A proto PCB arrived earlier this week. This is the 2nd generation board for the High Level electronics to measure noise. This instrument is for teaching physics students about noise and is modular. The students must make connections between the modules to make it all work. Something like this.
It has two switch selectable SV filter sections (10, 30... 3k and
300, 1k,...100k Hz) A gain section, Gain of 10 to 10^4 with switch selectable fine control. And a multiplier section. (The first generation board had some conceptual errors that lead to a design with gain peaking at the highest gain settings.)I populated the gain section and it looked great. There was no gain peaking and at the highest gain (10^4) the 3dB point was at about
1.5MHz. I should mention here that the output from the final opamp is coupled to the output BNC connector with coax. With bare wire capacitive coupling from the output to input leads to gain peaking. The output BNC is also isolated from the front panel, with the return currents flowing back along the outer braid of the coax, and grounded back at the output opamp. If the output is not isolated there is again gain peaking, due, I assume, to the return currents flowing back along the shield and making the input and output connectors have slightly different voltages. (The only ground connection between the shield and pcb is made at the input to the gain stage.)So far, so good. I then powered up the filter section and plugged it into the gain stage. Crap!!! Gain peaking at the highest gains. And if I run the High Pass output from the filter into the gain stage it breaks into oscillations. (Gain peaking is at about 1.5MHz as you might expect and the oscillation frequency is 1MHz.)
I've spent the past few days banging my head against this PCB. The filters and gain section were made with a common ground plane. (The multiplier section aready has a separate ground plane and power filter.) I dremeled a slot between the planes and ran wires back to the power supply ground input connection. (No real change.) I won't bore you with the rest of the gory details, except to say that I did learn that capacitive loading of the output made matters worse. This was not capacitive loading of the final opamp. If I grounded the input of the gain stage or drove it from some external generator there was no gain peaking when I added more capacitance to the output. I was adding 220 pF to the already existing 3 foot coax cable running to the scope
Today I powered up the filter section from a pair of 9V batteries. When I used the shield as the ground for the filter section every thing was great. No gain peaking. When I used the power supply ground from the gain stage as ground it was the same old oscillating story.
So I then built this configuration
+-------+ |Shield (V+)----R1R1---+---+ Vin | | | | | | C |filter | | C | | | +---+ GND---+------+ | | | +-------+Where R1 was 100 ohms and C was a parallel combination of 47uF tant,
1uF ceramic and 0,1uF ceramic.This worked fine, but I'm not sure I like the return current from the filter section flowing through the shield. (And then back through the gain stage.)
So then I made this,
+-------+ |Shield (V+)----R1R1---+---+ Vin | | | | | | C |filter | | C | | | PGND---LLL----+---+ GND---+-R2R2-+ | +-C2C2-+ +-------+ |Where R1 and C are as before, PGND is the power supply ground, L is
100mH, R2 is 10 ohms and C2 is 1 uF.This worked also. (Though I still have the high frequency currents flowing through the shield.)
The values were picked from what was lying around, It was late Friday and I decided to declare victory and go home.
There is a symmetrical R and C on the negative supply line.
If you've read this far, Thanks.
Is there something obvious I might have missed? Some way to make it better?
I'm not sure which of the above configurations to use.... My fear is that neither may end up working. I worry about sending the ground currents from the filter section through the gain section.
Thanks again,
George H.