# Filter Set up

#### Sonny120

Joined Nov 5, 2023
13
Good Night,
I have been trying to create 2 filters which I would cascade to make a basic band pass filter. I thought I understood the theory however it seems I do not. What I was trying to get on the output was a signal that would be filtered to only give a range of 0.5 - 5 Hz.
The input for this is a voltage that would be in micro amps after coming from a sensor then a current to voltage op - amp circuit. So the input voltage to this circuit would be uV. Attached would be diagrams of the circuits I was attempting to use.

On the left was supposed to be a 2nd order low pass filter and the right was the 2nd order high pass filter.

The equation used for the 0.5 Hz high pass filter:
$f=\frac{1}{2\pi RC} , C = 1 \mu, f = 0.5 Hz \\ R=\frac{1}{2\pi fC} =\frac{1}{2\pi *0.5*1\mu} =318309.89 \Omega \\ R\approx 320k\Omega$
This equation would find the 2 input resistors as they were set to be equal.

The equation used for the 5 Hz low pass filter:
$f=\frac{1}{2\pi RC} , C = 1 \mu, f = 5 Hz \\ R=\frac{1}{2\pi fC} =\frac{1}{2\pi *5*1\mu} =31830.99 \Omega \\ R\approx 32k\Omega$
This equation would find the 2 input resistors as they were set to be equal.

I am kind of lost as how to make it critically damped as I know the gain should not rise to more than 3 however in the pictures shown it was around there. These were just done to test please let me know how to better this.

This was done using multisim.
- Sonny120

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#### Sonny120

Joined Nov 5, 2023
13
Here is a copy of the ac sweep of the circuit where green is the low pass filter and red is high pass filter

#### Ian0

Joined Aug 7, 2020
9,531
Critically damped is a Q of 0.5. I don't know what relevance the "gain of 3" has.
If you make the op-amps unity gain, then it has a Q of 0.5.
However, because of the op-amp's phase shift at high frequency, you would be better off using two cascaded single pole filters with a unity-gain buffer between them.
By the way, the TL084 is neither a low-noise nor low-offset op-amp. A microvolt signal would get lost!
What transducer are you using?

#### Sonny120

Joined Nov 5, 2023
13
Here are the current circuits in case anyone ever needs them.

Their responses are

Originally these were made to filter frequencies out from an ALS-PT243.
My initial process was to convert the current to voltage using a current to voltage converter op-amp circuit something like the following:

Then i would send the signal through the filter to clean it up. All of this is done so I could try to make a pulse oximeter. My fear with this process is that I would then have to measure a DC voltage from the sensor.

This was done using multisim.
- Sonny120

#### Ian0

Joined Aug 7, 2020
9,531
But don't use a 741 either - it's twice as noisy as the TL084, and the offset is just as bad.
If you are trying to measure a small DC voltage, you need a LOW-OFFSET op-amp.

#### Sonny120

Joined Nov 5, 2023
13
But don't use a 741 either - it's twice as noisy as the TL084, and the offset is just as bad.
If you are trying to measure a small DC voltage, you need a LOW-OFFSET op-amp.
I was given these (741 and 084) to do the project so there must be a way? If I amplify it input signal first would that solve it? But I would be using the same 0p -amp to amplify the input signal.

#### Ian0

Joined Aug 7, 2020
9,531
I was given these to do the project so there must be a way? If I amplify it input signal first would that solve it? But I would be using the same 0p -amp to amplify the input signal
If you are looking for a pulse, then your circuit might ignore the DC level, so the offset won't cause too much problem, but if you are trying to measure the DC level, then you are going to have to use the offset-null terminals to get rid of it.
No-one in the real world would do that, as it introduces a time-consuming procedure that is open to error, and likely to need re-calibrating at a later date. The 741 was introduced in 1968 - would you expect to be using a computer that old? Computers have improved immeasurably since then - and so have op-amps.

#### Sonny120

Joined Nov 5, 2023
13
I know however it's just a basic uni project I'm more worried about the ALS sensor since it's the first time i'm using one. And I can't figure out how to get a proper output. This has been the bane of my existence for the past week.

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#### Ian0

Joined Aug 7, 2020
9,531
What do you get if you use the circuit on page 6?

#### Sonny120

Joined Nov 5, 2023
13
I wouldn't be able to test the circuit until Monday when I get back into the lab. However the instructor was calling it noise. When i was reading off the oscilloscope with a 1k resistor as a load resistor. With the lED powered from a 3.3 V source I was getting 20uV when the ALS was covered with a finger around 8uV. I thought that was enough to show proof of concept but apparently not. They were saying I didn't size the resistor correct because i used 1k to test.

#### Ian0

Joined Aug 7, 2020
9,531
Try 100k

#### Sonny120

Joined Nov 5, 2023
13

#### Audioguru again

Joined Oct 21, 2019
6,620
Your original schematic had its parts far apart (in different cities?) so I cropped it.
I am glad to see that you fixed the oscillators that had way too much gain to be filters.

Why are your new simulations showing a frequency as high as 100GHz and levels as low as 100nV??