MSP430 SD16 and unipolar input

Anyone use this peripheral on a regular basis? I'm using an
MSP430F2013 and trying to work out if it's possible to reclaim the -ve
input pin for something useful. It seems to me that if I set P1SEL for
that pin to GPIO mode, and set the SD16 into unipolar mode, I should
be able to use the pin as an I/O, but it doesn't seem to be working
properly. I'm not sure if that's because what I'm trying is
impossible, or simply that I'm not setting up SD16 properly.

My code is real simple:

P1SEL |= 0x09; // P1.0 = A0+, P1.3 = VREF

SD16AE = SD16AE0; // enable analog input on A0

SD16INCTL0 = SD16INTDLY_0 | SD16GAIN_1 | SD16INCH_0;
SD16CTL = SD16XDIV_1 | SD16DIV_1 | SD16SSEL_0 | SD16REFON;
SD16CCTL0 = SD16UNI | SD16SNGL | SD16OSR_256 | SD16DF | SD16SC;

// wait for conversion
while (!SD16CCTL0 & SD16IFG) { }

sample = SD16MEM0;


Any advice?

(PS: Generic hatred towards TI's documentation, it's so darn difficult
to cross-reference. And the SD16 peripheral is particularly hard to
understand!).