Filter design software

[uoficowboy]

Michael,

The usual stuff, as you say:

Physical separation where possible e.g. between input & output.
Ground planes.

If you have 4 layers that makes things easier because you can put a good integrity ground plane between the components and any possible tracking which may need to cross over the signal lines (e.g. power).

If you have two opamps with four poles it can be worthwhile using two single opamps although I must admit I have done some 8 pole filters with quad opamps (space forced that approach) and they worked fine.

Filter the power supply to each opamp separately - a small resistor in the power rails of each opamp and a BIG capacitor. At your frequencies it can be beneficial to use 100uF on each opamp. I have certainly found that on circuits that work down to low frequencies (and 500Hz is low).

I must say I don't often worry too much about isolation. Good layout practice, avoiding quad opamps and lots of decoupling seems to be enough. I have designed circuits with 100dB of gain and more and if the isolation wasn't any good I would have had an oscillator. And that was with a layout that had to zig-zag the signal path due to space constraints. A lot of RF receivers will have 120dB of gain or more and the same problem would occur if isolation was that difficult.


Keith.

Hi Keith - thanks for your response. Can you tell me what you mean by isolation exactly? I wasn't entirely sure as to what you were referring to there.

I'm not sure if I'll have the luxury of a 4 layer PCB here or not. If I do, is there any benefit to a shield GND plane, rather than a power ground plane? I'm sure my terminology is wrong - but what I mean is I wonder if there is an advantage to having an internal ground plane that has only a single connection, as opposed to a normal ground plane that would have vias from all over connecting to it. Hopefully my description makes sense.

Your suggestion of a 100uF decoupling capacitor is very interesting. I thought you generally only wanted large capacitors like that to handle very sudden and large spikes in current, ie motor drivers. In a low current application like this I am confused as to why they are beneficial. Can you tell me why this is? It is very counter intuitive for me.

For high sensitivity applications like this in the past I've typically used an RLC filter, or sometimes even an RLFC filter (with F being a ferrite). I've typically used a couple different capacitors in parallel (ie 10uF | 100nF | 100pF) to block out a wider range of frequencies. Does this make any sense, or is a RC better for power filtering?

Lastly - it sounds like you're saying that a dual OA IC is OK, but a quad OA IC is significantly worse. Wouldn't a dual suffer from pretty much the same problems that a quad would suffer from? Is it just that since there are so many more signals on the IC that you get more cross coupling?

Thanks, and my apologies for the pile of questions.

-Michael

 

[keith1200rs]

By isolation I am meaning the coupling between the output and input of an amplifier and filter. When I say I don't worry about it, that is because with good design and layout it doesn't seem to be an issue - at least not to achieve 100dB to 120dB or so.

I have never used separate shield planes, just ground planes although they often have splits in them.

The large (100uF) capacitors have proven to be useful when you have low frequencies. Your filtering of signals coupled down the supply lines depends on series impedance (either resistive or inductive) and the terminating capacitance. So, for a given series impedance, increasing the capacitance improves the attenuation of the unwanted signal.

Your 10u/100n/100pF combination is good, although 100pF is something I would mainly reserve for RF frequencies up to 1GHz rather than 10kHz. Look at the SRF of the capacitors to see which are the most useful at the frequencies you are working at. I think 100nF will still have an SRF of 1MHz, I would have thought.

I don't really have hard & fast rules of single/dual/quad opamps. When you get to a quad you have potential cross coupling between four different stages which could be tricky to judge the effect of. As I say though, I have used quads in asymmetric 8 pole (5+3) bandpass filters with no problems, although I never measured the stopband attenuation (I never actually got to test them at all - I designed & simulated them & the customer went straight to PCB. They satisfied their requirements which was 110dB rejection). There is usually a figure for isolation between opamps in multi-packages, but as I say, it is difficult to know how that will work out. It may be easier to determine if you simply have a couple of independent amplifiers running through the chip.

Keith.