Sallen Key Filter Question

[newbie_hs]

The below image is from LTC1966 datasheet.
May I know the use of RB

 

[danadakk]

Mitigate bias current generated offset ?

https://www.ti.com/lit/wp/sboa590/sboa590.pdf

https://www.analog.com/media/en/training-seminars/tutorials/MT-038.pdf

Regards, Dana.

 

[KlausST]

Mitigate bias current generated offset ?

I thought the same - at first.

But for bias compensation one would chose identical resistance for both inputs. Thus I expect: 38.3k + 169k = 207.3k.
294k seems a bit high.

Thus my second thought: maybe it uses the input capacitance to improve stability.

Sallen-key filters are known to be stability critical.

I don´t know

Klaus

 

[danadakk]

From datasheet :

Using input C to manage stability.....not sure I would rely on that in a production
environment. T affects on board parasitic C ......and fdbk R drift/tolerance, one
would have to run the numbers.

And - https://ez.analog.com/data_converters/precision_adcs/f/q-a/109765/ad7747-capacitance-drift-over-time


Regards, Dana.

 

[BradtheRad]

The OP schematic is not identical to a conventional sallen-key. RB is not current-carrying in a significant way. Changing RB does not affect rolloff curve in simulation anyway.

Usually we take an input from some sort of divider network (resistive, or RC, etc.) In fact simply by lifting the right end of C1 and attaching it instead to the inverting input. That makes the schematic behave. It's intended to be a high-pass filter, correct?

 

[FvM]

Rb is used for the OPs without built-in bias current cancellation, there's no other useful purpose for it, I think. The resistor value is most likely a typo, respectively R1 and R2 have been modified without thinking about Rb.

LT2050 is apparently also a typo, the type doesn't exist.

 

[std_match]

The typical output impedance of the LTC1966 is 85 kOhm, so the 294k RB value is expected (85k + 38.3k + 169k).

 

[FvM]

The typical output impedance of the LTC1966 is 85 kOhm

Thanks, this explains resistor choice. I didn't expect high output resistance.