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Need help for single supply anti-aliasing filter.

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picstudent

New Member
Hello

Just before feeding to ADC I plan to introduce a low pass anti aliasing filter to my voltmeter.I have one spare LM358 opamp available. (The other half is used for offseting to my range).

Which should be the cut off frequency? My device will be placed in a telephone exchange equipment room normally. Response time can be low. No high sampling speed is needed.

circuits I previously used were dual rail.

Thanks
 

MikeMl

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Most Helpful Member
Since you say that you are using one stage of an LM358 (presumably operated from a single rail supply), put the anti-aliasing filter between the level shifter and the ADC input. If the DC level is right for the ADC now, it will be right for the anti-aliasing filter, too.

Use a two-pole, unity gain, low-pass Sallen-Key filter like the one shown below. I used 10Hz as the cutoff frequency. If you want something else, download the free program from the TI website, and recalc.

ps, I just saw the other thread about the level shifter. Put the anti-aliasing filter ahead of the level shifter. That way, the range of the signals through the anti-alias filter is guaranteed to stay between 3.5 and 5V, which is well within its common-mode range.

Also, to enable the level-shifter to pull all the way to ground, without the slight non-linearity shown in Eric's LTSpice sim, put a 10K resistor from the output of the level-shifter to ground, effectively in parallel with the ADC input.

This is one case where I wish you hadn't started a new thread. If I had seen the original thread I would have come to the correct solution much easier...
 

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ericgibbs

Well-Known Member
Most Helpful Member
Hello

Just before feeding to ADC I plan to introduce a low pass anti aliasing filter to my voltmeter.I have one spare LM358 opamp available. (The other half is used for offseting to my range).

Which should be the cut off frequency? My device will be placed in a telephone exchange equipment room normally. Response time can be low. No high sampling speed is needed.

circuits I previously used were dual rail.

Thanks

hi,
Is this the thread is related to our previous discussion on ADC level shifting.?

What is the analog signal source.?
 

picstudent

New Member
This is one case where I wish you hadn't started a new thread. If I had seen the original thread I would have come to the correct solution much easier...

Actually I was thinking that having separate thread for each issue is the norm because it will be helpful for those who search on a later date. But I got the point now.

Thanks for the circuit and the program. The TI program is not specifying power rails. Any way
now I can do it.
btw positioning filter before amplifier(level shifter)? Is it Ok to do so? I think what you suggest is noise coming through power supply is to be properly handled by keeping within common mode voltages and the noise from the next amplifier is either not serious or to be handled by software filter.

Erric
This is the same circuit I was discussing in the other thread.
ADC input is actually a ~50VDC coming from battery array (with or without charger ON) through a voltage divider. (Telecom equipment room).

Thanks a lot for the help

Roy Thomas
 

crutschow

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What is the signal noise level and the A/D resolution and input range? I'm concerned about the noise from the LM358.
 

picstudent

New Member
What is the signal noise level and the A/D resolution and input range? I'm concerned about the noise from the LM358.

No dangerous high resolution involved - 8 bits only

Actually I drop 40V--60V(Original input) to 3.3V to 5V then (resistor divider)

Offset this to 0 to 5V then

LP filter then

8 bit ADC

The entire system will derive its power using a switching regulator based on LM2576HV-ADJ from the 40V to 60V supply.

see this thread

https://www.electro-tech-online.com/threads/help-needed-for-opamp-offsetting.96749/

Thanks
 

MikeMl

Well-Known Member
Most Helpful Member
You didn't mention the sampling rate. The corner freq for the anti-aliasing filter should such that it's response is down more than 20log(1/256) db = -48db (1/256 because it is an 8bit AD) at the ADC's sampling rate.

If the level-shifter is not biased from its own supply (i.e. if its reference voltage is a capacitively bypassed zener or other active reference, rather than a voltage divider off the 12V), then the LM358 will not pass noise on its supply pin to the ADC input due to its PSRR.
 
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picstudent

New Member
You didn't mention the sampling rate. The corner freq for the anti-aliasing filter should such that it's response is down more than 20log(1/256) db = -48db (1/256 because it is an 8bit AD) at the ADC's sampling rate.
Let me study that. I am measuring a DC voltage to display.
I may take 500 samples / second at maximum. Will take average of say 100 samples and update display. Also the LP filter will have a settling time which slows down the response.

Thanks a lot.
 
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