Differential mode analog active filter?

The only concrete suggestions I have are negative -- if it's a sampling converter, watch out that whatever solution you adopt is low impedance on the output, and damps transient currents well. The high-performance, differential input SARs that I have worked around (but not on) inject a bit of charge back into their input pins when they sample; this charge is more or less random, and if it's effects persist until the next sample then it's just another thing that degrades accuracy.

I'd help you more, but on the projects that I've done using high performance ADCs I've had analog circuit designers who were eager to interpose themselves between me and the signal acquisition chain, so I haven't had the opportunity to design one yet.

Have you looked at passive LC filters with symmetry and no ground connections? I think the matching problem with this sort of design involves the stray capacitances to the common, not the tolerance of the parts. Of course, the filter will not remove common mode energy, so you are relying completely on the differential A/D input to remove the common mode signals.

When I was at IDT, we had a flash converter with performance very dependent on the op amp driving it. My recollection is on the DUT board we used some damn expensive Comlinear part which cost multiples of the DUT itself.

I'd design the filter single ended using leapfrog design. Such designs tend to have the lowest THD due to the op amp being used with one input at ground.

I've done fully differential leapfrog designs in switchcap implementation, where I have full control over the op amp. Obvious to most but just to be sure everyone is on the same page, the fully differential designs need to set the output common mode of each op amp.

If all you want is an anti aliasing filter then if you make a filter with no reference to ground then you should not introduce any CMRR problems. maybe just a differential mode torroid choke and a capacitor and some resistance for damping.

However if you want to reject common mode noise to improve CMRR this might be more difficult, but ordinary LP filters on each input should help and not affect the CMRR below their cutoff frequency.

Colin =^.^=

It takes more to make a high performance design than just throwing high performance and expensive parts at it. You seem to have no concept of SINAD degradation of SNR, nor is it important to you to mention bandwidth. You either decouple the 4th order Bessel requirement from the anti-alias function or drown under the weight of the competent invention needed.

The required inductance is at the order of several mH. Besides the inevitable problems with the mismatch, the inductors of that kind will act as antennas picking up noise.

That's is exactly what happens when the conventional single ended active filter is used for the differential operation with the floating ground. The bootstrapping of the power rails helps, however it is cumbersome and the performance is limited.

Straight to the point! Actually, it is a bit more complex. In the short, the analog filter has to suppress only the critical bands near the multiples of the sample rate; the rest of the filtering is done by the DSP. In the "low" bands, I can rely on the common mode suppression of the ADC. The "high" bands are attenuated by the other filters in /+/ and /-/ paths; the cutoff is high enough not to worry about the CMRR degradation in the band of interest.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

This is little problem.

Toroidal designs are pretty good in that respect. That you need a Bessel design and not an elliptical or Chebychev (lower Q resonators) helps a lot.

With a floating differential filter, I would enclose it in a shield driven by the common mode voltage.

You can have a torroid with two equal windings wich should not present any problems.

but above the cuttoff frequency of the differential filter the CMRR will not be so badly affected by the mismatch of two low pass filters before it.

Colin =^.^=

Hello Vladmir,

This is an interesting problem! I recommend you read James Karki's app note SLOA54D especially the section about Multi Feed Back topology and enhanced CMRR. (Maybe you have read this already?) By the way, the THS4509 kicks ass!!!* It is an amazing fully differential amplifier. Look at the GBW product on that baby. I used it for an application and was extremely impressed with its performance. Oh yeah, use the most accurate caps and resistors you can find! but you know that. Good luck.

regards, Kadir "Solid Gold" Suleyman

Lessee 0.1% match preserves 60dB so ummm...0.001% match for 100dB CMRR?

Yeah good luck measuring the capacitors that accurately, right? I didn't read that he needed 100dB CMRR just that his ADC was capable of this and he didn't want to 'sacrifice' the performance of his ADC. The graph of CMRR performance using the test circuit of the 4509 data sheet shows 90dB @ 100kHz using 5V supplies. At higher frequencies it is worse but it sounds like for his application the higher frequencies are of less concern. He never specified bandwidth. For a part that costs less than $4 in 1k quantity, I think the 4509 has pretty darn good performance but he's not going to get 100dB CMRR performance out of it without a having board manufacturable only by thaumaturgic obeissance.

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Here is an example of the kind of inductors I am talking about:

Take a look at the gyrator-based filter configurations on the SED page of my website. You should be able to easily extend these to differential.

...Jim Thompson

Thank you very much for the good advice. Yes, the gyrator topology works well for the differential filter.

VLV

And you can synthesize BIG inductors ;-)

...Jim Thompson

I've been thinking about your differential input A/D converter and wondered if you might be able to make use of a differential amplifier with a differential output as a front end, something like this:

___ ___ in+ -|___|-+----|___|-+ | | | |\\| | +----|-\\ | | | >--+ out- | +-|+/ | | |/| | | | | ___ | | ___ in- -|___|----+-|___|-+ | | | | | |\\| | | +-|-\\ | | | >--+ out+ +----|+/ |/|

This circuit removes the common mode part of the signal to the extent that the two resistor pairs match in ratio. You can turn this into a one pole low pass differential filter by paralleling each of the feedback resistors with capacitors, or extend the concept into a pair of MFB 2 pole Bessel filters.

I came up with this variation on the single opamp subtractor, today, but I don't remember ever seeing it before. Surely, something so simple and useful has a name. Does anyone recognize it?

Well, if it is new, you only have 364 days left to file for a patent. ;-)

Cheers! Rich

AD have started pushing diff in, diff out, amps (eg ADA4938) for their fast ADCs but it's anyones guess what's inside. In LTspice I'm seeing curious results both in .AC and .TRAN, if the Rs are in the 10 to 1000 ohm area. The circuit is perfect using a couple of VCVS' (1Vpk inputs) but LTspice "ideal" single pole opamps, 25ma limit, Vos=0, seem to develop an offset output signal and 10% second harmonic. I've set something wrong but don't know what.

I see a positive feed back loop.

...Jim Thompson