuration
- at fc 85kHz, opamp GBP should be above 8.5mHz

Comments:
- I see ringing/oscillation on my sim, with your values (see phase plot exceeding 180deg rule)
- slight peaking, approx. +0.4db @ 24khtz
- consider increasing input coupling from 1uF to 4uF (improves low-freq phase response)
- consider Bessel filter configuration (better group delay and overshoot characteristics, look up "Blauert and Laws")

Hope this helps....

Attached:
- zipped sim Bode, transient plots and suggested Bessel schematics and comparison plots (823kB)
http://premium1.uploadit.org/bichi00...sult-Plots.zip

REFS:
Active Low-Pass Filter Design (Rev. B)
http://focus.ti.com/general/docs/tec...tName=sloa049b

Active Filter Design Application
http://focus.ti.com/docs/toolsw/fold...filterpro.html
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Old 5th March 2009, 11:32 PM   #4
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Sallen & Key needs very low output impedance to drive it's input. Do you have a buffer before the C14/16 input caps?

Otherwise it won't give you the frequency response you expect.
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Old 6th March 2009, 06:44 AM   #5
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I have no buffer before this circuit.. Directly connected to PCM2702s outputs..
Anyway,
This is what I see in my simulation;
Click the image to open in full size.
The red line is frequency response and the blue dotted line is the phase..
I cannot see any oscillation, do you notice any?
And, concerning the source impedence; I have added a series 10K to the signal source and have no difference on both phase and frequency plot!
May you explain this issue a bit more please..
Thanks a lot for your comments..
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Old 6th March 2009, 07:26 AM   #6
Dxvideo is offline Dxvideo  Turkey
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By the way,
Dear Bichi, I ve just noticed that you made a simulation for me.. Thanks a lot for this.
As I see there is no problem with my simulator since I have exactly the same results..
The slight peaking on 24hz you mentioned is calculated by me to reduce the slope affect on 20Khz band.. (May be because I like treble details too much ) Otherwise there a -0,5dB affect on 20Khz point.. Or the worst is I have to extend the frequency response till 200Khz to reduce this... So to put a peak on 24Khz seemed a good idea to me..
Anyway, another thanks for the software.. Now I am using it and already calculated some other combinations..
Any special recommendation on filter type for especially audio circuits?
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Old 6th March 2009, 07:33 AM   #7
bichi is offline bichi  United States
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DxVideo:
- you will not "see" real-time oscillation, on Bode or Phase plots.
- indicator is phase rotation, over frequency. (phase margin)

Tips:
- input a frequency, say 7.5kHz at 1Vrms and watch it on "virtual" sim oscilloscope on your simulator.
- or take a look at the "transient" plots I made for you, and compare your values with the Bessel values.
- or input the Bessel values on your simulator and see the differences.

Easier:
- input your values on "online simulator tool," Sallen-Key and look at line: "Oscillation frequency."
- if the design is not prone to oscillation, it will state: "This system does not oscillate."
- I see "Oscillation frequency: f = 63527.1933944[Hz]" with your values, indicating a possible oscillation problem.

Page Navigation to Sallen-Key Tool:
Tech Tools -> Engineering Design Utilities -> Filter Tool -> Sallen-Key Active Filter -> Sallen-Key Low-pass Filter Tools

Online Sallen-Key Low-Pass Filter Design Tool:
http://translate.google.com/translat...%3Den%26sa%3DG

Good Luck!
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Old 6th March 2009, 07:40 AM   #8
bichi is offline bichi  United States
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Quote:
Originally posted by Dxvideo
By the way,
Dear Bichi, I ve just noticed that you made a simulation for me.. Thanks a lot for this....
...Any special recommendation on filter type for especially audio circuits?
Ah, just saw your reply and you are welcome!
- ummm, I prefer Bessel with MFB topology, instead of Sallen-Key or Butterworth.
- if you search "Audio Bessel Filter," you will find detailed papers/theories and opinions.
- lastly, I like the sound Bessel filters produce, especially filtering/reconstructing after high-performance (up to 192kHz sample rate) Delta-Sigma type DACs.
- last comment, SMT (surface mount), careful layout and decoupling required for best performance.
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Old 6th March 2009, 08:00 AM   #9
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Click the image to open in full size.
This is the new one.. Based on TI Filter Pro calculations for
Bessel+MFB (as you recommended) and x2 gain...
By the way,
I am already designed a PCB for it.. However I am not good at working with SMD resistors and caps And the PCB is mostly fullfill with "normal" components.. Do you think it causes any problem?
PS: I will post my PCB design in the day.
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Old 6th March 2009, 08:24 AM   #10
bichi is offline bichi  United States
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DxVideo,

- hard to predict "leaded component" performance vs. SMT, without seeing and testing physical design.
- 1% resistors, NPO/COG 5% capacitors are preferred. (SMT PPS film caps sounded "harsh" to me)
- capacitors should not be below 20pf and larger value resistors (50k+), can add Johnson (thermal) noise.
- attention to groundplane is key.
- opamp power rail decoupling is an important factor (10 to 100nF, X7R ceramic, close to opamp power pins to groundplane)
- larger, power rail decoupling capacitors (10 to 100uF electrolytics) should be less than 1cm from opamp power pins.
- depending on your power-supply decoupling, quality and location, some prefer low-ESR, electrolytic without smaller ceramic.
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