Multi-purpose dual op-amp PCB design

[OlegSh]

For completeness and future reference attached are the actual PCB pictures and schematics.

Regards,
Oleg

 

[AndrewT]

RCF is missing from the +IN route in all three sch.
It is only present in the -IN route.
It should be parallel to CA, CB, CNI

 

[OlegSh]

It is intentional since I did not plan this PCBs to be used for making active filters or generators, thus no positive feedback path.

 

[AndrewT]

It's not positive feedback.

It should be parallel to CA, CB, CNI

And your GND symbols are all mixed up.
Differentiate between Power/Decoupling Ground and Chassis and Signal Return.

 

[OlegSh]

There is a resistor from +IN to GND parallel to CA, CB and CNI, it is just named RCA, RCB and RCNI respectively. It made it simpler to name components in the limited space on the PCBs.

 

[AndrewT]

RG/RCNI loads the + & - lines
R1A'/RCA loads the + & - lines
R1b'/RCB loads the + & - lines

But RCF only loads the -IN line. The other half is omitted.

Notice how RCFA||CFA and RCFB||CFB and RCF||CF

 

[OlegSh]

Sorry Andrew, but I can't follow you. Could you please elaborate in simpler terms? Let's take a single op-amp board as an example. Could you please name the wire joints or IC pins which should be connected by the missing resistor?

 

[OlegSh]

There is one probably misleading component on the PCBs: RI(A,B) and RI(A,B)' on the dual op-amp PCB or RI/RG on the single op-amp PCB. This is just one resistor which can either go to GND (in the non-inverting configuration) or to the input connector in the inverting or differential configuration. There are three pads for this resistor on the PCBs. May be that is why it looks missing on the non-inverting input side...

 

[AndrewT]

post41 said

RCF is missing from the +IN route in all three sch.
It is only present in the -IN route.
It should be parallel to CA, CB, CNI

 

[OlegSh]

So, if understand it right, one end of the missing resistor should connect to the +IN (pin 3 and/or 5 of dual op-amp IC), where should the other end connect?

 

[AndrewT]

in parallel to the capacitor in the three diagrams

It should be parallel to CA, CB, CNI

 

[OlegSh]

But there is already a resistor parallel to CA, CB and CNI from +IN input to GND. It is RCA, RCB and RCNI correspondingly. I must really miss something here.

 

[AndrewT]

But there is already a resistor parallel to CA, CB and CNI from +IN input to GND. It is RCA, RCB and RCNI correspondingly. I must really miss something here.

RG/RCNI loads the + & - lines
R1A'/RCA loads the + & - lines
R1b'/RCB loads the + & - lines

But RCF only loads the -IN line. The other half is omitted.

Notice how RCFA||CFA and RCFB||CFB and RCF||CF

 

[OlegSh]

Andrew, if we omit RX and CX network then -IN of the op-amp sees RIA(B) (note that RIA(B) "prime" as well as RI and RG is the same single resistor, just turned either towards the input connector or to the ground) and RCFA(B)//CFA(B). Next +IN sees RNIA(B) and RCA(B)//CA(B). Thus each op-amp input sees exactly two resistors and one capacitor. I really do not see where another resistor can go...

 

[AndrewT]

I don't know why you put in Rx+Cx, but there may be ocassions where they are required.

IN-3 sees shunt R-,. series RI,. shunt RG,.. Shunt CF and shunt RCF,
IN-1 sees shunt R+, series RNI, shunt RCNI, and shunt CNI.
But is missing the equivalent of shunt RCF

your GND symbols are all mixed up.
Differentiate between Power/Decoupling Ground and Chassis and Signal Return.

 

[OlegSh]

Rx-Cx network has been suggested by johnc124 to stabilize OPA1688 in unity gain configuration in agdr's thread. Since I wanted my PCBs to be universal I made the pads for Rx-Cx such that a single component, either cap or resistor, can be soldered across the pads if necessary (e.g. resistor for precise THD measurements).

As for the missing resistor quest: series RI and shunt RG is only one resistor. The two are never used together (at least to my limited knowledge). If the op-amp is used as inverting or differential amplifier then only RI is populated (RG is empty). In the non-inverting configuration only RG is populated if the gain other than unity is needed (RI is empty since connector pin IN-3 is not used in this case). So I really do not see where an extra resistor from the non-inverting input can connect!? If you could make a sketch on top of my schematic and post it here I would really appreciate it since my imagination fails here...

 

[AndrewT]

I agree about the noise gain stability for low gain opamps as suggested by Johnc124.
I used it for a +20dB chipamp where the uncompensated gain margins are too low for good audio reproduction.

I give up on repeating myself about the missing resistor.

When are you going to sort the mess of the common grounding, using one symbol for everything, including the signal returns?

 

[marce]

Just use a ground plane its all low level....

 

[OlegSh]

I give up on repeating myself about the missing resistor.

May be I'll get it one day.

When are you going to sort the mess of the common grounding, using one symbol for everything, including the signal returns?

The designs are using ground planes and are laid out such that the local decoupling supply and return currents as well as the output currents are traveling on one side of the PCB - near the power entry terminal. At the same time input signals and their returns do not cross those paths (this is fully accomplished on the dual op-amp PCB and almost complete on the single op-amp PCB).

 

[OlegSh]

Just use a ground plane its all low level....

Thanks for looking, marce! The PCBs are shown here. They use GND planes as you suggest.

Marce, since you are here, may be you could explain me what Andrew tries to tell me about missing resistor from the non-inverting op-amp terminal?