which opamp is good for 250Hz Low pass sallen key filter

[awan]

op amp gain non inverting sallen key

I need to design 250Hz sallen key filter and looking for good opamp. I only have single supply at 3volts. I used AD824 and my design works for +3volts and -3volts supply. But it doesnt work for +3volts and 0 supply although the data sheet says that its a single supply opamp.

I cant push the input to 1.5votls as I need to use several stages and also have a gain from the filter.

Thanks

 

[FvM]

sallen key lowpass filter

It most likely doesn't work with a AD824, because of the limited common mode input range. But any (low voltage) OP with input and output rail-to-rail capability should be O.K..

I cant push the input to 1.5volts

I'm not sure, what you mean with this statement. Obviously, a single supply AC amplifier or filter needs in- and output levels biased by some amount to provide bipolar voltage swing without saturation. A bias to mid-supply is a common solution, depending on the circuit topology, also a virtual ground at 1.5V may be suitable.

 

[awan]

sk-lpf

The input is from a Sensor and so the signal will not saturate the output.

As the signal frequency is low; I dont think its possible to couple the DC bias.

 

[FvM]

how to adjust the gain of a sallen key filter

An output without suitable DC bias can be saturated even with no signal or mV signal.

Generally, you can use any coupling you like to, also with single supply. But have to care, that input and output voltage are within the OP common mode range specified for normal operation, and that the intended output voltage swing can be achieved. One prerequisite is to select a suitable OP, but the circuit has to be correctly designed for single supply operation, too.

 

[awan]

sallen-key low pass single supply op amp

What do you mean by DC bias? The OP-AMP is supplied the DC voltage either dual supply or single supply; but do the inverting and non-inverting inputs also need a DC?

 

[LvW]

single supply op amp with dc offset

I need to design 250Hz sallen key filter and looking for good opamp. I only have single supply at 3volts. I used AD824 and my design works for +3volts and -3volts supply. But it doesnt work for +3volts and 0 supply although the data sheet says that its a single supply opamp.

Besides the question of opamp choice, common mode etc. - at first, you have to tell us something about your requirements. Low pass or band pass (you only spoke about a "filter") ?
I suppose, you mean lowpass - and in this case you must be aware that for single supply operation capacitor coupling at the input and output of the filter is mandatory (because of the dc bias).
But then - it is no lowpass anymore ! Please clarify and refine your requirements.

 

[FvM]

sallen key low pass

Input rail-to-rail capability usually allows an input voltage range including the supplies, respectively a few mV outside the rails. Explicitely, I only said about a necessary output bias in my last post. It depends on your circuit, if it achieves an output bias without input bias, a standard Sallen-Key topology however doesn't.

 

[awan]

sallen-key + gain + 1.5

Its a Low Pass filter for 0-250Hz. I understand the dual-supply operation; but need help in converting it into a single supply.

Incase, of Single Supply what are the necessary conditions to consider. The Data Sheet for the OP-AMP mentions that it can be used for Single Supply. But the filter simulation doesnt work; so i am thinking that I am missing some important concept in it.

Thanks

 

[FvM]

low-pass filter with bias opamp

But the filter simulation doesnt work; so i am thinking that I am missing some important concept in it.

Yes, it's seems so.

As a first step, you have to define a suitable range for the filter output voltage. With single supply, it can't be but positive, as said. If the input voltage has zero bias, the filter obviously must achieve a level shift, but a standard Sallen-Key topology doesn't provide the option, so you have to modify it.

As you're effectively adding an offset to your signal, it may be important, that the respective voltage is stable, noise filtered, whatsoever not to reduce the circuit's performance. Otherwise, dual supply would be still the better choice.

 

[LvW]

sallen key unity gain

Its a Low Pass filter for 0-250Hz. I understand the dual-supply operation; but need help in converting it into a single supply.

The classical circuit for a unity-gain S+K lowpass filter using single opamp supply is as follows:

Use two equal-valued resistors in front of the first filter resistor in order to bias the circuit with 0.5Vdd. Because of unity gain of the opamp the same dc voltage appears ath the output - so it can swing in both directions.
Then, you must use an input capacitor to decouple your signal source from the bias voltage - with the consequence that the ideal lowpass function is lost.

It is a matter of philosophy: You can save on power supply but, instead,you have to enlarge/complicate the circuit. You never can earn something for nothing. It´s always a compromize.

 

[awan]

I understand the unity gain LPF with single supply voltage but is it possible to get gain rather than unity gain.

In case i need to get gain with single supply; then the resistance R3,R4 controlling the gain would be connected to virtual ground(0.5Vcc) rather than the actual ground. Am I correct in this? Or we cant have gain from S-K LPF with single supply voltage.

 

[E-design]

Use the multiple feedback sallen key topology for better performance. Make sure the op amp you select will have good low output impedance in the required stop band to avoid re-entry. Pick your RC values with low C and high R combinations. This will reduce feed through effects.

 

[LvW]

Use the multiple feedback sallen key topology for better performance. Make sure the op amp you select will have good low output impedance in the required stop band to avoid re-entry. Pick your RC values with low C and high R combinations. This will reduce feed through effects.

I know the multiple feedback topology and also the Sallen-Key topology.
However, I never have heard about a combination of both.
E-design, please can you describe the structure ?

 

[E-design]

Typing error. Should read "Use the multiple feedback over sallen key.." That is if a inverting response is not a problem in the application. Also you can get away with using a lower GBP op amp with the MFB

 

[LvW]

I understand the unity gain LPF with single supply voltage but is it possible to get gain rather than unity gain.
In case i need to get gain with single supply; then the resistance R3,R4 controlling the gain would be connected to virtual ground(0.5Vcc) rather than the actual ground. Am I correct in this? Or we cant have gain from S-K LPF with single supply voltage.

AWAN, you can get what you want - nearly ! With or without gain, but one thing is not possible (and up to now you did not answer this question):
For single supply the opamp output dc voltage (bias point) must be somewhere between 0 and Vdd (normally, in the middle). This requires an output capacitor !
At the same time the input must be biased around 0.5Vdd - and when your sensor cannot deliver this dc bias voltage (I doubt), you have to provide it separately.
Then, an input cap is necessary with a rather problematic dimensioning because your passband goes only until 250 Hz.
That means you cannot have a lowpass down to dc.
IS THAT A PROBLEM OR NOT ?

 

[awan]

Hi LvM;

You have summarised it correctly. The sensor cant provide a DC bias at 0.5Vdd and so i need to add a buffer with a decoupling Capacitor at the input. But since my signal is at such a low frequency, i dont understand as to how to achieve it.

Is there some method to go around this problem?

 

[FvM]

As I mentioned before, you can add a DC offset in the filter. Simply connect a bias resistor to the first lowpass node
and a reference voltage (or Vcc). Adjust the resistors to keep the filter characteristic and correct the signal attenuation of
the resistive divider by a respective gain.

Here is an example for a 2nd order butterworth filter adjusted to unity gain.

 

[awan]

Rather than adding a DC bias to the low pass node.

For the inverting node in S-K the resistors and capacitors are connected to Ground. Can I change this Ground to Vcc/2 and do nothing to the input of Low Pass Node (i.e. the Non-Inverting Node).

But I think this will cause the Non-inverting node to be at DC of Vcc/2 also and thus couple with the Sensor Signal.

 

[FvM]

You can simply estimate the effect of different bias circuits, if you consider, that the feedback enforces Vin+ = Vin-. The capacitors can be ignored in the analysis. You'll see, that there's no way than introducing a positive bias to the signal path at Vin+ (or a negative bias to the Vin- path)