[sdiy] Reality versus simulation?

[Tom Wiltshire]

Another interesting idea. You think I might do better with 5.6K and 1n for the lowpass?

The output is from a PIC, so they're supposed to be able to sink/source 20 or 25mA (depends on the chip/package). I've pushed them to this in other circuits and found that they can actually do it.

On 27 Nov 2015, at 12:04, Phil Macphail <phil.macphail at liivatera.com> wrote:

> Is your digital output sufficiently low impedance to be ignored? Chances are it can't really source and sink the 10mA your schematic indicates.
> 
> Phil
> 
> Sent from my iPhone
> 
>> On 27 Nov 2015, at 12:05, Tom Wiltshire <tom at electricdruid.net> wrote:
>> 
>> Hi All,
>> 
>> I've been playing with the following circuit. It's a couple of simple passive filters to follow a digital noise generator. The LPF is set around 25KHz to smooth the digital pulses, and then the HPF acts as a DC blocker at about 15Hz.
>> 
>> http://electricdruid.net/images/PentaNoiseFilter.pdf
>> 
>> Now, the interesting thing is that in simulation, this works fine, and I have a flat frequency response between the two filters. However, in reality, there's a pronounced downward slope to the frequency response (high frequency rolloff) which amounts to about -10dB at 25KHz. This is measured by feeding the noise into my oscilloscope's FFT function. I know the rolloff isn't coming from the 'scope or the FFT, since feeding the digital noise in directly gives me a flat response out to 100KHz or so.
>> 
>> So what's going on with the real-life filters? Can I improve their performance?
>> 
>> Thanks,
>> Tom
>> 
>> 
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