How is IR measured?
When the AT makes IR measurements in either front panel measure mode or when the PC Editor's 'measure' button is pressed during program development, the AT makes several measurements at which time it is automatically selecting the optimum measurement range.
The AT returns the result that is taken using the best of several possible ranges. That is, using the range that most closely fits the measurement. When the test terminals are open circuit the AT automatically selects the largest range. A typical compensated measure mode open circuit measurement will be in excess of 100GOhm.
In normal program mode (e.g. when you download a program and press run) the AT is NOT auto ranging for IR measurements. This is done so as to produce the fastest possible test speed.
To avoid measurement accuracy being compromised, the AT does not simply choose the maximum range. It chooses an optimum range based on the maximum resistance limit that you have entered into the Editor. For example, when programming a 25Mohm limit the AT consistently read 125MOhm for an open circuit. For a 50MOhm limit, the AT will read 250MOhm for an open circuit.
In summary, during measure mode the AT is auto-ranging. In program mode the AT uses a fixed range in order to provide the most accurate result in the shortest possible time.
This has been done due to the fact that at very high resistance values (into the G Ohms) the AT measurements will become unstable because it's measuring extremely small currents.
To stop this instability the AT will not allow a measurement to read above 5 times the maximum IR limit. This 'blanking level' therefore changes with the maximum value until it reaches 100GOhms where it will blank independently of the maximum limit.