Can changing amplifier in Sullen-Key filter affect the frequency response much?

[loop123]

Consider this Sullen-Key low pass filter.



If you change the amplifier to LF411CP (with the same pins). How much would the frequency response change?

When I increase the value of the capacitor from 1000pF to 6800pF, the frequency get lowered. Why is that?

I found a Ltspice file using the third order filter (see attached). I changed it to the 2nd order Sullen-key by deleting some parts and changing the values. Please check if the entry is correct. Also the voltage source is reverse. This is ok for AC, isn't it? Second. What amplifier is in the Ltspice attached Ltspice file?. How do you change it to the LF411CP? I want to see if the frequency response would change. If it's hard to change. Please change it to LF411CP and attached the edited Ltspice.

 sullen-key.asc (1.21 kB - downloaded 55 times.)



What does ".ac oct 20 10 10000" mean? If I removed it, I can't run the simulator anymore.

Thank you.

[Benta]

Perhaps you should add power supplies and an output to your simulation schematic?

[loop123]

Perhaps you should add power supplies and an output to your simulation schematic?

It ran. I just want to know the frequency response of the output. You mean it didn't have any power supplies? But why did the plot run? I don't know how to add power supply since I just edit the existing file with third order filter. I don't know how to use Ltspice except to delete components, change values and run the simulator.

[magic]

Yes, it ran. But do you see the vertical scale?

At -160dB you are simulating parasitic coupling through the feedback network or things like that...

[loop123]

Yes, it ran. But do you see the vertical scale?

At -160dB you are simulating parasitic coupling through the feedback network or things like that...

There is a V1, AC1 in the left side. What is it? I thought it was the power supply. Where and how do you add the power supply? I don't know how to use LTspice Pls download the file in original thread and add it and upload it back so I can run the frequency response plot. Thanks.

[Solder_Junkie]

It ran. I just want to know the frequency response of the output.

While I have both LTSpice and Tina, I prefer the latter for simple circuits using generic components...

See attached using the original circuit (1n), and also another run using 6n8. Note the "blip" in the response curve. The "knee" is around 38 KHz with 1n and 18 KHz with 6n8. It makes no difference which op amp you use.

SJ

[loop123]

It ran. I just want to know the frequency response of the output.

While I have both LTSpice and Tina, I prefer the latter for simple circuits using generic components...

See attached using the original circuit (1n), and also another run using 6n8. Note the "blip" in the response curve. The "knee" is around 38 KHz with 1n and 18 KHz with 6n8. It makes no difference which op amp you use.

SJ

Please change the capacitor C2 to 6800pF (instead of 1000pF) and share the frequency response. Thanks.

[Solder_Junkie]

Please change the capacitor C2 to 6800pF (instead of 1000pF) and share the frequency response. Thanks.

Check my earlier post, that is what the 6n8 (6800pF = 6n8F) image shows...

SJ

[loop123]

Please change the capacitor C2 to 6800pF (instead of 1000pF) and share the frequency response. Thanks.

Check my earlier post, that is what the 6n8 (6800pF = 6n8F) image shows...

SJ

In a 2 pole Sulley-key low pass filter. What components must you change to adjust the frequency response? You mean both 1000pF and 6800pF give both about 50,000 Hz frequency response? What must you change to make it only 4000 Hz response?

[loop123]

Please change the capacitor C2 to 6800pF (instead of 1000pF) and share the frequency response. Thanks.

Check my earlier post, that is what the 6n8 (6800pF = 6n8F) image shows...

SJ

Anyway, here is my complete question. How does putting a 2 pole sulley-key filters exactly remove the ripples in the following circuit?  If someone used an LF411CP amp instead and with values of C2=6800pF and C1=33pF with similar resistors. How would it affect filtering the ripples? This is the specific information I seek. Please simulate them and see the result as it's very important to me. Thanks.

[Solder_Junkie]

Texas Instruments have a design tool: https://webench.ti.com/filter-design-tool/

You can add filters in series, such as this one: https://www.qsl.net/g4aon/pdfs/aonrx_afpre_smd_v3.pdf
Which gives the attached response curve.

Just play around with modelling software and see what results you can obtain.

SJ

[jonpaul]

SALLEN-KEY!

Since 1970s much better active fil topologies are avail and Lin Tech, Maxim made IC specific for AF.

Easite to build a breadboard and MAKE THE CIRCUIT than to bother with simulators, GIGO.

Just place a 8 pin DIM socket, collect a bunch of opamps and play

Suggest NE5534 wideband low noise audio opamp.

