ADS1018-Q1: ADS1018-Q1

Hello,

Let me copy a response over from a similarly designed device, the ADS1015.

First, the operating analog input range is GND to VDD. The absolute maximum for the inputs is GND-0.3V to VDD+0.3V. If your inputs go beyond the absolute maximum, the you may damage the device. It’s likely that the ADC will continue to work as the inputs go beyond the rails a little, but again you risk damage. This is always important to keep in mind.

I'm not completely familiar with internal reference design but the reference value is a scaled from the bandgap. My guess is that the reference is always 1.024V. I'll explain how the ADC works, and you can see how the input or reference gets scaled to get the different full-scale values.

For any ADC, the input voltage is compared to the reference and you get an output data based on the comparison. Let’s say the reference is 1.024V, the input sampling capacitance is 1pF, and the reference sampling capacitance is 1pF. If the input is 1.024V, then the ADC will read out positive full scale. This would be what we read for the ±1.024V FSR range.

To get different FSR values, I can change the scaling of the input capacitance. If I want an FSR ±0.512V, I can just double my input capacitance to 2pF and keep the reference sampling at 1pF. Here when the input is 0.512V, I read the full scale because now I have twice as much charge in the input sampling. For the ADS1015, The smallest input range is ±0.256V, and the input sampling cap is 4x of the reference sampling cap.

To get larger values for the input range, I could reduce my input sampling or I could also increase the reference sampling cap. If I want to make the input range ±2.048V, I would keep the reference at 1.024V, but double my reference capacitance to 2pF. To get the maximum FSR setting of ±6.144, I would use 6x the reference capacitance compared to the input capacitance. By changing either the input capacitance or the reference capacitance, I can get all settings for the FSR.

This is why I can set the FSR to ±4.096V and run the device down below 3V. The internal reference never is never larger than the supply. Note that if the supply is 3V and the and the FSR is ±4.096V, the maximum input should be about 3V and the maximum output code is 5DCh. Again, the supply does not limit the internal reference, and the internal reference doesn't change regardless of the FSR.