shifting bits

I'm trying to read the data from an ADC. I'm getting the error message error C193 '+=' bad operand type. If I use '=+' I don't get the error but my data is shifting. My result is a 0 or 1. It should be in the range from 0 to 255. CONVST is the convert command I'm sending to the ADC, SCLK is the clock pulse I'm sending to the ADC, DOUT is the data coming from the ADC, and ad_in is my variable. Any help would be appreciated.

Here it the part of the program that's giving me the trouble.


void read_ad(void)
{
data byte i;

SCLK = 1;
CONVST = 0;
ad_in = 0;
CONVST = 1;

i=8;

do
{
SCLK = 0;
ad_in<<=1;
SCLK = 1;
ad_in += DOUT;
}
while(--i);
}