OpenScope Grounding

[Nth]

My son is doing an NPN transistor type diff gain circuit/CMRR breadboard for his class. He's using the +3V and -3V supplies and the AWG from the openscope MZ board.

Seems to me he needs to ground the scope 1- and 2- leads to the black GND, otherwise floating, hence wacky results.

He has in fact been floating these "-" leads and indeed got some weird results. 

Since he is using positive and negative supplies, the breadboard has no GND connection explicitly. So the path to ground is maybe thru the supplies, or some weird high impedance AC path thru the scope "+" leads.

Shouldn't he simply build a ground plane on his breadboard where he shorts scope "-" leads to the AWG and DC supply GNDs?

[JColvin]

Hi @Nth,

Attaching the Scope 1- and 2- leads to ground is usually what we recommend for singled ended measurements to help keep these artifacts out, although as you can see in the schematic, https://s3-us-west-2.amazonaws.com/digilent/resources/instrumentation/openscope-mz/openscope-mz-sch-revg.pdf, on page 2 and pages 4/5 the negative inputs are already connected to ground. The positive leads each have a 1 MOhm impedance, though they are only DC coupled (more details on the Analog Inputs starting on slide 42 of this presentation: https://s3-us-west-2.amazonaws.com/digilent/resources/instrumentation/openscope-mz/21037_ad2_finalslides_1_.pdf).

In the end I still agree that a ground plane on the breadboard for the negative leads on the oscilloscope would be a good approach.

Let me know if you have any more questions.

Thanks,
JColvin

[Nth]

Thanks so much. Turns out the specific issue was that in trying to create an NPN phase splitter to test out the diff Amp, he had ONLY caps on the collector and emitter of the phase splitter. These in turn were connected to the respective bases of the two NPN transistors of the diff Amp. 

When he hung the channel 1 scope probe on either of the outputs of the phase splitter, the diff Amp output on channel two "worked", or at least he got an output signal that matched the sine wave input.

When he disconnected the scope probe from the output of the phase splitter, the circuit essentially shut down. 

Sooo, we put a 1Mohm resistor on each of the phase splitter outputs with the other end tied to scope ground, and voila, the circuit works correctly, and no need for the scope probe.

Somehow, an exclusive AC couple from the phase splitter output to the diff transistor base had issues. We are thinking he essentially had no good DC bias for the diff Amp base (es). So the scope probe was providing some path to ground to maintain a DC bias at the base of the diff transistor.

I know this is starting to wander into the basics of a diff Amp experiment, but curious if you know what could have gone wrong here.

Also, for completeness, he uses only capacitor coupling from the AWG into the base of the phase splitter, with +3 and -3 on VC and VE, respectively.

We're thinking it's probably a bit sloppy to rely on the AWG for biasing the phase splitter base, but this does not seem to compromise the circuit.

Thanks again for the links, and your time.

Nth

 

[Clyde]

A pencil sketch of the schematic would be great for readers to help you.  Does not need to be elegant and you can shoot it with a smart phone.  Include the power supply please.

Clyde