Electronic Design

Charge Pump Helps 50-MHz Op Amp Crush Harmonic Distortion

Input crossover distortion has long been the bane of rail-to-rail op amps. The input stage for most R-R amps consists of p-and n-channel differential pairs (Fig. 1a). The result is that the stage's offset voltage varies with the common-mode input voltage. The nonlinearity as the input signal passes through the crossover point limits the amplifier's total harmonic distortion (THD) (Fig. 1b).

The idea of using a single differential pair whose supply voltage is boosted by a charge pump isn't new. (For details, see www.imec.be/esscirc/papers-97/75.pdf for a paper by Philips Research Laboratories for the 1997 European Solid-State Device Research Conference.) But it had never been implemented in a commercial product until Texas Instruments' OPA365 (Fig. 2a). The result is 0.0006% THD + noise performance (Fig. 2b).

The 50-MHz OPA365 charge pump boosts the positive supply by about 2 V to supply the input tail current of a pMOS differential pair. The charge pump's self-contained-regulation loop contributes to the device's 4.5-nV/√Hz noise performance. Common-mode rejection ratio (CMRR) values are a guaranteed 100 dB minimum and 120 dB typical over the entire input range (100 mV beyond the supply rails).

The OPA365 comes in an SOT23-5 package. It costs $0.95 in 1000-piece lots. An SO-8 package and the dual-version OPA2365 in DFN-8 and SO-8 packages will be available in the fall.

Texas Instruments

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