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In my design I am trying to use to the ADC Delsig to measure the battery capacity of an on-board battery in my system by converting the battery input voltage to a digital value using the ADC Delsig component in the PSoC and then calculating the remaining battery life based on the output.
The battery I am measuring has an input range to the PSoC of 2.2 - 2.8 VDC. My board has Vdda connected to +5VDC, so I configure the input range of the delsig to be single ended with an input range of "Vssa to Vdda." However, when I make these settings, the Vref of the delsig is automatically set to "Internal Vdda / 4" or 1.25 volts.
How can the ADC delsig accurately measure a voltage input range of Vssa to Vdda when the reference voltage for the ADC is set to Vdda / 4? Doesn't the reference voltage for the ADC need to be greater than the maximum input voltage?
Looking at the datasheet for the ADC Delsig, it specifies that when input range is set to Vssa to Vdda, the ADC delsig behavior is ratiometric. Is this saying that that both Vref and input values of the ADC are being divided by 4, so the output equation for the ADC will be:
d = ((Vin/4) / Vref) x 2^n where d is the n-bit output of the ADC and Vref = Vdda / 4?
I would just like to get a definitive answer because it's not 100% clear to me from the delsig datasheet.
Yes, as you have pointed out, the measurement made is ratiometric. The reference value used is selected based on the Input range selected in the Delsig ADC component(for VSSA - VDDA range) and voltage(VSSA) configuration is CYDWR file. When VSSA - VDDA input range is selected, the reference is configured to be VDDA/3 or VDDA/4(this depends on CYDWR setting). The input to be measured (input to the ADC) is attenuated to obtain input/4 or input/3. The attenuated input is then compared against the reference and digitised.