Variable-sampling-rate sigma-delta modulator for instrumentation and measurement

We have designed, built, and tested a second-order, variable-sampling-rate, sigma-delta analog-to-digital converter (ADC) which operates over an input frequency range from 1 Hz to 8 kHz. Experimental results show that this circuit performs well and maintains the same signal-to-noise ratio over an input frequency range from 1 Hz to 1 kHz although it was built with individually packaged operational amplifiers and analog switches. For input frequencies above 1 kHz the circuit still performs well, but the signal-to-noise ratio varies with input frequency