LTC1569-7 shows half of the amplitude of the input signal at 8 times the cutoff frequency. What is the reason?
Need more info please.
What is the schematic around the 1569-7? At this point, we cannot know what the cutoff frequency is supposed to be.
And it is not obvious - can you show what you mean in your plot with half the amplitude of the input signal?
Also, you may confuse the reader by using linear scale on the frequency axis. Better to plot LOG scale to try to match datasheet curves.
thank you for your reply，according to datasheet，aliasing phenomenon will occur at 64 times the filter cutoff，but now，it occur at 8 times the filter cutoff。
Happy to try to figure out what might be going on. But we should make sure we agree with what is an issue, and what is THE issue.
I think you are probably right: the -25 dB is not aliasing. But if it is not aliasing, then any discussion about the datasheet aliasing is not relevant.
-25 dB is less than 1/10th. Close to 1/20th, or 5 %. It is not 1/2.
Aliasing of a clock would appear as a spike.
And, aliasing only happens in this case when there is mixing. The clock frequency would mix with a possibly input frequency, and then show up as a spike in the pass band. So aliasing is not relevant here. You would need an input, with a high frequency content near the clock frequency, to mix down.
So the real question in my view is not at all related to the datasheet's discussion of aliasing.
The real question is not related to 1/8 or 1/64 ratios.
The REAL question is why does your response go up, at 50kHz, all the way to -25 dB? The response should not come back up so high. The response should go down, have a few bumps, then remain down.
If you could provide more about your circuit, then we might figure out why the response goes up to -25 dB, rather than remaining down at -50 dB.
I'm sorry to reply your letter so late，when i want to reappear phenomenon of picture，i find i can't.Up to now,i have't find solutions.This is my design circuit，R3 is 10k.
Hello again. Thanks for sharing the circuit.
At first I thought your R3 (Rext) was too high, but then you mention it is 10K.
The datasheet recommends 40K max.
Also, 2V at GND is recommended. Again, you have properly done this.
The schematic looks ok.
We do not understand why the roll-off would suddenly "come back up." In the worst case, please send a board to Analog Devices and the company will power it up. But hopefully it will simply work for you.
Thank you for your patient answers，I recently research the feature of switch capacitor filter.so,LTC1569-7 not influenced my whole job.thank you for your help again.
No problem. Good that you are considering the LTC1569. It is part of a collection of products that are very interesting, with capabilities not found anywhere else. Clock tunable (like 1569), others with integrated R's and C's for super high accuracy in creating high Q filters.
New similar phenomenon occur again，the filter cutoff is 500Hz，ltc1569's forward and behind is second order voltage-controlled lowpass circuit.
Hello again, and thanks for the plot.
The LTC1569-7 has a 32:1 Nyquist, I am told. For 500 Hz cutoff, the Nyquist should be 16kHz and below.
Your plot seems to show evidence of "out of band" content aliasing. This out of band content seems like it must be information above 16kHz, if what I am told is correct.
- Are you driving into 50 Ohms of a spectrum analyzer?
- Your description of circuit "forward and behind" are not informative enough. Please let me know what actual circuit components go before and after the 1569.
- Try to change pin 5 to all three possible cases. /1, /4, /16. and take your frequency plot again and provide the data.
- Have you looked at the frequency response of the signal going INTO the 1569-7? Please let me see it.
- Have you ordered a LTC1569-7 eval board from the factory?
Retrieving data ...