silly circuits

I wrote a program that finds resistors in our stock that hit a specified ratio. Because it brackets thevenin impedance, it accidentally also finds parallel resistor combos.

I should fix it to do series too.

And I can write ECOs to change parts after a board is built.

The two passive sections have much better ultimate rejection than a second Sallen-Key, which is often a win.

Cheers

Phil Hobbs

{snip]

Agreed. ...Jim Thompson

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The unity-gain S-K with equal resistor values is pretty good--sensitivity values of about 1, vs. 0.5 for LCs or gyrators. It's the high-gain VCVS ones that have the really horrible sensitivities.

Cheers

Phil Hobbs

But I thought you only had one value of resistor. ;-)

Williams and Taylor lists a bunch of equi-ripple approximations to classic filters, which can be handy.

Using quad r-packs can lock one down. If I suspect that a part might have to change, I use a discrete R or C.

The game is to keep the BOM line count down.

Different worlds. Our job is to keep the cost down. R-packs are expensive but if they can replace four resistors, the placement cost makes up for it. There is no advantage to keep the BOM line count small, though I try to anyway.

We are paying 0.69 cents for a 1K 1% quad resistor pack, about 1206 overall size.

You can get quad capacitor packs too FYI (don't know if the first circuit has equal values)

We are paying less than 1 cent for a resistor pack and with the resistor pack you get very good tracking between the resistors in the package, so some circuits perform a lot better

Cheers

Klaus

We have theorized that the tempcos should match very well inside a pack, but haven't measured it yet.

I think we pay about 1/4 that for a 0408 4x Rpack and about 1/40th that for an 0402 resistor. It costs something like .25 cents to place either.

That may or may not be true. The Rpacks I've been using don't specify any tracking. If it's not specified, I can't use it.