What comes into my mind is that your regulator GND wiring might be wrong; picking up the ripple on the power GND line.
The GND pin should be attached to the 5V (logic) load and not anywhere on the high current path.
I think you are right. I considered this and split the paths of the powerside and the control side, but they still share a few inches of wire which is part of the battery.
1) Increase the value of C1 to 100uF or 220uF Low ESR 105C Electrolytic capacitor.
2) Add a ceramic capacitor between pins 1 & 2 on the regulator. The capacitor should be physically close to the regulator IC.
Will do, thanks for the advice
The waveforms look so close its probabily power supply ripple is getting right through. R-Cs will help as long as they're placed well below 20kHz. 10ohms and 10uF is around 2kHz. Increasing the output capacitance of the LDO too much can make the PSRR worse because the open loop gain crosses over much earlier, but too small output capacitance can result in stability problems. If you have an assortment of caps try messing around with a range of values and see what you get. If you're worried about stability try adding a 1ohm in series with your 10uF cap on the output.
This regulator is not an LDO (I think?) but I understand that your suggestions also directly apply to the regulator I am using. I will try your suggestion, hopefully i have a resistor small enough sitting around. Thanks
There's a clue in the 7805 data sheet. Its power supply rejection figure is quoted at 120 Hz, but the noise on your 12V supply is nearly three orders of magnitude higher in frequency than that.
It looks to me as though the 7805 just can't respond fast enough, and you need a regulator with much wider bandwidth and/or a passive filter network on the 12V input to the regulator.
LC filter on the input will help a lot, very few regulators will handle high frequency noise on the input well, often passing it straight through or worse oscillating.
Yes, it appears like the control side waves are responding directly to the supply wave. What is confusing me is that the waveform are mirroring. When the high side voltage goes up, the low side voltage goes down and vise-versa.
don't put the cap on the battery terminals, put it as close as you can to the input of the regulator!
I have the 10uF capacitor right next to the regulator. Are you suggesting I put the gigantic 4.7mF capacitor next to the regulator? I will try that. Thanks
You're gonna need a better regulator. The 7805 is very basic, and isn't going to like all that high frequency stuff.
Im afraid this may be the case. Do you have any suggestions for a relatively cheap regulator? Something like an isolated switching regulator?
Thank you all for your replies. Once I get home from work I will try your suggestions and report back.