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Measuring LDO with passive/active notches/filter
Measuring LDO with passive/active notches/filter
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Old 8th February 2020, 04:35 PM   #51
1audio is offline 1audio  United States
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Measuring LDO with passive/active notches/filter
QA just published an app note that is quite relevant: Notch Filters



– QuantAsylum
which I will need to explore. I have a Picoscope 3206 which I may try and a Bode 100 if I get motivated to set it up.

I used to use a Picoscope on the output of an AP. It worked quite well. The picoscope software is really good. With a good notch you should not need more than 8 bits.
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Old 8th February 2020, 06:15 PM   #52
MagicianT is offline MagicianT
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I'm testing Fliege notch right now.
Pro:

1. single pot adjustment central frequency about +-10%;
2. Q easy to set, so harmonics magnitude is correct.
Contra:
1. tendency to oscillate with high GBW OPA;
2. adjustment changes max attenuation level.
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File Type: jpg ne5534-ti-1.JPG (831.7 KB, 125 views)
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Old 13th February 2020, 11:23 AM   #53
Preamp is offline Preamp  Germany
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Measuring LDO with passive/active notches/filter
Created a little layout for the SV with two dual SMD OPA1612 and a single DIP LT1028 for additional gain after the notch. One of the OPA's is configured as an input buffer this time, as the input impedance of 1k might be a little low already and too much of a load for Victor's oscillator or any other DUT.
Crammed everything onto 5x5cm, including a relay to switch between two sets of capacitors for 1kHz and 10kHz use. Footprints are there for SMD or THT passives, which may come in handy during experimenting with the best performing values. The small size qualifies for el-cheapo chinese manufacturing, but I think this time I'll opt for some European board house, so I don't have to wait another month or so.
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Old 13th February 2020, 11:45 AM   #54
diyralf is offline diyralf
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Quote:
Originally Posted by 1audio View Post
QA just published an app note that is quite relevant: Notch Filters



QuantAsylum
which I will need to explore. I have a Picoscope 3206 which I may try and a Bode 100 if I get motivated to set it up.

I used to use a Picoscope on the output of an AP. It worked quite well. The picoscope software is really good. With a good notch you should not need more than 8 bits.

Seems to be a combination of Jim Williams oscillator and a T-notch on a common board. I doubt they get the right performance out of SMD junk parts.
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Old 16th February 2020, 04:15 PM   #55
HpW is offline HpW
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Quote:
Originally Posted by 1audio View Post
There were questions about the noise characteristics of the AD725D notch filters.
Attached is a scaled plot of the 725D measuring the victor oscillator in rtHz. Marker 3 I could not get to connect to HD3 of the oscillator.

I'm looking though my options for a way to plot the frequency response. The QA does not have a suitable way (not enough resolution).
Here an 3x of your given asc file. The question rises how the 725D compensates the H2 attenuation as about 6.3dB

See below freq. response, 2Khz details, noise analysis..

Hp
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File Type: png 725D A3-3x-Freq.png (11.7 KB, 64 views)
File Type: png 725D A3-3x-Freq-2kHz.png (10.0 KB, 63 views)
File Type: png 725D A3-3x-Noise.png (11.8 KB, 18 views)
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Old 16th February 2020, 05:46 PM   #56
1audio is offline 1audio  United States
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Measuring LDO with passive/active notches/filter
I'm not sure how accurate my .asc file is. Some aspects are confusing like how to represent the cap neutralization etc. not to mention the potential for errors on my part.

The unit does not seem to have attenuation on H2 so I well might have an error.

I hope to have an extender board soon so I can measure the first notch filter independently. We don't have a schematic of the 725D. The one I worked from was the 725B. The different generations all have small adjustments to some values. My modded first gen 725 gives essentially the same results at the 725D. The critical change seems to be the opamp driving the feedback node.
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Old Yesterday, 04:23 PM   #57
HpW is offline HpW
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Quote:
Originally Posted by 1audio View Post
I'm not sure how accurate my .asc file is. Some aspects are confusing like how to represent the cap neutralization etc. not to mention the potential for errors on my part.

The unit does not seem to have attenuation on H2 so I well might have an error.

I hope to have an extender board soon so I can measure the first notch filter independently. We don't have a schematic of the 725D. The one I worked from was the 725B. The different generations all have small adjustments to some values. My modded first gen 725 gives essentially the same results at the 725D. The critical change seems to be the opamp driving the feedback node.
Well, the freq. figures looks ok! Also the R7/R8 feedback looks ok!

This means, for this values, the Q is not that great. May some compensation happens elsewhere..
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