1/ If you take 2 batteries of 9 volt and connect the plus from one battery to the minus from the other. You have now 18 Volt on the not connected contacts.
But if you take the middle as a reference point, then you have +9 Volt
on the battery where the - is connected to the + of the other battery (common point), and -9volt where the + is connected to the common point. It is easy to understand, because the - and + signs are on the battery.
Thats why you have a negative voltage compared to the reference point.
Similar you can build a dual powersupply with a common point called
ground.
Code:
- < -battery+ > < -battery+ > + = 18 Volt power supply
or - 9 Volt ground + 9 Volt
2/ DS1804
From the DS1804 datasheet:
H - High-Terminal Potentiometer. This is the high terminal of the potentiometer. It is not required that
this terminal be connected to a potential greater than the L-terminal. Voltage applied to the H-terminal
can not exceed the power-supply voltage, VCC, or go below ground.
L - Low-Terminal Potentiometer. This is the low terminal of the potentiometer. It is not required that this
terminal be connected to a potential less than the H-terminal. Voltage applied to the L-terminal cannot
exceed the power-supply voltage, VCC, or go below ground.
So this digital pot DS1804 CAN NOT BE USED in your application.
( on -8 Volt and + 8 Volt )