Good point. I was in fact expecting, that the circuit is correctly biased to mid-supply. In this case, there should be no problem with overall DC-coupling and a gain of 1.25, I think. A not so obvious point is impedance of Vref, which is working as a virtual ground. It must have very good bypassing to avoid signal coupling through this path.what is the value of Vref?
Did you check the dc operational point (opamp dc output voltages) ?
The prototype board was built exactly as in schematic, using OP462 15Mhz op-amp. Could in part be layout, this was a quick prototype board is all.Assuming you're actually using a 15MHz op amp to build a 60KHz lowpass, like in the schematic, it should be performing better than what you're getting. .....I'm betting it's a layout issue, or the op amp GBW is too low.
"Feedthrough capacitance" is almost certainly not your issue. If it is, then scaling your components to give smaller Rs and larger Cs should mitigate that effect.
You haven't described your measurement procedure. In general measuring large attenuation like -60dB can be quite difficult without careful methodology or good instruments. You may just be hitting the noise floor of your measuring device.
I believe, the explanation for the effect observed by you is as follows: "tail" effect.
This is a known disadvantage of the multi-feedback topology.
There are other filter structures (e. g. positive Sallen-Key) which do not suffer from these unwanted effects.
Plugging OP462 real parameters in the filter design tool however suggests, that output resistance has a neglectable effect in this case.
My guess is cross-talk involved with the quad OP package. It's mentioned in the datasheet, but not specified for the frequency range of interest, if I understand right. An additional critical point related to crosstalk are possibly the bias compensating resistors, that should be bypassed just in case.
what is the value of Vref?
No, but I will.Did you check the dc operational point (opamp dc output voltages) ?
I think, single supply operation for three dc coupled stages (with noninverting gain of 2) is rather problematic.
Yeah, it's possible for capacitance between the input and output to have that effect, but that capacitance isn't part of the op amp itself, but rather stray capacitance in the signal traces.The reason I believe it is feed-through capacitance is because of the Texas Instruments application note SLOA049A - “Active Low-Pass Filter Design” pg. 16. (see attached). I just found the book "Op-Amps For Everyone" recommends using capacitor values of 1nF to a couple uF for filter design (pg. 425) to minimize the effects of parasitic capacitances, so I will probably change this.
What measurement bandwidth are you using? That will determine the noise floor.I'm using the Omicron Bode 100, which should have a noise floor around -100 dB.
To summarize (and make sure I understand correctly), the following suggestions have been made:
1) Increase capacitor sizes and decrease resistors to minimize the effects of parasitic capacitances.
2) Ensure DC Operational point is correct.
3) Add bypass capacitors to R6,R10, and R13. (I assume to minimize parasitic capacitances on the input).
4) Build each stage using a single op-amp chips (OP162) instead of the quad package (OP462) and measure the frequency response in order to determine if crosstalk is the problem.
5) Consider using the Sallen-Key topology.
Did I miss anything?
Attached is the filter design and a plot of the measured vs. theoretical frequency response.
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Two different methods can be used: (a) FDNR technique and (b) leapfrog topology (leapfrog-like coupling of integrator stages)
Are you required to use single supply?
To 2) Yes, absolutely necessary. For this purpose (multifeedback topology) capacitive decoupling beween the 3 stages is required, see link in posting #7
This may be a basic question, but I don't fully understand why this is needed? Help me in my ignorance.
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