hi,
That is Quad OPA, all the 4s are +V and 11 are =V ,,,are internally linked.
E

 

hi,
That is Quad OPA, all the 4s are +V and 11 are =V ,,,are internally linked.
E
View attachment 221929

I see, so it is as if all 4s are linked to +15V, and all 11s are linked to -15V. is that right?

 

hi,
Look at this marked up image.
E

 

hi,
Look at this marked up image.
E

Thank you so much for the efforts.

 

One problem with that design is that the common-mode rejection depends upon how well the resistors are matched.
For 1% resistors the common-mode rejection could be as high as -40dB.

IC instrumentation amps use laser trimming of the internal resistors to get much higher common-mode rejection.

 

I would have called that a differential amplifier (but with input buffers). I wouldn't have called it an instrumentation amplifier unless it had resistors between pins 1 and 2, and pins 6 and 7; with the gain set by a resistor between pins 2 and 6.

 

All the TS asked for was clarification of the OPA supply line pins.

 

All the TS asked for was clarification of the OPA supply line pins.

Since when did we stop at just answering the stated question?

And he did ask to "understand the problems in the following design".

 

It is a special case of an IA. Normally yes, there are resistors in those feedback loops AND a gain setting resistor between the two inverting inputs. THAT is the IA that we are most familiar with.

 

It looks like someone did a poor job of making a quad opamp in their CAD software, so that each individual opamp shows the supply connections. Where did that screenshot come from?

 

each individual opamp shows the supply connections. Where did that screenshot come from?

Looks like LTspice.
LTspice does not have any information about how many op amps are in a package since it only does circuit simulation, not PCB layout, so the supply pins are shown for each op amp used.

 

Looks like LTspice.
LTspice does not have any information about how many op amps are in a package since it only does circuit simulation, not PCB layout, so the supply pins are shown for each op amp used.

Pulsonix (pcb design) does the same thing. It's a pain to have to connect Vdd and Vss to each inverter in a 74HC04!

 

OK, so now I can provide a complete summary of the flaws. First, the CAD model is wrong in showing power connections on each section of a quad package. THAT was the CAD error. The other errors were in the resistors that were left out, which made it not really an instrument amplifier. In an actual instrument amplifier there are feedback resistors from the output to the inverting input of both of the input amplifiers, AND there is the single gain-setting resistor then connected between those two inverting inputs. Without those three components the assembly is not an actual instrument amplifier, it is something else. It could be called an abnormal special case, at best. The comments about resistor matching are OK, because one of the manin applications of an instrument amplifier is to reject the common mode voltage at the input.