The MAX14921 samples the voltage on all the cells at once. Once sampled, you can go in and measure the individual voltages via an ADC, one at a time. There is an "AOUT Voltage Droop Time" of 1mV/ms specified in the Electrical Characteristics table.
- Does this apply to the voltages (caps) that aren't currently being read by the ADC?
- If not, how bad is the droop on these capacitors? Is this a concern for customers using a slow ADC (i.e. Sigma Delta).
Regarding the AOUT Voltage Droop Time in the EC table.
- Yes, this occurs for all the cell voltages on the stack. This is because, when set into sample mode, the MAX14921 captures the cell voltages across the capacitors, disconnects the capacitors from the cells, and, essentially, connects a switch to each cell for the reading. Internal leakage currents will slowly discharge the capacitor, even when it is not being read. The leakage current has a maximum value of +/-1uA. In short, if the customer samples the cell voltages and then waits a long time to read the voltages, there is a possibility of accuracy errors, but if they sample voltage and read through the cells relatively quickly, then there shouldn’t be any problems.
- The leakage on the capacitor increases as the cell voltage increases, so we recommend that the customer reads the highest voltage cell first, as that capacitor will discharge faster than capacitors for lower-voltage cells.