Gary, you do not list design goals, ie. I want 16 bit absolute accuracy ove T & V,
that guides you as to whether you can achieve it.
The errors you have to deal with are (Sensor, PSOC IA, PSOC A/D) :
2) Non Linearity
5) Gain errors
Example, lets say you want 16 bit system resolution, and accuracy of 1 lsb. So
in PSOC with ref = Vbg +/- Vbg = 1.3 nom, then 1 lsb = 2.6 V/ 2**16 = ~ 40 uV.
The drift in just the ref, 40 mV over T & V, in lsbs = 40 mV / 40 uV = 1000 lsbs. This does
not include all other errors.
The IA gain error, at G = 1, is .2 % typical, that corresponds to a 9 bit system in absolute
accuracy. Or ~ 128 lsbs of error in a 16 bit system.
Typical Vdd noise in a PSOC designs is 200 - 400 mV, and IA CMRR typical is 60 db, so
that is 400 uV input referred, not counting frequency rolloff in IA CMRR, and that rep-
resents 10 lsbs.
So without doing a full analysis over T & V, and not having worst case numbers on many
specs, IA, A/D, already we are at 1138 lsbs of error !
So you start with your goal, that in turn establishes how many lsbs of error, and then you
look at each component in system to see how the components contribute.
The only way to achive very high absolute accuracy is to do a production cal on system,
with PSOC running a cal routine talking to high precision instruments that force hi precision
V and T to PSOC and establishing a table of errors, which then can be converted into a power
equation or least squares error fit equation to be used by PSOC for normal application correction
of sensor readings.
The only other partial fix is possibly use an SCBLOCK, under API control force inputs to a known
voltage or ref, and capture offset, and subtract out of measured. But this is only a partial work-
around, is does not compensate for Vref drift, or a lot of other issues like long term component drift.
If you do not need absolute accuracy, just high non linearity, than NL errors and noise are your
Or live with the errors and non worst case specs PSOC 1 is lacking. PSOC 3/5 improves on this,
but still a work in progress.
Last edited by danadak
on Wed Mar 21, 2012 3:46 am, edited 1 time in total.
Field Application Engineer
KB1RHB Mostly listen