Op-amp circuit question (DAC o/p)

[jcx]

I see Mooly finally has found an important piece of the "puzzle"

the circuit is a fully differential amplifier made with the dual op amp and a fully differential multiple feedback low pass filter

single fully differential op amps like THS4130 are popular for high speed signal processing like A/DSL

but you will find fully differential op amps also include a a output common mode servo on the chip

http://focus.ti.com/lit/an/slyt165/slyt165.pdf

the posted circuit fixes the outputs to be symetric with respect to gnd - which means the typical mid supply offset of the DAC outputs causes DC current to be drawn from the DAC +/- output buffers - not always a good thing

using just one side of the differential output you are relying on lots of components ratio matching for common mode rejection/differential-to-single ended conversion accuracy

there can be stability issues using the 2 op amps from a dual to build the fully differential amp - the phase margin may not be high enough - especially with the odd internal compensation of the 5532

[SoNic_real_one]

It was said a the second post.

Originally Posted by SoNic_real_one

That's the negative feedback. Instead of applying the neg feedback only on one input in an assymetrical way, they take the output, invert it and apply the feedback on positive input too. In this way the main Op-Amp has symetrical signal and impedances on the inputs. Maybe hepls with interferences via ground since it will isolate the ground plane, but I didn't tested nothing like that.
I would use the second half of a dual OpAmp for that.

[Glenn2]

Great stuff guys. Exactly what I hoping for, to learn about the circuit in question.

[gvelim]

Originally Posted by Mooly

I got interested in this and turned this up,
Differential Multiple Feedback Bessel Low Pass Filter

which is the same configuration. As I suspected earlier that second opamp is vital to the filter operation. The opamp circuit in the above link has both inverted and non inverted outputs.

You may also find the below article informative w/ relation to the thread topic
"A Differential Op-Amp Circuit Collection" : http://www.ti.com/lit/an/sloa064/sloa064.pdf

..."Proper operation of high-speed op-amps requires proper decoupling techniques. That does not mean a shotgun approach of using inexpensive 0.1-µF capacitors. Decoupling component selection should be based on the frequencies that need to be rejected, and the characteristics of the capacitors used at those frequencies"