This is from the innovate web site forum.
Ground offsets 101
Hi,
A lot of people have problems understanding correct grounding and how it effects AFR to voltage conversion of the analog outs of an LC-1.
Lets first look at the basics of what is going on there electrically:
Think of Voltage as the pressure in a line (measured in Volts, symbol V).
Different to pressure though, there is no “absolute” Voltage. Voltage can only be measured as “pressure” difference between 2 points.
Current is the amount of flow of electrons, like gallons per minute in a water line. Current is measured in Amperes or amps (symbol I).
When water flows through a line, there’s a pressure loss along the line because the of the friction of water against the pipe walls and other restriction. Same goes for electrical current. Everything that conducts electricity has a resistance to that current flow and therefore causes a Voltage (pressure) loss along that conductor. The resistance is measured in Ohms (symbol R).
The Voltage (pressure loss) along a conductor follows Ohms law:
V = R * I (Voltage loss is resistance times current).
Different to a home plumbing system, the electrical current runs closed loop. What goes out of the + side must come back on the ground (-) side.
This “return” current is what creates ground offsets.
When the engine is running, the alternator acts as “pump” for the electrons. The battery is then just an electrical power consumer, like everything else. The “head” side of the alternator is its B+ terminal. The “suction” side of the alternator is the body of the alternator itself, that usually has good contact with the engine block.
All the current for all electrical systems in the car has to flow through the ground strap of the engine. If for example the total electrical current load of the car is 10A, these 10A have to flow through the frame and then through the ground strap back to the alternator/engine block.
If there is electrical resistance in that path, as there always is, a voltage loss is created along that path. For example if the path resistance is 0.1 Ohms, at 10A a voltage loss of 1 Volt is created. This means that the frame sits at +1V when measured against the engine block (remember, voltages can only be measured as difference between 2 points).
Now assume the ECU is grounded to the engine block, but your LC-1s system/analog out ground is grounded to the frame. The LC-1 measures its output voltage referenced to its “ground”, as that is the only zero reference it has. The ECU measures its O2 input voltage against a different zero reference, namely the engine block. As the LC-1’s zero reference sits 1V higher than the ECU reference, the ECU will read 2V on it’s input. Of course, you could compensate by programming the LC-1, but that is not a very good solution because:
a) The LC-1 cannot output what it regards as “negative” voltages. It references everything to its ground, and therefore 0V is at low as it can go. 0 Volt would even then be read as 1V by the ECU/datalogger/display.
b) The 1V ground offset as described in the above example depends on the current in the return path. With a 0.1 Ohm resistance in the return path and 10A electrical current you will have 1V. With 20A current you would have 2V. With 2A electrical current you would have an offset of only 0.2V.
c) The current load in a car, and therefore in the return path, can change very dramatically from 2-3 Amps to 50-100 Amps in milliseconds due to ignition systems, fans, a/c clutch and so on. These changing currents will create corresponding changing ground offsets which then is often wrongly attributed to "noisy analog outs". This "noise" can often be seen when displays intended for NBO2 sensors are used. On LED instruments, multiple LEDs will light up at the same time.
A better way is to ground the LC-1 to the same reference point where the receiver of the data, wether its ECU, datalogger or display instrument is grounded. This way all devices are “on the same page” and ground differentials somewhere else have no effect.
This is also the reason the LC-1 has different grounds for heater (large current) and system/analog out ground. The potential ground offsets created by the heater current when they are connected together can create its own problems. By connecting the heater ground to the same grounding metal as the rest you will minimize that effect. Therefore the best way is to ground the LC-1’s heater to the same area as the other grounds, but on a seperate bolt/lug. The contact points of bolts and lugs can account for 75-80% of the resistance of the connection and can change dramatically when corrosion sets in.
