Hi TI,
We are building a pressure sensor with PGA305 and had a few questions about the coefficients.
These are the steps we are taking, but not arriving at the right answer. Can you help us?
1. We start by calibrating PGA305 using the Guided Calibration in the PGA305 EVM GUI software. We have connected a dc power supply to the INP+ and INP- pins to simulate the bridge. The P_Gain is set as 10.
These are the parameters we use to calibrate (for testing and learning purpose):
0.100v = 0x331A36 = 3349046 should equal 4mA = 0x7F2 = 2034 on DAC.
0.150v = 0x4C4528 = 4998440 should equal 12mA = 0x17E5 = 6117 on DAC.
0.200v = 0x6566DE = 6645470 should equal 20mA = 0x27D9 = 10201.
After we finish the guided calibration in PGA305 GUI software, these are the calculated values:
- h0 = 0x188221 = 1606177
- g0 = 0x29805B = 2719835
- n0 = 0xFE89C1 = 16681409
- TADC Gain = 0x1
- TADC Offset = 0x0
- PADC Gain = 0x1
- PADC Offset = 0xE6F043 = 15134787 / 2^14 = 923,7541
- h1, h2, h3, g1, g2, g3, n1, n2, n3, m0, m1, m2, m3 = 0x0
When we set the PGA305 to compensation mode, then measure the current, it looks correct.
At 0.100v we get ~4mA and at 0.200v we get ~20mA.
2. Now we want to understand it, so we insert these coefficients in the 2nd grade polynomial function:
- p(x) = n0*x^2 + g0*x + h0, we get:
- p(x) = 16681409x^2 + 2719835x + 1606177
Then we try to use it with the Padc values:
- p(3349046) != 2034
- p(4998440) != 6117
- p(6645470) != 10201
In the gui user manual, its mentioned that the values are normalized at 2^14. We even tried:
- p(3349046 / 2^14) != 2034
- p(4998440 / 2^14) != 6117
- p(6645470 / 2^14) != 10201
And:
- h0 = h0 / 2^14
- g0 = g0 / 2^14
- n0 = n0 / 2^14
- DAC * 2^14
- DAC / 2^14
- x = x +- PADC Offset
And no luck, the result doesn't match the parameter values.
We want to understand this simple 2nd order polynomial first, before we look at the 3nd order polynomial and all its complications.
Are we missing some steps, or maybe not formatting the adc or dac data correctly?
Any help and guidance will be appreciated.
