Voodoo Main Discussion

[Qiaozhi]

I don?t see any difference in the end result on the breadboard by removing the SAH cap. A slight ripple makes it through the first amp. Maybe your simulation is correct.

The extra capacitor is not necessary in the Surf-PI derived circuit because the sample gates and the opamp integrator together form a "sampling integrator".

In the Voodoo circuit the filters are differentiators, so there's a capacitor at the output of the sample gates to create a sample-and-hold circuit. Without the SAH capacitor the signal suffers a large drop in amplitude during the TX period.

[Orbit]

My warm recommendation of a soldering iron in hand! The simulation is sometimes not one hundred percent accurate!If that were the case, every project would be done for a day what they would do for half a year?

[Altair]

The net should be like a "T" with a resistor, capacitor and another resistor at which OP amp is connected. See others IB schematic.

[dfbowers]

I don?t see any difference in the end result on the breadboard by removing the SAH cap. A slight ripple makes it through the first amp. Maybe your simulation is correct.

[green]

Some op-amps hate any C on their -input and will oscillate. Which op-amps did this to you?

Without seeing exactly what the circuit looks like I am unable to suggest why no change.

When I posted earlier I had tried adding a capacitor to one of my existing circuits. Don't remember which amplifiers oscillated. Redrew the schematic to be closer to Don's. Adding the capacitor doesn't seem to effect the simulation. Maybe doing something wrong?

[waltr]

Tried adding the hold capacitor in a spice simulation. Sampled a input signal. No change at amplifier out with some amplifiers, oscillation with some others. Wondering what I'm missing. First time I've seen adding a capacitor from common to -input.

Some op-amps hate any C on their -input and will oscillate. Which op-amps did this to you?

Without seeing exactly what the circuit looks like I am unable to suggest why no change.

[green]

Here is the sample with and without the 100n cap. Perfect sample for non ferrous detection is on the declining slope of the oscillations. The net signal will decrease in amplitude for non ferrous and phase shift right plus increase for iron... same for the ground signal so it seems best to sample on the declining slope between 90 - 180 degrees.

Tried adding the hold capacitor in a spice simulation. Sampled a input signal. No change at amplifier out with some amplifiers, oscillation with some others. Wondering what I'm missing. First time I've seen adding a capacitor from common to -input.

[ivconic]

Could you also include a series idiot diode in power line , needs slightly bigger holes thanks

Sure,np.

[dfbowers]

U2b,R24,C31 = Sample And Hold, SAH.
Although i am not sure about the value.
100nF shall do.
So C31 is "Hold capacitor".

Here is the sample with and without the 100n cap. Perfect sample for non ferrous detection is on the declining slope of the oscillations. The net signal will decrease in amplitude for non ferrous and phase shift right plus increase for iron... same for the ground signal so it seems best to sample on the declining slope between 90 - 180 degrees.

[6666]

No sweat:

[ATTACH]54142[/ATTACH]

Could you also include a series idiot diode in power line , needs slightly bigger holes thanks

[ivconic]

What does C31 do?

U2b,R24,C31 = Sample And Hold, SAH.
Although i am not sure about the value.
100nF shall do.
So C31 is "Hold capacitor".

[ivconic]

What does C31 do?

Don, I would have expected a capacitor after the CD4066B to hold the sampled voltage; it's really missing?

I put 100nF, it can be higher value.

[green]

No sweat:

[ATTACH]54142[/ATTACH]

What does C31 do?

[ivconic]

Ivconic, I think is better left space for a capacitor on PCB (if you agree)

No sweat:

[Altair]

Work fine without one. It?s getting averaged out but ideally should have one.

Ivconic, I think is better left space for a capacitor on PCB (if you agree)