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sinara-hw / zapper

8-channel high-voltage source/PZT driver
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Zapper - filter stage #6

Open JakubKajzer opened 4 years ago

JakubKajzer commented 4 years ago

Filter stage

I have designed a switchable low pass filter (3db BW is 5Hz) to reduce noise and bandwidth. I have used two in series capacitors and resistors so they can withstand 200V. I have used a double photoMOS relay and DIP switch to turn on/off filtration.

Filter.pdf

dtcallcock commented 4 years ago

Don't you need balancing resistors to add capacitor voltage ratings like that?

You can get 450V rated 1210 C0G caps and 400V rated 0805 resistors. A bit non-standard but still <$1 components.

JakubKajzer commented 4 years ago

I've checked some Vishay resistors and they were 150V rated, so I decided to place two in series, but you're right, I found 250V rated 0603 capacitors and 350V rated 0603 resistors so I'm going to change the schematics.

dtcallcock commented 4 years ago

The AQW210EHA has a typical Ron of 18Ohms. This should be considered when selecting the series output resistor for the opamp.

How about using DIP switches directly to avoid this and remove a point of failure? You could also then use DPDT to disconnect the filter from the output completely. At these frequencies, parasitics shouldn't cause problems. They can't hot switch 200V but I think it's fine to require the board to be off when switching.

JakubKajzer commented 4 years ago

How about using DIP switches directly to avoid this and remove a point of failure?

I think we don't need to turn on/off filter often, so no possibilities of hot switching shouldn't be a problem. I've changed also the filter components.

Filter.pdf

gkasprow commented 4 years ago

@JakubKajzer please commint all the Altium source files.

adamkolodynski commented 4 years ago

This way of switching off resolve a major problem with zero offset filter I proposed in OPAMP issue. Bypassing the whole filter allows us to place large resistance in series with op-amp output. I used dual ADA4807-1 which requires only 1mA/channel and two 0805 68nF capacitors. In a result filter with the same -3dB frequency has a better slope in lower frequencies(to 1 kHz) and it doesn't kill op-amp output current(time of setting the voltage on capacitive load). FilterZeroOffset.pdf

Is CM choke is necessary? This one is rated only 80 V and it's hard to find 200V rated. This has 300VDC but it's very expensive 10$/channel unless we use chokes for AC power lines e.g Schaffner RN which are cheaper but bigger.

gkasprow commented 4 years ago

CMCS make sense when we have fully symmetrical load. Piezos are referenced to GND so CMC will do more harm than help.