MT-075 TUTORIAL

MT-075 TUTORIAL

Differential Drivers for High Speed ADCs Overview
DIFFERENTIAL DRIVER BASICS
Many high performance ADCs are now being designed with differential inputs. A fully differential ADC design offers the advantages of good common-mode rejection, reduction in second-order distortion products, and simplified dc trim algorithms. Although they can be driven single-ended, a fully differential driver usually optimizes overall performance.
The reduction in second-order distortion products inherent in differential designs can be illustrated as follows. The distortion products are modeled by expressing the transfer functions of the circuit as a power series.
Taking a generic expansion of the outputs and assuming matched amplifiers, we get:
VOUT+ = k1(VIN) + k2(VIN)2 + k3(VIN)3 + . . . Eq. 1
VOUT– = k1(–VIN)+ k2(–VIN)2 + k3(–VIN)3 + . . . Eq. 2
Taking the differential output:
VOUT+ – VOUT– = 2k1(VIN) + 2k3(VIN)3 + . . . Eq. 3
where k1, k2 and k3 are constants.
The quadratic terms gives rise to second-order harmonic distortion, the cubic terms gives rise to third-order harmonic distortion, and so on. In a fully-differential amplifier, the odd-order terms retain their polarity, while the even-order terms are always positive. When the differential is taken, the even order terms cancel as shown in Eq. 3. The third-order terms are not affected.