Decimator Loop Buffer Opamp Question

[lars-musik]

Again, I am working on this Noise Gate that already had me wondering scratching my head (here).

Now I have some very basic questions. To have the "normal" Decimator working as the "G-String"-Version, a loop was implemented. To my understanding the signal chain was cut for that purpose and two buffers used to get it right. The "Send" is buffered by U7.1 in Galego's schematic. For the "Return" another opamp is brought into the circuit (U1.1) and only one half used. 



Two questions here:

1. The unused opamp (U1.2) isn't properly terminated, right?

2. Couldn't I replace the return U1.1 buffer with a simple jfet- buffer? That would lower the part count and facilitate my 1590a efforts. Like this?

[garcho]

1. The unused opamp (U1.2) isn't properly terminated, right?

http://www.electronicproducts.com/Analog_Mixed_Signal_ICs/Amplifiers/Properly_terminating_an_unused_op_amp.aspx

https://www.maximintegrated.com/en/app-notes/index.mvp/id/1957

[lars-musik]

Thanks, this was what I had in mind. So this one isn't terminated correctly and I'd rather have something like that:





Which pushes the part count even higher (I am still heading towards a 1590a built). Has somebody an opinion on my initial question 2?:

2. Couldn't I replace the return U1.1 buffer with a simple jfet- buffer? That would lower the part count and facilitate my 1590a efforts. Like this?

In yet another schematic of this circuit, the superfluous opamp is used in a way I do not understand. Could somebody explain what is going on here?






Thanks in advance!

[duck_arse]

what's going on there is a voltage follower feeding a voltage follower. something of a pointless exercise, except that all parts are correctly terminated. you would probably just use a single opamp package instead of a dual in this inst.

"something like this", yes. correctly terminated and actually being useful ( ..... if the output is VR).

[garcho]

you could always use that properly terminated op amp as a buffered Vref and not feel bad about not using it, and then have rock-solid Vcc/2.
the JFET buffer is probably fine although a TL07x op amp has better input/output impedance and will be less noisy. I don't see how a discrete buffer lowers the part count, it requires at least one more resistor than an op amp unity buffer, no?

[lars-musik]

Thanks for clarifying that! The "voltage follower follows voltage follower" is new to me but might it just the way to go for me in this case.
A buffered Vref is already present (nicely hidden in the middle of this schematic), so I guess there's no point in setting up another.

[garcho]

nicely hidden in the middle of this schematic

doh!