Electrical – Multisim and DAC (summing op amp)

Yes, as you suspect the CA3130EZ is not unity gain stable without compensation capacitor(s). On page 3 of the datasheet states that a capacitor (called \$C_c\$) of at least 47pF be placed from pin 1 to pin 8 of the device. \$C_c\$ provides Miller compensation, and reduces bandwidth of the part. Circuits shown with unity gain also typically have a 2kOhm || 0.1uF feedback network, and a 25pF load. See the Bode plot of open loop response on page 14 for details of effects of various values of \$C_c\$ and \$C_L\$.

It looks like both filter sections are designed with the -3dB point at the same frequency, or very close together, so this filter is doing what it should.

In the crossover region, both sections contribute to the output, so it is higher than either alone. The slight peax at the crossover frequency would be 3dB if both signals were in phase(so they added coherently), so presumably they aren't. EDIT : apparently the small separation between -3dB points, rather than phase, accounts for this peak being less than 3dB.

For a classic design without that bulge, read up on the Linkwitz-Riley crossover, commonly used in loudspeakers where you want HPF and LPF outputs to sum to unity.

I don't know what you were expecting but if you wanted a notch you'd have to separate the -3dB frequencies, then the depth of notch will depend how far apart they are, and it won't be a deep notch.

If you wanted a deep notch, one approach is the Twin-T filter which can be made as narrow as you want.

Or start by specifying the frequency, notch width (at -3dB), and notch depth you want, and research filter design techniques to meet that specification.