john jardine wrote: (snip)
I don't think there is any way to use this concept with non inverting stages. I think each half must have a virtual ground node. Multiple feedback filters have this, so they work fine.
But with a loop gain of 1/4 I think.
It still be positive and will introduce squirrels... I'm an expert at getting inadvertent squirrels ;-)
...Jim Thompson
-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |
Haven't you ever been introduced to a squirrel you liked?
I really don't see why you wouldn't do a leapfrog design. It is very straightforward, plus the built in phase reversal availability would come in handy. A number of books cover low pass leapfrog techniques. Adding transmission zeros is a little tricky, but if you understand signal flow graphs, it is also straightforward. Not totally obvious, you will need to dynamic range adjust all the op amp nodes.
[SMI-I-I-IRK :-]
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
You see, John, the whole idea of the differential filter is using just one set of reactances between /+/ and /-/ paths. So the limited tolerance of the capacitors will not impair the CMRR.
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
Yes indeed!. Had to pin an input to 0V to see the path.
Well, it took me a little pondering to figure out what was wrong with this circuit. I had stumbled onto it while plinking around with LTspice, an it seemed to work a miracle. It produced a perfect conversion from single ended to differential (subtracting the inputs from each other and producing symmetrical outputs centered on zero volts) but it had no zero volt reference connection anywhere in it. I couldn't figure out how it knew where ground was. So I tried to come up with the simplified equations dscribing it, but there was only one. Vi1-Vi2=Vo1-V02, with i and o indicating input and output nodes of the two opamps. This matched the result, but again, offered no clue how the circuit knwe where zero was, since there are an infinite number of combinations that this equation satisfies.
So I went back to the simulator and added 3 mV of offset ot one opamp. the outputs saturatedat the supply rails. Bingo. I had an indeterminate circuit that only appeared to work, because themodel used perfect opamps.
I can regain the operation I wanted, by adding a third voltage divider between the inputs, and tieing the two + inputs to that, to get rid of the implicit solution of their voltages, but that also adds a third ratio tolerance to the solution. But that still might be good enough to use as a speaker driver.
When things are too good to be true, they usually aren't.
(snip)
Oops. Make that a 3 resistor divider between the 2 inputs and ground, with the two resistors to the two inputs twice the resistance of the one to ground, so that the rresult is half of the common mode voltage of the two inputs. I'm now up to 7 precision resistors. Sigh.
The lack of reference point flummoxxed me. But all the same, I had a worthwhile half hour.
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Actually that penalty is paid only on transitions between single-ended and fully differential. Just the same, for the desired performance "lab set" capacitors and resistors would be the norm.
-- JosephKK Gegen dummheit kampfen die Gotter Selbst, vergebens. --Schiller
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