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SDADC full scale

Question asked by pacan.bogumil on May 17, 2013
Latest reply on May 23, 2013 by pacan.bogumil
Hi
We are using STM32F373 for the first time in our units and a problem with SDADC full scale occurred. We use it in a simple resistor measurement circuit:
SDADC.JPG
Circuit compares Rx with a know Rref resistor. Controller is supplied with 3V3, max. Vref (when Rx open) is 2.5V, all grounds together, external reference. Followers separate analog circuit from controller.
What we found is SDADC output value is max. (0xFFFF) even if channel voltage is still below reference voltage e.g. 2.475V on channel, 2.492V on Vref. So we can't use full scale, which is very important in this configuration. Lower voltages are ~ok.
What we tested up to now:
- different buffers (with input bias current <2nA typ.), single supply
- with/without SDADC calibration
- 2V5 -> 2V0
- different gains, channels, SDADCx, single channel/multiple channels (we use 8)
- voltage on all pins with multimeter (err 0.1%) and scopes - all 'pure' DC, no HF
The only setting which helped a little bit is gain=0.5. We can measure in full scale (channel from 0 to Vref) but then we loose one bit (15bit left, I think we could live without it) and values are 0...~33712 (> 32768) (this is not good).
My question is have You seen similar behaviour on Your boards or have You any tips what else can we check.
Second question is about SDADC calibration. Automatic sets bits SDADC CONF0R OFFSET0 to ~0x5A0. We tried set it manually, but it seems it's not a linear function of measurement offset (e.g. OFFSET0 = 0..1..2 gives values 0..2..1). What is exactly meaning of CONFOR0 OFFSET bits?

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