Sourcing parts for Really, Really low frequency filters

Question

I'm doing a project at work that has a couple of esoteric requirements, particularly for a 0.1 Hz (0.07 Hz really, due to part availability limitations) Highpass filter (in the data acquisition system).

Right now, I'm using a 22uF film cap, and 100K resistor, and the whole affair works quite well. However, the film cap is enormous (1.240" L x 0.532" W), and the resultant PCB is really a very large (there are many channels).
I really don't want to go too much higher for the R in the filter, since it's going into an op-amp. With the existing system (OP27, need the really low 1/f knee), +-10nA bias current, you get \$10*10^{-9}A*100,000\Omega = 0.001V,\$ or 1mV of offset due to bias current.

WIMA used to manufacture some compact 22uF 16V film caps, but they have EOLed them without a replacement.

Unfortunately, the application is a bit extreme. The caps need to be able to handle extremely low temperatures, and hard vacuum, which I think means electrolytics are out.

Does anyone manufacture large, low voltage film caps (voltages in question are +-5V, nothing major)? Alternatively, does anyone know how electrolytics fare in vacuum?

Answer 1

I'd forget analogue techniques and use DSP. At 0.1 Hz virtually any MCU could be used, but I'd use a dsPIC as I have the MDS dsPIC filter design utility. It actually writes the code for me. It'll be cheaper, smaller, and operate in a vacuum without any problems.

Answer 2

Why does it have to be a film cap? Why not a ceramic? I'm no expert on vacuum, but I think they should be able to handle that fine.

According to my calculation, you only need 16 µF with 100 kΩ to get a 100 mHz rolloff. In any case, a couple of 10 µF 20V ceramics in parallel with good dielectrics should work. Using them over a small part of their voltage range keeps the capacitance reasonably constant.

Answer 3

Instead of using an RC highpass filter, why not use an RL highpass filter - but instead of using a real inductor use an inductance gyrator. You can use active components (and a couple much smaller capacitors) to simulate a massive inductor connected to ground to give you your low frequency cutoff point, and it will save you a lot of board space and the other problems of using a large capacitor. Here are some notes on gyrators.

Edit: Here's a gyrator RL filter design for a cutoff frequency of 0.1 Hz, using 2 opamps, resistors and a 0.1uF capacitor to simulate a 1000 H inductor. The gyrator design is based on the one here by Jim Thompson.

Answer 4

Many EKG and EEG machines have "a very small AC signal of interest, modulated onto a very large, (slowly) variable" unwanted near-DC signal -- the "baseline wander". Since the heartbeat can drop as low as 40 Hz, we typically want a linear-phase highpass to cut off everything below around 0.5 Hz. a b

Perhaps you could use the same techniques they use for their highpass filter:

• Servo loop: Instead of passing the signal through the capacitor of a high-pass passive RC filter, they use an active filter that integrates the DC component and subtracts it from the signal ("servo loop"). A active lowpass filter somehow tweaks the main signal chain to produce a highpass effect. My understanding is that this approach can be scaled up to extremely high resistances -- say, 10 MOhm and 1 uF to get a roughly 0.015 Hz high-pass corner frequency -- without the noise that such high resistance values normally cause.

• digital filtering: some people say that baseline wandering is easier to filter out in software than in hardware.a b c

The Imac Engineering people claim they have a hipass corner frequency of 0.03 Hz. (See the "Highpass Filter Simulation" page -- how do I link directly to that page?)

The INA322 datasheet in Fig. 9 "Simplified ECG Circuit for Medical Applications" uses a servo loop driving the REF input to produce a highpass effect.

Figure 37 of the INA333 datasheet has another servo loop.

Figure 69 of the AD8420 datasheet has another servo loop: 0.5 Hz high-pass.

Figure 70 of the AD8295 datasheet has another servo loop.

Figure 5 of "Getting the most out of your instrumentation amplifier design" has another servo loop.

The ECG prototype from Matthew Shieh has another servo loop.

Answer 5

EPCOS have Metallized Polypropylene Film (MKP/MFP) series capacitors listed on their website. Digikey have these capacitors up to 110µF!!