PSoC5 PGA output does not go to neg rail | Cypress Semiconductor
PSoC5 PGA output does not go to neg rail
Hi, I am trying to develop a strain gauge amp for a custom load cell in an architecture project. I'm just reading the ADC, small signal amplification, offset and gain correction papers. As a test, I tried to measure the PGA output with G=1 from bench power supply input. Attached is a pencil sketch of the simple measuring circuit: The dev kit is the PSoC5 CY8CKIT-050. The PGA input pin is set to P0 and connected to the positive terminal of the power supply. The PGA output is set to P0 and connected to the HI terminal of the multimeter. The LO terminal of the multimeter and the minus terminal of the power supply are both clipped to the VSSA ground terminal of the dev kit board.
If some 50 mV are input, the output follows happily down to something like 24mV where it remains. I naively would have expected it to go down all the way to the negative rail. I didn't measure the offset but only read the power supply display, since it really doesn't matter if I can't go even near the full scale ouput of a load cell bridge (which at 2mV/Vex at 2.5Vex would ouput at most 5mV). The data sheet of the PGA module says about the output swing: Voh min = VDDA - 0.15V and Vol max = VSSA + 0.15V which is (assuming that Voh, ol stands for V output high and low) the opposite of what I would have expected. The technical reference manual says: RR to within 50 mV of Vss or Vdda, which is not RR at all.
So, what am I getting wrong here? I makes no sense to check offset and gain calibration if the most basic thing doesn't behave as expected. Any clue and nudge in the right direction would be highly appreciated. A