Software

Question

Hi, 
 I cannot get the RAM zero pressure offset override function to work on the MAX40109 EV kit. I have followed the Sensor Offset Calibration sequence detailed on page 53 of the Datasheet (19-101790,Rev2). The following are settings applied to the MAX40109 EVKit GUI (v1.0.97): 
 GUI settings recorded below: 
 
 Notable settings applied: 
 
 Pressure Cal: Bypassed (Raw Mode) 
 Trim Resistor: Connected 
 Current Source: Connected 
 PGA Mode: Bypass Mode 
 RAM Override Value: 32767 
 ZPO Override: RAM Override 
 
 No matter what my value on RAM Override value (0, 20, 100, 1000, 16383, 32767), it never changes the result. None of the above settings, with the exception of "PGA Mode:Bypass Mode" and "Pressure Cal: Bypassed Mode", seem to make any difference to the result. I have also tried setting the Bridge Drive mode to current mode (as opposed to voltage mode), and haven't been able to make any offset changes. 
 Can you please assist me with this - where I am going wrong? 
 Regards, 
 Chris

chris100 · Answer

After some trial-and-error, I've answered my own question: the polynomial needs to be created before you can override the offset via RAM. The offset affects uncalibrated pressure, so it isn't obvious that the polynomial needs to be changed first, but it does. 
 You can initialise the polynomial by writing coefficients using the "initialise" , followed by "write" functions. These functions are temporary and don't use up the limited burn cycles according to the MAX40109 eval system datasheet (Rev1 pg6). Write "1" to k1 and g0 coefficients for a simple pass-through 'uncalibrated = calibrated' validation. They will reset on power cycle unless you use the "burn " function. 
 Regards, 
 Chris