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Old 22nd July 2012, 02:38 PM   #1
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Default Help with Zero Field input circuit

Hello, I need some help with this circuit. It is a zero field input circuit using an opamp. The problem I'm having in the simulation is that the S/N is really bad, only 50dB, and I can't figure out why or what to do to make it better. I can't find much information on the internet about it either, except for Lundahl.
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Old 22nd July 2012, 02:41 PM   #2
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OK, I just happened to change R3 from 18 to 9 and the S/N went way up, to 100dB. Then I changed it to 1 ohm and it was at 105dB. I'm still scratching my head on this one. The resistor R3 is supposed to be about the same value as the DCR of the secondary of the transformer, which in this case is 9 ohms.

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Old 22nd July 2012, 02:44 PM   #3
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Here's the Lundahl circuit from one of their data sheets. Pin 5 is the shield, pin 6 is minus and pin 8 is plus.
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Old 22nd July 2012, 03:07 PM   #4
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In order to get good S/N (125dB in this case), the feedback resistors have to lowered so far that the opamp can't drive it, so I added a buffer (just grabbed one). This circuit seems unusually sensitive to the absolute values for the feedback resistors. I suppose it's because there's a lot of current flowing through them, according to the simulation.
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Old 22nd July 2012, 06:11 PM   #5
DF96 is offline DF96  England
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It may be because of the positive feedback, which can be fiddly to adjust and add noise.
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Old 23rd July 2012, 06:51 AM   #6
tvrgeek is offline tvrgeek  United States
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In addition to DF's astute comment,

They list the secondary as 28 Ohms. If your transformer model is not right on, it will throw everything off. Spice is only as good as the model.

You may just want to breadboard it and see. Start with their values.

I hate to second guess, but you did not say why you are in need of a transformer input. They have some advantages, but they also have issues.
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Old 23rd July 2012, 03:02 PM   #7
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Thanks for your replies. I do believe that because I chose a different transformer (different specifications in the spice model for a nonlinear transformer) that I had these difficulties with noise. It seems that the DCR for the windings plays a big role in the values for the feedback resistors that give a reasonable S/N. I guess that makes sense because the secondary winding is part of the feedback network.
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Old 23rd July 2012, 03:32 PM   #8
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These types of circuits that depend on exact matching on device parameters are always very sensitive to small deviations.

I remember that Audio Precision uses a similar circuit for their analog generator output transformer (it's actually a Bruce Hofer patent). They have the resistor representing the DC wiring resistance actually inside the winding so it tracks the temperature. I am not 100% sure and too lazy to check it but I seem to remember that it is the primary DC resistance they were cancelling.

Edit: decided to check it anyway ;-)

This is what Bruce sayz:


jan didden
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Old 23rd July 2012, 03:56 PM   #9
jcx is online now jcx  United States
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I believe one way is to make the sense "R" a "hair pinned" bifilar winding of copper along side the compensated winding so that it tracks the temp, tempco of the transformer winding as closely as possible

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Old 23rd July 2012, 09:22 PM   #10
tvrgeek is offline tvrgeek  United States
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Quote:
Originally Posted by janneman View Post
These types of circuits that depend on exact matching on device parameters are always very sensitive to small deviations.

I remember that Audio Precision uses a similar circuit for their analog generator output transformer (it's actually a Bruce Hofer patent). They have the resistor representing the DC wiring resistance actually inside the winding so it tracks the temperature. I am not 100% sure and too lazy to check it but I seem to remember that it is the primary DC resistance they were cancelling.

Edit: decided to check it anyway ;-)

This is what Bruce sayz:


jan didden
Now that's a significant statement.
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