This is not a simple chip to use. These parts of the data sheet may help you understand what is happening. It is internally an analog device and needs clean power. I would highly recommend you download the Allegro data sheet, it has a lot of information that will help you. Below is a short section of the data sheet, The first paragraph should give you a hint that the output voltage will be about 2.5V with a 5V unipolar power supply.
"Quiescent output voltage (VIOUT(Q)). The output of the device
when the primary current is zero. For a unipolar supply voltage,
it nominally remains at VCC⁄ 2. Thus, VCC = 5 V translates into
VIOUT(Q) = 2.5 V. Variation in VIOUT(Q) can be attributed to the
resolution of the Allegro linear IC quiescent voltage trim and
Electrical offset voltage (VOE). The deviation of the device output
from its ideal quiescent value of VCC / 2 due to nonmagnetic
causes. To convert this voltage to amperes, divide by the device
Accuracy (ETOT). The accuracy represents the maximum deviation
of the actual output from its ideal value. This is also known
as the total output error. The accuracy is illustrated graphically in
the output voltage versus current chart at right."
I realize this does not answer your question but it does give you a good starting point to resolve your problem. Hopefully you or a buddy has some analog experience, that will help a lot. Oh Yes clean power supplies are imperative for this to work.