Akerberg Mossberg notch filters and how in the world do they even work - Page 1
Forum section: electronics / projects

author: Berni
author_rank: Super Contributor
timestamp: December 17, 2014, 02:16:58 pm
content: While doing audio THD measurements i have eventually reached the limit of my ADC chip at 0.0002% THD. To solve the problem i came up with an idea to build a notch filter to remove the 1KHz test tone so that i can gain up all of the tiny harmonics and have a closer look at them. Since the DAC under test usually has a differential XLR output and my measurement ADC board also takes in a differential signal over XLR it made sense to also build the filter differential.After some googling i decided upon a design i found in TIs Appnote on fully differential opamp circuitswww.ti.com/lit/an/sloa064a/sloa064a.pdf( page 9 ). To make sure of its performance i built it up in Switcher CAD (aka LT Spice now). Worked pretty nicely. But as i was drawing it i was thinking to myself how does this even work when everything is a positive feedback loop, but i wont argue with spice, if it was stable no matter what i did to it then it will work for real so lets build it.Some time later i get PCBs and i populate it and when i turn it on i notice the output keeps flying off in to the positive or negative supply rail as soon as it gets power. I check and recheck everything if i made a mistake somewhere and nope its exactly as in the app note. Some fooling around with simulations later i reproduced the exact problem. Turns out if i tell LT Spice to start the supply voltage at 0V so my caps are not already charged to there bias levels the exact same thing happens. Later i also found out that applying a sharp rise time edge (under 1us ) on the input causes the same effect.While trying out things in the simulator i kept seeing nothing but positive feedback loops. As far as i understood it the opamps should always fly off and crash in the supply rail. The way caps are wired across U1 is a positive feedback at high frequency. The feedback resistor to U1 from the output is also positive feedback because U2 inverts the signal. Only negative feedback loop i can see is around U2.Can somebody explain how this piece of black magic stays stable? Also does anyone have any suggestions how to keep it from flying off on power up?

author: KJDS
author_rank: Super Contributor
timestamp: December 17, 2014, 04:04:16 pm
content: Have you tried swapping over the outputs of the first amp to see what difference it makes?

author: Berni
author_rank: Super Contributor
timestamp: December 17, 2014, 04:31:42 pm
content: I tried that in the simulator and now it survives power on and sharp edges. Thanks!So then there must be a mistake in TIs appnote?

author: tonyarkles
author_rank: Regular Contributor
timestamp: December 17, 2014, 04:33:44 pm
content: Quote from: Berni on December 17, 2014, 02:16:58 pmI don't have any solid recommendations for how to fix it, but I'd like to thank you for the solid chuckle this morning. I've been there brother...

author: Berni
author_rank: Super Contributor
timestamp: December 17, 2014, 05:14:40 pm
content: A little moding to the PCB later and yup it works like that in the real world too. The whole circuit makes hell of a lot more sense now too.Thanks TI for the very informative appnoteI have seen silicon bugs before and copy paste mistakes in datasheets, but a straight out wrong schematic in a appnote? I am guessing someone put this together in a simulator there and went hey it works, just like i did.Still quite remarkable that the circuit performs exactly as expected with swapped opamp pins.

author: KJDS
author_rank: Super Contributor
timestamp: December 17, 2014, 05:17:16 pm
content: Quote from: Berni on December 17, 2014, 04:31:42 pmOnce upon a time, it was customary to put deliberate errors in technical papers. If a reader understood what was going on then the error was obvious.I've seen soooooo many + and - signs swapped over that it's something you learnt to look for. I've not looked closely at that circuit, but it just seemed the most obvious thing to try, and as you already had a model in a simulator then it's only two minutes of your time to see if it worked.

author: xygor
author_rank: Regular Contributor
timestamp: December 17, 2014, 05:21:58 pm
content: Maybe it needs a DC path at the input. Try shorting c1 and c2.