[sdiy] tempco puzzle

[Seb Francis]

Hi,

I'm building a Mixer/VCA with an SSM2018 I got lying around.  It has a CV tempco of -3500ppm/C, so I need to build the CV input stage with a 3500ppm/C tempco.

The required gain of the CV input stage is -0.25, and I have 1K 3900ppm/C tempcos.

The question is .. how to do it with *minimum* extra components?

The only way I can work out uses 3 inverting apamps:
CVs => Inverting unity-gain summer
=> Inverting unity-gain
=> Inverting 0.25 gain with the 1K tempco + 150R 100ppm/C resistor in the feedback loop

I've been racking my brains for hours trying to work out a more elegant way to do this, but have got nowehere (maybe I'm trying too hard over 3 opamps, but it's nice to find the neatest solution ;-)

Single opamp + potential divider with tempco doesn't give a near linear coefficient because the gain is not small enough.

There must be a neat way to do this!

Seb