A Mixed Signal (Analog-Digital) Integrator Design

Presented is a design of a mixed signal integrator, consisting of a traditional analog integrator (op-amp with capacitor in feedback) augmented with comparators, logic, counter registers, and a precharged capacitor that can be switched into, and out of, the input of the analog integrator. This device integrates with analog, but stores results in analog and digital. Whenever an analog integrator voltage equals or exceeds a threshold voltage, an amount of charge equivalent to the threshold voltage is cleared from the feedback capacitor of the analog integrator, and the counter register is incremented or decremented accordingly. With the threshold voltage representing one in a counting system, these actions convert the integrator contents to ±1. At any instant, the counter register holds the integer portion of the integral, and the analog integrator holds any fractional portion, permitting the integral to be internally stored as a real number. The mixed signal integrator accumulates integral beyond supply rails without op-amp saturation; has analog integrator bandwidth that extends into the limited gain low frequency region; avoids drift due to low pass filter effects and unbalanced input biases; and exhibits no sampled data or discrete system effects. Tests on a PCB prototype verified the design.