Here are the conditions:

Function Generator:
Freq: 1.00 kHz
Vpp: 2.00V

CH1: Vac (Function Generator, input)
Ch2: Vr (Resistor measurement, output)

I've heard that if you split the ground (using different grounds) for both the resistor and the diodes, that this should fix the problem but it does not work for me.

Any ideas what I may be doing wrong?

Thanks

 

Would it be possible to see a schematic of your circuit and test setup?

 

Would it be possible to see a schematic of your circuit and test setup?

This is the setup of my circuit. But I have the node (between D4 & D2) grounded.

 

Grounded how?

Is the output of your function generator ground referred?

Where are you clipping the ground for your o-scope probe?

 

I'm not sure if this helps or not.

 

Grounded how?

Is the output of your function generator ground referred?

Where are you clipping the ground for your o-scope probe?

I'm grounding it to a nearby Power Supply, (not referenced to either Oscilloscope or Function Generator.

 

Your o-scope probe is (almost certainly) ground referred. Are you sure the function generator output is not ground referred? Many of them are.

Disconnect everything from all of the instruments and then use an ohmmeter to measure the resistance between the power supply ground, the function generator's output "ground", and the scope's probe "ground". Are you showing relatively low resistance between any of them?

 

Your o-scope probe is (almost certainly) ground referred. Are you sure the function generator output is not ground referred? Many of them are.

Disconnect everything from all of the instruments and then use an ohmmeter to measure the resistance between the power supply ground, the function generator's output "ground", and the scope's probe "ground". Are you showing relatively low resistance between any of them?

Okay,

Yes, I am showing low resistance between all of them.

 

There's your problem. You have a ground loop that is shorting D2.

You need to pick ONE point in your circuit to connect to ground. In this case, that choice has been made for you by the function generator. So that is your ground for your entire circuit, including your scope probes.

So use two scope probes to connect to opposite sides of your load, grounding both probes to the D2/D4 junction. Then use the scope's math abilities to display Ch1 - Ch2.

 

Are all of the diodes functioning correctly? If one is not functioning it will look like a half wave rectifier.

Also, are all of the diodes connected correctly, the image does not clearly show the connection of the lower left diode. Try a continuity check for all of the connections.

 

Also, both of your scope channels are AC coupled. For this kind of measurement you want them to be DC coupled.

 

There's your problem. You have a ground loop that is shorting D2.

You need to pick ONE point in your circuit to connect to ground. In this case, that choice has been made for you by the function generator. So that is your ground for your entire circuit, including your scope probes.

So use two scope probes to connect to opposite sides of your load, grounding both probes to the D2/D4 junction. Then use the scope's math abilities to display Ch1 - Ch2.

This fixed the problem. Thank you for your help!

 

This fixed the problem. Thank you for your help!

Glad to hear it. This is a common problem that most people (including me) fall prey to at some point. We tend to be pretty sloppy about grounding in general and most of the time it just means that we have more noise than we could otherwise achieve. But at times it can really bite us and I would bet that most people, even experienced people, do not know how their test equipment is grounded. In particular, they assume that, like their multimeters, that their scope probes are differential.

Now that you've been bit, you are largely immunized because this issue won't be too far away from your thoughts as you make future measurements.

 

Now that you've been bit, you are largely immunized because this issue won't be too far away from your thoughts as you make future measurements.

Wise words, these. Don't take them lightly. It seems the simplest things, like defining what "ground" is, can be the hardest to diagnose. We take for granted we have it right and are blind to checking it with the same rigor we apply to the "complicated" things.