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Message: [QUOTE="bcarso, post: 242828, member: 1113"] Chris, what do you have surrounding the "JLM Hybrid" circuit? In particular, what is the d.c. reference for the (noninverting) input? Is the circuit just the four-resistor diff amp configuration? If so what are the resistances? EDIT: looking at the TI spec on the 2604, I see a typical bias current of 100pA and offset current of +/- 4 pA. Although no maxima are given, to have one amp produce an error of 2V at any conceivable circuit output is a stretch. The loop [i]is[/i] closed when the servo is applied even in the "naked" JLM schematic, although there has to be something to reference the n.i. input to common*. Clearly you have it closed by additional components since it behaves with no servo at all. The 47k however, and the method of implementing the servo action, could well be inappropriate for a given overall circuit. Note for example that, in a four-resistor diff amp, at midband frequencies the 47k acts like a resistor to common, and unless accounted for somewhere else alters the circuit differential and common-mode gains. *at low frequencies though, only, hence will have huge gains above without additional feedback to limit. [/QUOTE]

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