Implementation of SallenKey bandpass filte

1. Use filter pro to design your filter. This will give you all the component values needed to make the filter work properly. See link below.
2. Pay special attention to the Gain Bandwidth Product. Filter pro will tell you the required gain bandwidth product.
3. Test your filter design in Tina Spice. If you are not familiar with Tina, it is a software simulation program that TI provides at no cost. See link below.
4. Once you confirm operation with simulation, you can build it.   If the simulation works you shouldn’t have any issues with the real circuit.
5. Also attached is a presentation on active filters that may be helpful.

http://e2e.ti.com/support/amplifiers/precision_amplifiers/w/design_notes/3076.filterpro-v3-1

http://www.ti.com/tool/tina-ti?keyMatch=tina&tisearch=Search-EN-Everything

 

Successful_Active_Filter_020713.pptx

Hi,

Thanks for your reply.

We are using FilterPRO to design the SallenKey filter and pSpice for simulating the schematics obtained from FilterPRO.

We designed filters with various Qs ranging from 0.5 to 5.

In simulation we are getting proper results but in the practical implementation we are not at all getting the results.

All the filters are behaving like all pass filters.

Below is a list of things to consider:

Below is a list of things to consider:

1. The gain bandwidth of the OPA551 is a little low (3MHz). Normally, the gain bandwidth is set a few decades above the filter bandwidth. What did filter pro recommend for the gain bandwidth? (15 to 50MHz). This is especially true if gain is greater than 1.
2.  Also look at the “Maximum Output Voltage Swing vs. Frequency” curve. Page 6 on data sheet. Make sure that you are not violating this limitation.
3. An all pass response is a very unusual and unexpected response. To confirm, do you mean that the gain remains constant over frequency? If the response is really all pass, I suspect a wiring or measurement error.
4. I do not have a copy of PSPICE. If you use TINA spice, I can look at your simulation results.
5. Can you provide a schematic?

Hi
Thanks for your suggestion.
Till now we are using only single supply.
Now we started using dual power supply i.e positive and negative.
This solved our problem.
But the rejection is not steep at higher frequencies.
Please see the schematic and suggest necessary action for improving the rejection at higher frequencies.
The circuit has center frequency at 3.7 MHz and a bandwidth of 2MHz.

I cannot see your picture attachment. So, below I will show simulations for Sallen Key and Multiple Feedback versions of the 159kHz Low Pass Filter. Note that the high frequency response on the Multiple feedback is a little better than the Sallen Key. Also, I used 1% resistors and 10% capacitors. The filter cutoff could be more accurate with higher precision components, but I thought these were reasonable. Finally, note that the OPA551 is a somewhat unusual amplifier for an active filter amp, as it is a power amp. Nevertheless, it should work. Look over my schematics and see how they match your results.

 

 

Multiple Feedback result: