Voodoo Main Discussion

[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.

[ivconic]

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

Sure,np.

[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.

[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]

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?

[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.

[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.

[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?

[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.

[green]

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.

Which capacitor are you referring to? I've been looking at C31 top of schematic. Doesn't look like a differentiator. https://www.geotech1.com/forums/atta...2&d=1609969536

[ivconic]

This need to be checked by "neutral eyes":

[Qiaozhi]

Which capacitor are you referring to? I've been looking at C31 top of schematic. Doesn't look like a differentiator. https://www.geotech1.com/forums/atta...2&d=1609969536

it's not a differentiator, it's a sampling integrator.
The differentiating filters I was referring to are in the Voodoo circuit.

[dfbowers]

Parts, parts, parts.. where does everyone order parts from? I realize we are from all around the world.
Here in the US I use Digikey, Mouser and for older stuff sometimes Jameco.
Reason I ask, for some sources we can share a shopping cart. I have some of the VooDoo components already ordered but I spent a few hours building the shopping cart. I can share what I have if someone would like to check it over.. or if you have an old shopping cart and have a working Voodoo.. let us know.
Don..

[6666]

This need to be checked by "neutral eyes":

Hi Ivconic , good work, I have had a look at the diode I requested but it appears to be drawn in parallel which requires a fuse to be placed in power lead which is ok it will work, but people seem to get it wrong and dont add the fuse

but what I was asking for was a series diode something like a 1N5822 40v 3A Schottky, if people dont want it they can short it out, what are your thoughts ?

[ivconic]

No sweat.