Hmm something must be wrong then, because according to the adafruit library, the default mode is single shot, and my input is only to A0 and GND, so I am not using the differential, and my input range measured with a DVM is +0.3 to +1.4v, but I am getting a -6000 to +6000 output.
Maybe I misunderstand you.
I'm not sure what the adafruit library default configuration is. Why don't you just ground the A1 pin and see if your measurement values on A0 change or not. The datasheet is the best source for working with this chip series. This one covers the ADS1113/4/5 which vary in number of input channels supported and a few minor differences. They all handle the single ended/differential input the same way.
If you are getting negative digital values then something is wrong in either as the library code is being set up or in how your sketch is interpreting the results. Single ended input should only be able to return digital codes from 0x0000 to 0x7FFF (just 15 bits of positive range numbers) only in differential mode can the digital results vary the full 16 bit range of 0x0000 to 0xffff with 0x8000 to 0xffff being the negative portion of the range when using ANin0 and ANin1 in differential mode.