This is for an anti-aliasing filter. I found a good ref here
page 16 which uses split supply with two resistors as per usual.
How does that compare with say using one of those ICs that generates a negative supply and using that instead? (so that the op-amp has +=0=- supply lines from a single positive supply)
That circuit on P. 16 of the app note isn't just a lowpass filter--it's AC-coupled at the input. That's why it needs its own bias resistors.
A plain-vanilla Sallen-Key lowpass is a DC-coupled noninverting amplifier with unity DC gain, making it a reasonable choice for antialiasing. Its output basically just follows its input, so you shouldn't need a separate source of bias.
You do need to pay attention to what happens to the op amp's output impedance at high frequency, though. "Rail-to-rail" outputs are intrinsically high impedance, and rely on feedback to get low closed-loop output impedance. (LDO regulators have the same issue, for the same reason.) That means that the op amp's output impedance rises more or less linearly with frequency, i.e. it looks like a lossy inductor. At some frequency that will mess up your filter characteristic, so you need to calculate it fairly carefully.
Some RRIO amps (e.g. the AD8605) use fast internal feedback around the output stage, which makes them quite a bit stiffer. Those ones make much better ADC drivers as well as better filters, compared with normal RRIO amps.
If you use an amp with totem-pole outputs, you'll need that charge pump to get the output to swing to ground, but with an RRIO with stiff outputs, you should be able to get away without it.
One very useful tip is to make sure that you have a small positive offset, i.e. that zero signal gives you an on-scale reading from the ADC.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 USA +1 845 480 2058 hobbs at electrooptical dot net http://electrooptical.net
[snip]
Look at p13, non-inverting. Amazing how bad apnotes, datasheets _and_ Spice models have become :-( ...Jim Thompson
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| James E.Thompson | mens |
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| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
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I love to cook with wine. Sometimes I even put it in the food.
"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...
So, fig. 3 was ok? Hey Jim, don't take any wooden nickels.
Harry
They just used R3 on the wrong opamp.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
I missed that one... I only checked the ones in the discussion. Personally I despise Sallen-Key due to their component sensitivity issues. ...Jim Thompson
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| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Not sure what you are saying here. I take a plain Salen Key low pass - but that needs +0- dual supplies. How do I run it from one supply? Just split t he 0v with two resistors? Then I get a bias of V/2 at the output hence the ac coupling at the output to get rid of the dc bias - and at the input as y ou pointed out.
Stupid matlabi. Stick with matlab. Don't poke your nose into the things beyond your comprehension.
Ok what about this one in Ficure 12
How about you just quietly returning to your coffin Vlad, I rarely if ever use MatLab.
That's the ever-popular Sallen-Key Oscillator.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
From the op amp's POV, there's nothing special about single-supply vs bipolar supplies--all it cares about is that the inputs and outputs stay in range. (Op amps don't have a pin called 'ground'.) A "single supply" op amp is one whose input common mode range includes the negative supply rail, and whose output can swing nearly to the negative rail as well. That makes it convenient to run from a single positive supply, but no magic is involved.
Since you're building a filter, it has to get its input from some earlier stage. I assume this input signal is also unipolar because you didn't ask about level shifting (e.g. going from a +-5V signal range to a 0-2.5V digitizer).
The basic Sallen-Key lowpass is a voltage follower at low frequency, so if its input signal range is centered around 2.5V, say, so will its output be.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 USA +1 845 480 2058 hobbs at electrooptical dot net http://electrooptical.net
You are over thinking this. For the Salen and Key g=1, the op amp is just a follower (buffer). In fact, the wiki does a better job than that app note.
One cap goes to ground, but any AC ground will work since it is just a capacitor. Obviously you would use the quietest ground you have.
For any other Salen and Key, you would need the psuedo ground, but not for g=1.
While Jim is right that the Salen and Key is a shitty topology due to component sensitivity, it may be suitable for anti-aliasing oversampled ADCs. That is, the ADC has its own anti-aliasing up to a point, thanks to the digital filter, but it is still subject to aliasing artifacts at the Nyquiest frequency of the sigma-delta modulator in the ADC. But thanks to oversampling, you don't need much of a filter (2 poles is fine) AND the corner frequency will be in the stop band of the digital filter. Basically the anti-aliasing filter corner will be far enough away from the in-band signal that you don't care too much about the crappy accuracy.
Here is the procedure:
1) Determine the sigma-delta modulator sampling frequency. 2) Divide that frequency by two to get the Nyquist frequency. 3) Pick an anti-aliasing type: Butterworth, Gausian, Bessel, etc. [Note the 2nd order Bessel doesn't ring if that is your criteria.] 4) Using a table or hacking around with spice, insure that at the Nyquist frequency, your filter is down (attenuating) sufficient for the noise floor of the ADC. If you use some sexy 24 bit ADC, that would be around 110db. 5) Since the Salen Key component sensitivity is horrible, see if you can live with the error. Alter the components for a worst case. If you are doing multiple channels, look at phase difference.If this Salen Key filter is for anything other than an oversampled ADC, you are just kidding yourself.
My ADC runs + o - 10v ie bipolar input and runs from a 5v supply. The source is from a stereo DAT player which I assume has been ac coupled at the output to remove dc.
Don't have sigma-delta adc on this board. I did on another one and it was e asy to deal with. Won't be a single op-amp, I will need at least a 4th orde r or higher and probably to sacrifice a lot of bandwidth - I know its not w hat I would have hope for. Before sigma-delta we managed ok for a good 30 o dd years. It is annoying that designers aren't putting in sigma-deltas as s tandard.
Would have been nice if you'd said that at the beginning. In that case, yes, you need the two resistors.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 USA +1 845 480 2058 hobbs at electrooptical dot net http://electrooptical.net
Those sigma-delta bastards made me find a new specialty. It killed the SCF business. The technology is good, especially for control systems since there are no missing codes.
It is annoying that designers aren't putting in sigma-deltas as standard.
The only place a shitty filter like a Salen Key is useful is before an oversampled data converter. In fact, if gain error is a big deal, I can make an argument that a g=1 SK is the right filter.
But it appears now you are not using an oversampled converter. The filter error is now part of the error budget. Expect lots of variance due to using the wrong filter topology.
Have you priced 1% caps?
I'm outta here.
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