Sorry to leave you hanging here pHaestus.
Nice work Groth.
Do you know of any good schematic capture shareware? I don't have any good way of creating schematics at home.
I hope you don't mind some suggestions...
Maybe you've thought of some of these things already, but I thought they were worth mentioning just in case.
Quote:
Originally posted by Groth
I have ten resistors from the same company (Ohmite), but I've chosen the 10 series for their higher precision.
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I'd go for minimal TCR. Actual resistance can be measured and compensated for in the following gain stage. Drift due to variations in ambient and airflow can be a real pain.
I don't know what value of shunt resistor you were going for, but keep in mind, the larger the shunt resistor, the more of your Vcore adjustment range you are giving up. On the other hand larger shunt resistors will give better signal to noise ratio.
Quote:
Originally posted by Groth
A length of twisted pair was attached to the shunt and led to the amplifier stage. I used a TI LF412 dual JFET-input op-amp, half as a unity gain difference amp, half as an inverting amp with a gain of 21. For the Abit board's turn, I'll add another LF412, so that I have 3 difference amps, one for each inductor, and an inverting summing amp with a gain of 75-80 or 110-120.
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I'd suggest adding a film 0.1 uF cap in parallel with the shunt resistors. This will bypass a lot of the high frequency transients that come through the capacitance of the switching inductor. These transients might screw up the the amplifier output somewhat, unless the amplifier is fast enough to track them.
I'd suggest using
OPA227 Op-Amps. (OPA2227 or OPA4227 for duals or quads) With your present setup (20 mOhm shunt, 9A max current) the input offset voltage of the LF412 can cause a +/-1.7% error in the first difference amp alone. The main advantage the LF412 has is low bias current, and because of the extremely low output impedance of the shunt, bias current is a nonissue. The OPA227 has substantially better: input offset voltage, input voltage noise,
common mode rejection ratio, and unity gain bandwidth.
I can get these at my employer's cost. If you don't mind working with surface mount packages, (SO-8) I'd be happy to send you some. (I might be able to get the dip package in singles.)
I've got more to bring up, but it's late...