Bandpass Filter and Oscillator ICs with THD < -140dBc at 10Vppd for Testing High-Resolution ADCs

Abstract

The growing demand for high-resolution (18 to 20bit) precision ADCs has increased the need for very low THD <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(&lt; -140\text{dBc})$</tex> testing hardware that can simultaneously characterize multiple ADCs cost-efficiently in a small form factor. One of the options is to use an active bandpass filter (BPF) to attenuate the harmonics of a medium-accuracy <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\text{THD}\sim$</tex> −80dBc) sinusoid from a DAC or a bench-top generator. Another option is to generate a -140dBc THD sinusoid using an analog oscillator consisting of a BPF in a positive feedback loop with amplitude stabilization. In either case, the filter's distortion must be kept below -140dBc at a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$10\mathrm{V}_{\text{ppd}}$</tex> output swing, which is the typical full scale of ADCs operating with a 5V supply. Among active filters, the second-order Tow-Thomas active-RC topology with opamp-based integrators provides the lowest distortion levels [1]. Negative feedback with a high loop gain suppresses the components' distortion in the forward path, but increasing the loop gain does not completely suppress the distortion. This is because (a) distortion contributed by passive components in the feedback loop is not suppressed by the loop gain or (b) further increase in the global loop gain is infeasible because it compromises stability. The distortion-generating components must be identified, and their distortion contribution suppressed using local negative feedback or cancelled by subtracting the distortion component. In this work, we present techniques to mitigate distortion arising from the two limitations and demonstrate a filter with THD <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$&lt; -140\text{dBc}$</tex> for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$10\mathrm{V}_{\text{ppd}}$</tex> inputs.