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Cenlasoft
10-27-2011, 02:56 PM
Hello,
I have a tank circuit where the resonant frequency is 430 khz. I am using the prop with sigma delta to monitor voltage change, but if I connect the sigma delta circuit to the output of the tank circuit, I get loading. I was thinking of using an op amp to buffer the two circuits. I want to know if anyone knows of an op amp (hopefully single supply) that can handle 430 khz. Thanks for your help.
Curtis

Beau Schwabe (Parallax)
10-27-2011, 03:35 PM
What's the peak voltage within the tank and what is your L value? ...Also is at least one leg of your coil gnd or vdd?

Peter Jakacki
10-27-2011, 03:50 PM
Curtis, if you have the right input resistor (not the feedback one) then there really shouldn't be a problem as the loading would be quite minimal. I hope you are not feeding this directly into the input caps as the demo microphone circuit does as this is okay for that circuit but not for general-purpose use. Do you have a drawing of the circuit that you can share?

Cenlasoft
10-27-2011, 06:40 PM
@Beau,
Thanks for your response. The peak voltage is 3.3 v from a square wave on pin0. The inductance of the coil is .398 mh. I have attached the schematic.
@Peter,
Thanks for your comments. You were right, I did attach the output of the tank circuit to the cap on the sigma-delta. Look at the attached schematic and tell me what you think.
Any suggestions would be appreciated.
Curtis

Cenlasoft
11-04-2011, 01:12 PM
Hi,
Is there anyway to buffer a tank circuit from a sigma-delta AC circuit? I tried many techniques with any luck.
Thanks,
Curtis

Toby Seckshund
11-04-2011, 02:32 PM
You can take off a tap connection. The coil is the usual place but it can be done with the capacitors too.

Beau Schwabe (Parallax)
11-04-2011, 03:57 PM
Toby Seckshund,

"You can take off a tap connection. The coil is the usual place but it can be done with the capacitors too." - That was where I was going with asking what the overhead voltage within the tank was. Along similar lines you can have a secondary coil that couples into the primary and take your measurements from the secondary instead. This method at least allows you to easily 'tune' the tank in a way that's inclusive of your sensing load.

Cenlasoft
11-04-2011, 04:03 PM
This tapping, do you mean wind a secondary coil and place it inside the primary to act as a transformer?
Or do you mean physically tapping the coil?
Thanks,
Curtis

Beau Schwabe (Parallax)
11-04-2011, 04:15 PM
Cenlasoft,

A tap would be electrically connected to the existing coil and act like two coils in series... there are two ways to do this. 1) the tap could be part of the LC tank in which L1+L2 = the total L requirement in your tank or 2) L1 + C is your tank and L2 happens to share a common connection to L1 ... in this case L2 is coupled to L1 by proximity. The other way where L1 and L2 are in series, each coil contributes to the total L and shows proportional characteristics of the LC tank depending on the value of L1 and L2.

The second option is essentially a transformer, and by removing the common connection in the option mentioned above, you can create a galvanic isolation between L1 and L2. If your 'load circuit' on L2 is constant, then L1 with C can be tuned to encompass/negate the loading effects of L2.

Toby Seckshund
11-05-2011, 02:32 PM
Given that the coil is 0.398mH and the frequency is 430KHz that would mean that the capacitance would be 344pF. Just as a quick thought I worked out that a 470pF cap and a 1000pF cap connected in series would give 319pF, as the coil would have some self capacitance this wouldn't be too far off.

With the 1000pF at the ground end of the combo this would give a "tap" the equivilent of 1/3 up the coil and hence 1/9 of the impedance allowing less damping. It might be posible to use the caps that form the input of the ADC circuit dirrectly,if they were changed to 500pF each ( I have only ever had 40 pin DIP Props and so have never had that much luck with the ADC arrangement)

The LC is being driven via a 10K resistor and even this would be damping the resonance, it could also be fed to the "tap" point

frank freedman
11-05-2011, 10:04 PM
Hello,
I have a tank circuit where the resonant frequency is 430 khz. I am using the prop with sigma delta to monitor voltage change, but if I connect the sigma delta circuit to the output of the tank circuit, I get loading. I was thinking of using an op amp to buffer the two circuits. I want to know if anyone knows of an op amp (hopefully single supply) that can handle 430 khz. Thanks for your help.
Curtis

Go to either the TI or Nat. Semi web sites and look over the op amp offerings and compare for GBW and input capacitance etc. Then run the simulation on your circuit buffered by the op amp. I think someone on the prop forum mentioned that they had done a complete design using the WebBench simulation on the National Semiconductor site.

Frank