LMF100 is changing the frequency given to the input

Knupp_08

Other Parts Discussed in Thread: LMF100, LM358, LMC6482

Hello

We have built a circuit using the LMF100 as a bandpass-filter. We would like to filter a frequency of 1.25Hz, so the given clock frequency is 125Hz. We have the following schematic of the circuit that is attached.

To test the bandpass-filter, we connected a sinus signal of 1.25Hz of a function generator (Agilent 33210A) to the input of the circuit. The frequency of the function generator is absolut stable. On the output we connected a oscilloscope. On the output of the bandpass, the cycle duration of the sinus signal changes with a delta of maximum 1.5ms. So it means that the frequency signal on the output is not excact 1.25Hz anymore. It always changes up and down. Do you have any suggestion, why that delta can be so high? What could be the problem?

Thanks for your help and support.

Regards,

Knupp Reto

schematic_bandpass.pdf

over 8 years ago

0 Michael Stout over 8 years ago

TI__Genius 16890 points

Hello Knupp,

It looks like you are using Mode 1 for the filter that uses pins 1 through 5. Is that correct? What Mode are you using for the filter that uses pins 16 through 20?

Also, I want to make sure that you know that this part is not recommended for new designs.

Mike

0 Knupp_08 over 8 years ago in reply to Michael Stout

Prodigy 190 points

Dear Mr. Stout

Thank you for your reply. First of all I have to tell you that I did not design the filter. It was designed years ago and I have to analyze it now to improve it. I am not the expert about the LMF100 and how it is used in the schematic I sent you. I found an old document with calculations of Mode 3, Mode 1 and Mode 1a. So I guess that some of those Modes are combined together. I added the file with the calculations.

We also know that this part is not recommended for new designs. Thanks for the information.

Regards,

Knupp Reto

Filter LMF100.xls

0 Michael Stout over 8 years ago in reply to Knupp_08

TI__Genius 16890 points

Hello,

That is what is so confusing about the schematic. The components connected to pins 16 through 20 are not connected according to any of the modes in the datasheet. It kind of looks like mode 3 but the input is going into the wrong pin (pin16 instead of pin17) and the output is coming out of the wrong pin (pin 18 instead of pin 19).

The filter using pins 1 through 5 looks ok and also matches the values in the spreadsheet you sent. The only thing I can see wrong with this filter is that it is being driven with a 47k ohm resistor and there is a note in the datasheet that says the input should be driven with a low impedance (<1k).

Mike

0 Knupp_08 over 8 years ago in reply to Michael Stout

Prodigy 190 points

Hallo Mr. Stout

Thanks for your help. The pin SA/B is connected to V+. From that point of view, it can only be Mode 1 or Mode 1a in my opinion (because nothing is connected to LP-pins). So I believe that Mode 1a is connected in series with Mode 1. I changed the schematic now. How does that look to you, does it make sense to connect them like that? Or how would you change the schematic depending on the given requirements?

And how would you choose the resistors? Because you mentioned the low impedance (<1k) that is needed.

schematic_bandpass_new.pdf

0 Michael Stout over 8 years ago in reply to Knupp_08

TI__Genius 16890 points

Hello,

Yes, I think that schematic looks better. The first stage is Mode 1a, as you mentioned, and the second stage is Mode 1. The bandpass output of the first stage is connected to the input of the second stage. I think the resistor between the two stages is ok as it is setting the BP gain.
Does this setup work now?

Mike

0 Knupp_08 over 8 years ago in reply to Michael Stout

Prodigy 190 points

Hello Mr. Stout

Finally I could change the hardware an try out the new setup like the schematic I sent you. Unfortunately, the result is still the same. The problem still exists. Do you have any other solution for the problem?

Kind regards,

Knupp Reto

0 Knupp_08 over 8 years ago in reply to Knupp_08

Prodigy 190 points

Hello Mr. Stout

Any news about the open problem?

Kind regards,

Knupp Reto

0 Michael Stout over 8 years ago in reply to Knupp_08

TI__Genius 16890 points

Hello Knupp,

Here are some troubleshooting step you can try.

Probe at the pins of the part (scope probe touching the pin) all of the pins. Are all the pins at the expected DC voltages? Does the clock look correct?

Disconnect the LM358 from the LMF100. Does the signal at the output of the LM358 look ok? Connect the output of the LM358 back up to the LMF100.

Disconnect BP1 from BP2.

Check the signal at the output of the LM358. Check it at the input S1B pin. Check it at the output BPB.

Reconnect the output of BP1 to BP2. Check the signal at BPB. Check the signal at INVA. Check the signal at BPA.

At what point does the signal go from being good to being bad?

Mike

0 Knupp_08 over 8 years ago in reply to Michael Stout

Prodigy 190 points

Hallo Mike

Thanks for your troubleshooting steps. Unfortunately, it is not so simple to say whether the signal is good or bad on the different points. Because I can only see on a rectangle signal, whether it is good or not. And at some point of the schematic I only have a sinus signal.

I definitely need more support of you. Did you verify the problem yourself? Did you see how the signal on the output is shifting all the time?

I rebuild the schematic again this morning to make some tests. I connected 2 function puls generators on the S1B- and CLK-inputs. All other pins I have checked, they have the correct dc voltage. The output singal is still shifting its edge! If I connect the function puls generator after the LMF100 on pin R20, than the output singal is not shifting anymore, so the LMC6482 seems to work fine. Furthermore I was using only bandpass 1 connecting the function puls generator directly on R15. The edge on the output signal was still shifting all the time!

I have attached pictures of the oscilloscope to better document the problem.

We need urgent help! Thank you.

Regards

Reto

Summary_bandpass_problems.pdf