The source voltage is actually not important as long as it's not high enough to cause heating of the resistances inside.
OK, a load cell is a differential Whetstone resistance bridge. There are two output voltages, and only the difference between them matters. The signal is VERY small, only millivolts for full scale. The signal is ratiometric to Vdd, and voltage references for the ADC should NOT be used- Vdd must be the reference.
In short, for a number of reasons, the PIC's (and I assume 89c51) ADC is generally unsuitable unless your precision and offset requirements are very loose.
There are two types of load cells- the 4 wire type has a pair for Vdd/GND (doesn't matter which one you connect to which) connection and 2 output wires. The 3 wire type has two measuring resistances inside and you must add external pullup (or pulldown) resistors which MUST be high precision, stable resistances if you want to avoid offset errors or even saturation of the scale's ADC. You can tie the common wire to Vdd or GND, it doesn't matter, just use the pull resistors to the opposite rail.
It's difficult to build a true differential amp with good accuracy out of discrete components because it's VERY sensitive to offset errors. Best thing in the world is a differential ADC like the LTC2480, which is cheap and you can get free samples.
The load cells
instruite refers to are complete scale units with an amplifier built-in. No load cell sensor itself has an output like that, unfortunately. I'm assuming you just have a sensor you took out of a scale or something?
https://www.electro-tech-online.com/members/instruite.html