Hello Paul,
Your two screenshots appear identical (both from SigmaStudio 3.9), so I'm not sure if you're referring to the coefficient calculations, their location in parameter memory, or a different problem. However, first-order HP filter info in version 3.10's help file does disagree with the Toolbox wiki at: http://wiki.analog.com/resources/tools-software/sigmastudio/toolbox/filters/general1storde
The wiki version is the correct one.
We can make a test by typing coefficients into the General Second-Order Filter, which works as a first-order filter when a2 = b2 = 0 --- its diagram reduces to this:
I was able to match responses with the stock First-Order HP filter by entering coefficients as shown below. Note that the formula for b0 should use e (2.718....), although the approximation 2.7 may work. Also the sign of a1 is always a bit confusing since the DSPs need to have their "a" coefficients negated. If the filter doesn't work, try flipping the sign(s) of a(n).
Best regards,
Bob