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
www.ti.com