Hi Gentlemen,
![Smiley :)](https://www.eevblog.com/forum/Smileys/default/smiley.gif)
I've been playing with Conrad Hoffman's 4th Order Low Pass software, and naturally my first filter didn't work...
I thought I was handy and didn't think I needed a gain for my test circuit.
So I connected the opamp output to the inverting input, well... the frequency characteristic wasn't right at all.
Everything checked at least three times, replacing opamp, putting the parts back on the breadboard again...
So I didn't think any further when building and that the passive section " attenuates " and that this must be compensated by the gain of the opamp.
So, after also configuring the software several times and I always got the same values for the components, I placed two equal values of the feedback resistors, TADA!
Now I had a filter that was almost perfect...
The setup is that it becomes a filter for measuring noise from power supply's and voltage references and that I want to keep the consumption as low as possible,
because the whole thing will be powered from batteries, but more about this later.
Testing filters at low frequencies is usually more difficult, so I always take a frequency that allows me to measure and scale the whole thing to the final frequency.
The filter shown on the picture is for 10KHz low pass filter.
It is set up so that I only need to adjust the capacitors to 4.7uF for the 10Hz for which I want to use this filter.
![](http://www.bramcam.nl/Diversen/10KHZ-4e-ConradH-One-Opamp-LP.png)
The opamp I use here is probably not the opamp that comes in the final circuit, great chance that this will be an OPA140 or one of his brothers.
One more comment, the feedback resistance, so that of the output of the opamp to the inverting input is quite sensitive.
I mean that deviations in the indicated value of Conrad's Software must be accurately checked for flat frequency characteristics.
That is why in this schematic two resistors are drawn in series, R7 and R8.
For noise measurements this is not such a problem, but in other applications I would take it into account.
Tomorrow I will show a drawn version of the frquence characteristic.
I need some sleep now, I had a long flight today that lasted 12 hours and my battery is empty....
Kind regards,
Bram
PS,
Thanks Conrad!