Low Pass Filter Testing

testing

demonstrated in figure 2 and 3. How is that possible? The figures are plots of gain vs frequency!

frequency of the signal to 1KHz. I am probing across the Capacitor.

thats where the gain will be zero db.

Gain is 0dB when the input voltage = the output voltage.

Gain (in dB) is 20*log10(Vout/Vin)

You can check that with the oscilloscope.

Best regards, Spehro Pefhany

That is kind of a poorly worded document. They mention to use a scope, but show the output of a digital signal analyzer (DSA). Most DSAs have microHZ resolution these days, so testing the filter on an analyzer could be done.

If you look at the curve, the response is a bandpass. With 0dB gain (input equals output) at 2mHz, you are testing the high pass portion of the filter. The high pass corner is around 300mHz. The low pass corner is around 40Hz.

Of course the anti-aliasing is a function of the low pass portion of the bandpass filter.

In the millihz range, these tests I assume are done on a digital scope. I'd set it up dual channel and watch input and output. According to the response shown, they would be nearly 180 degrees out of phase.

Hi,

Figure 1 :

Figure 2 :

Low pass filter values: R = 10,000 ohm ; C = 273pF

Input signal ( sine wave ) Freq = 3.2 Hz, Amp = 50mV peak to peak

Figure 2 shows the comparision between the input sine wave ( yellow trace) vs blue trace across the capacitor.

Figure 1 shows the circuit diagram.

Question:

Why I am not seeing a reduced or attenuated or filtered continous sine wave across the capacitor. Why I am seeing samples of the input sine wave. I am using oscilloscope DS1102D.

Thanks

Erica

I am talking about the capacitor connected between ground and pin 3 of the ADC.

blue trace across the capacitor.

across the capacitor. Why I am seeing samples of the input sine wave. I am using oscilloscope DS1102D.

It's my guess you are reading something else, also connected to the capacitor, which ever one it is.. ?

It looks like digital noise and if so. If could be emanating from the following circuit, there after.

Run that test with nothing connected to the cap other than the minimum.

Jamie

) vs blue trace across the capacitor.

ve across the capacitor.

What's the RC 'corner frequency' of 10k and 270pF? Anywhere near

As Jamie said unhook the A/D and see what it looks like then. What's the switch doing in the circuit? Do you have ground on the 'scope hooked to your circuit? Rip up everything but the signal generator and R-C and make that work properly! I'd like to see a plot of amplitude and phase shift vs frequency. (Crank up the amplitude on the signal generator 50mV is wimpy.)

George H.

ADC.

What you're seeing is most likely normal.

For some ADC's, the input has internal activities acting directly on the input. This means you must take care to insure the reference you put there is not being shared elsewhere, otherwise you'll be getting this noise there, too.

Usually the specs on the ADC chip give you a specific requirement as what should be there.

After looking at that ADC chip, I don't see why there should be that much noise there. That does seem odd. You may want to be causes with your grounds and path ways.

Digital can propagate through the board like capacitor couplings. Try moving your ground clip of the probe doing the ADC input measuring directly on the common of the chip and use the 10:1 mode. If that clears things a bit, you need to work on getting better grounding.. Try not to daisy chain to many items on a single ground path.

Jamie

First of all, you have switched topics, right? Was the original question answered?

I got the impression the first question dealt with some COTS machine that you are trying to calibrate. That is, and established design that is in production. Now are we dealing with a design question?

There is plenty I don't like about this design. The RC filter depends on the impedance of the mux and driving impedance of the signal sources. Is this ADC buffered on the inputs? If not, then all bets are off because your signal source isn't stiff.

The corner freq of your low pass is around 58kHz (1/(2piRC)). Your

The ADC, what is the input doing? Is it a buffered input or is it a switching input. Many ADCs sample the input, switching a capacitor between the input and internal circuitry. An ADC which samples the input needs to have a low impedance source which can handle a nasty load. Your RC circuit on the input is anything but low impedance.

What does this mean?

-- Les Cargill

The other issue is I'm calling the cascade of a LPF and HPF a bandpass, which could add to the confusion since it is colloquially correct, but not technically.

An anti-aliasing filter is not a reconstruction filter. If you want to get picky, the reconstruction filter needs an inverse sinc shape in the passband, but often that is done in software.

It's pretty close - the impulse responses of the three should be related in a very predictable manner .

True enough. That's the canonical form; sometimes implementations deviate ( and will have aliasing ).

Oversampling yadda yadda...

-- Les Cargill

he

Nah. A "reconstruction" filter is not an "anti-alias" filter. One goes before an ADC, one goes after a DAC.

After thinking about it, I think he meant that (since it is in the first Nyquist zone) it is the high side rejection aspect that is important for anti-aliasing. That's true in this case, as it is in the first Nyquist zone. It isn't generally true for bandpass sampling, and so it sounded odd at first glance.

The impulse responses of anti-aliasing and reconstruction filters do not have to be related. These filters only have frequency constraints, as based on sampling theory, to avoid artifacts. Phase is another story.

Simply put, I could anti-alias with an all pole filter or an elliptic. Both would do the job, but the individual impulse responses would be different.

I mean the impulse responses of a lowpass filter, a highpass and a bandpass of the same basic construction ( e.g, all three are an elliptic or Chebyshev or... ).

Goes to your "colloquial vs. technical" comment.

Quite.

-- Les Cargill

Maybe I got lost :) but I thought we were talking about after a DAC. I just sorta rounded "anti-aliasing" up to being much the same thing as reconstruction in that role. Now, *those* are terms that get conflated constantly.

-- Les Cargill

The schematic showed the filter going to an ADC. A differential one at that. So grounding one side and feeding the other through a rather high impedance resistor seemed to me to be a bad idea.

You generally lose a bit of resolution on those differential input ADCs, so a differential drive is worth the effort.