RCR filter - can you verify my calculations

[ack]

Hi,

you are looking at my Alpha DAC's analog output (J11 is the female RCA, and the bent pin is the positive):



R245/C185/R246 form an RCR filter (and I have verified that the yellow C185 capacitor's "north" lead connects to ground). If the values of the resistors are 75ohms each and the yellow cap's 1nF, I am calculating a roughly 1MHz corner frequency (1.061MHz to be exact), or is it ~2MHz??? My goal is to lower that frequency with the addition of the white 10nF Vishay cap, which should bring the corner frequency down to 96.5kHz, if the stock cutoff is ~1MHz; otherwise I need to add another 10nF

Can someone verify my calculations?

Thanks

-ack

 

[DonH50]

You need to know the driving and load impedances to calculate accurately. Assuming an ideal (0-ohm) source (i.e. U40 is a lo-Z driver) and a hi-Z load then the second resistor in the pi filter is irrelevant and the corner frequency is simply 1 / (2*pi*75*1nF) = 2.122 MHz. Probably to filter out the DAC's clock from the output. The second resistor probably helps isolate the opamp (U40) from the load for both stability and short-circuit protection.

HTH - Don

 

[ack]

Thanks Don; I suspected the second resistor might be irrelevant to the filter calculation (BTW, load impedance is 15K), therefore, indeed the corner frequency is then ~2MHz; I am glad I asked. I also suspect the filter is there to attenuate the clock from the output. So as is, the mod's corner frequency is now at ~193kHz, double my goal's, and I think I still want to go lower.