Datasheet

32142D–06/2013

ATUC64/128/256L3/4U

Problems with QMatrix acquisition of small sense capacitor voltages can be solved by connect-

ing the negative reference pin (ACREFN) to a voltage divider that produces a small positive

voltage (20 mV, typically) to cancel any negative input offset voltage. With a 3.3V supply, recom-

mended values for the voltage divider are Ra (resistor from positive supply to ACREFN) of 8200

ohm and Rb (resistor from ACREFN to ground) of 50 ohm. These recommended values will pro-

duce 20 mV on the ACREFN pin, which should generally be enough to compensate for the

worst-case negative input offset of the analog comparators.

Unfortunately, such a voltage divider constantly draws a small current from the power supply,

reducing battery life in portable applications. In order to prevent this constant power drain, the

CAT module provides a voltage divider enable pin (VDIVEN) that can be used for driving the

voltage divider. The VDIVEN pin provides power to the voltage divider only when the compara-

tors are actually performing QMatrix comparisons. When the comparators are inactive, the

VDIVEN output is zero. This minimizes the power consumed by the voltage divider.