Enjoy,

j

[loop123]

Texas Instruments have a design tool: https://webench.ti.com/filter-design-tool/

You can add filters in series, such as this one: https://www.qsl.net/g4aon/pdfs/aonrx_afpre_smd_v3.pdf
Which gives the attached response curve.

Just play around with modelling software and see what results you can obtain.

SJ

My situation is. I got a device that has the ISO122 isolator and salley key but the manufacturer somehow used LF411CP, 6800pF, 33pF instead of using the OPA602, 1000pF, 220pF and I'm getting ripples like in the following:



If I'd order the original OPA602, 1000pF, 220pF and replace these. Would the ripples go away?  Do merely more poles can remove the ripples? how are they removed exactly?  Is it the values of the capacitors or resistors or combined? Please simulate them in any way you can so I can decide whether to order the OPA602.

[Terry Bites]

fc=1/sqrt(R1*R2*C1*C2) to make life simple set C1=C2, R1=R2.

The opamp type does impact performance. If the opamp bandwidth rolls off below or close to fc the response will not be as calculated.
See "GBWP"
The rule of thumb is to use an ampfier with more than 10x bandwidth of your low pass cut off. For a 10kHz lpf, the opamp must provide the required filter gain to at least 100kHz.

Here's a very thourough treatment of this filter type. You love maths right?
Its Sallen, not Sulley from Monsters Inc.

[loop123]

Please tell me the detail theory why adding more poles that makes it closer to the vertical ideal response can remove the ripples of the carrier frequency.

What unity gain finish module is available with the highest pole or order? 

Thanks.

[loop123]

It ran. I just want to know the frequency response of the output.

While I have both LTSpice and Tina, I prefer the latter for simple circuits using generic components...

See attached using the original circuit (1n), and also another run using 6n8. Note the "blip" in the response curve. The "knee" is around 38 KHz with 1n and 18 KHz with 6n8. It makes no difference which op amp you use.

SJ

I'm reflecting on your graphs and comments. Although the "knee" is around 38 KHz with 1n and 18 KHz with 6n8. How can you conclude it made no difference if the OPA602 or LF114CP is used?

[sharow]

My situation is. I got a device that has the ISO122 isolator and salley key but the manufacturer somehow used LF411CP, 6800pF, 33pF instead of using the OPA602, 1000pF, 220pF and I'm getting ripples like in the following:

You can see about 5 cycle between 19:01.00 and 19:01.10 .
100ms/5 = 20ms  or  50Hz.

[vk6zgo]

Yes, it ran. But do you see the vertical scale?

At -160dB you are simulating parasitic coupling through the feedback network or things like that...

There is a V1, AC1 in the left side. What is it? I thought it was the power supply. Where and how do you add the power supply? I don't know how to use LTspice Pls download the file in original thread and add it and upload it back so I can run the frequency response plot. Thanks.

V1 is the input signal.

LT Spice, in their wisdom, always use a DC generator symbol with + & - polarity markings on it, unlike the ac generator symbol which I am familiar with from decades of seeing it used.
That is why LTSpice annoys me so much.

If the generator is so marked, it is easy for someone to think it is a DC power supply source.

[RFDx]

My situation is. I got a device that has the ISO122 isolator and salley key but the manufacturer somehow used LF411CP, 6800pF, 33pF instead of using the OPA602, 1000pF, 220pF and I'm getting ripple ...

6.8nF and 33pF are not a good choice, this values give you alot of peaking (gain) at the corner frequency of the filter.

If I'd order the original OPA602, 1000pF, 220pF and replace these. Would the ripples go away?  Do merely more poles can remove the ripples?

The ripples can only be attenuated and not removed completely, depending on the order of the chosen filter. The single pole 50kHz filter inside of the ISO122 amplifier achieves an attenuation of 20dB per decade. This means the 500kHz modulation frequency used by the ISO122 will already be attenuated by 20dB at the output. The Sallen & Key filter following the output provides two additional poles and another 40dB attenuation. With a total of 60dB of attenuation, the 500kHz modulation frequency should be, according to the datasheet, below the noise floor.

As for the choice between the OPA602 or the LF411, the only difference is in the attenuation far above the passband (a known disadvantage of Sallen & Key filters) where the OPA602 has lower output impedance and gives a somewhat better result. See attached frequency and phase responses for the 2nd oder filter alone and that of the maximally flat filter including the pole inside the ISO122 amplifier.

You never mentioned what you expect of your "device". Do you really need 50kHz bandwidth? What kind of input signals do you have?

[loop123]

My situation is. I got a device that has the ISO122 isolator and salley key but the manufacturer somehow used LF411CP, 6800pF, 33pF instead of using the OPA602, 1000pF, 220pF and I'm getting ripple ...

