Use constraints on fit coefficients to express physical requirements, such as requiring intensity peaks in a spectrum to have positive amplitude.
Use constraints to prevent fit coefficients from taking on undefined values, like requiring x0 in ln(x/x0) to be positive. A constraint like this can mean the difference between success and failure in curve fitting.
Igor Pro allows you to specify minimum and maximum values for each fit coefficient or for linear combinations of fit coefficients.
Simple constraints on the value of a single fit coefficient can be entered directly in the Curve Fitting dialog:
More complex constraints are expressed as arithmetic expressions in a string array which can be edited in a table:
Igor Pro 8
Igor XOP Toolkit
Igor NIDAQ Tools MX