Sigma delta analog to digital converter with internal synchronous demodulation

1. A sigma-delta (ΣΔ) analog to digital converter with internal synchronous demodulation responsive to a sample clock, reference clock and conversion clock comprising:

a sample switching circuit responsive to an AC input to sample the AC input at the sample clock rate; said sample switching circuit including first and second input switches responsive to the reference clock for selectively, alternately sampling the positive and the negative AC input at said reference clock rate; and

an inverter circuit responsive to said reference clock and said sample clock for reversing the polarity of signals from said sample clock in synchronism with said reference clock to reverse the sense of the input switches and synchronously demodulating the AC input within the converter.

2. The converter of claim 1 in which said inverter includes an exclusive OR gate whose inputs are said reference clock and said sample clock.

3. The converter of claim 1 in which said sample clock has a higher rate than said reference clock and said reference clock has a higher rate than said conversion clock.

4. The converter of claim 3 in which said conversion clock rate is a whole number of half cycles of said reference clock.