6.8nF and 33pF are not a good choice, this values give you alot of peaking (gain) at the corner frequency of the filter.

I was mistaken. 332 capacitor means 3300pF. so it's really Sullen and key of 6800pF and 3300pF with the LF411CP. This means low pass of less than 10kHz. Compare this to the original datasheet Sullen and Key 1 000pF and 220pF with cutoff of 50,000kHz. What does this do to the ripples? Does it mean the ripples at all frequencies will be less when the 50kHz cutoff is used versus the say 7kHz cutoff? Or does it mean only ripples below 7kHz would be cutoff in the latter?

Also more poles means more abrupt cutoff. What has this got to do with suppressing the ripples? Kindly explain because I want to understand the mechanism.

It's just an amplifier. I don't need 50kHz but 7kHz may be too low for say audio use.

If I'd order the original OPA602, 1000pF, 220pF and replace these. Would the ripples go away?  Do merely more poles can remove the ripples?

The ripples can only be attenuated and not removed completely, depending on the order of the chosen filter. The single pole 50kHz filter inside of the ISO122 amplifier achieves an attenuation of 20dB per decade. This means the 500kHz modulation frequency used by the ISO122 will already be attenuated by 20dB at the output. The Sallen & Key filter following the output provides two additional poles and another 40dB attenuation. With a total of 60dB of attenuation, the 500kHz modulation frequency should be, according to the datasheet, below the noise floor.

As for the choice between the OPA602 or the LF411, the only difference is in the attenuation far above the passband (a known disadvantage of Sallen & Key filters) where the OPA602 has lower output impedance and gives a somewhat better result. See attached frequency and phase responses for the 2nd oder filter alone and that of the maximally flat filter including the pole inside the ISO122 amplifier.

You never mentioned what you expect of your "device". Do you really need 50kHz bandwidth? What kind of input signals do you have?

[loop123]

The following is what I meant above. Also the input is 1V even in the file at original message. It's just that when I drew the feedback from - Vin to output. A pixel must have been missed so it's not continuous and the voltage is only in nV instead of 1V. Why do you get only nV when output is not connected to -Vin?

[magic]

You are still simulating without power supplies to the chip, so don't expect miracles.
You need to create two DC voltage sources, one pin of each to ground and the other pin to the supply pins of the opamp. Press F2 and search for "voltage".

If you still don't know how to do it, press F1 to open built-in help or look up some LTspice tutorial, because you are going nowhere without it.

[loop123]

You are still simulating without power supplies to the chip, so don't expect miracles.
You need to create two DC voltage sources, one pin of each to ground and the other pin to the supply pins of the opamp. Press F2 and search for "voltage".

If you still don't know how to do it, press F1 to open built-in help or look up some LTspice tutorial, because you are going nowhere without it.

I've been running this for 2 days already can't do it. Can you or someone please add the power supply to the file attached. It has 6800pF and 3300pF. Why can it show low pass filtering below 10kHz if there is no power?

[magic]

I don't know how to use Ltspice except to delete components, change values and run the simulator.

This is your fundamental problem.
Learning how to add voltage sources, draw connections and maybe use net labels would take not two days but less than two hours.

Tip: F2, F3, F4, respectively.

[RFDx]

I was mistaken. 332 capacitor means 3300pF. so it's really Sullen and key of 6800pF and 3300pF with the LF411CP. This means low pass of less than 10kHz.

With 6.8n/3.3n the corner frequency is at ~4.7kHz (see attached screenshot).

Compare this to the original datasheet Sullen and Key 1 000pF and 220pF with cutoff of 50,000kHz. What does this do to the ripples? Does it mean the ripples at all frequencies will be less when the 50kHz cutoff is used versus the say 7kHz cutoff? Or does it mean only ripples below 7kHz would be cutoff in the latter?

I don't know what frequency the ripples have. The screenshot you provided was unclear because you cut off the units respectively the values of the time and amplitude scale.

If you use the 4.7kHz filter variant, the amplitude of the ripples that are far above the corner frequency will be less compared to the 50kHz filter. Ripples inside the filter passband can obviously not be attenuated at all.

Also more poles means more abrupt cutoff. What has this got to do with suppressing the ripples? Kindly explain because I want to understand the mechanism.

As already mentioned, you can only suppress ripples that are outside of the filter passband. A steeper filter (more poles) ensures ripple voltage(s) with lesser amplitude.

It's just an amplifier. I don't need 50kHz but 7kHz may be too low for say audio use.

So 20kHz bandwidth will suffice?