Low-Current/High-Resistance Measuring Devices: 6517, 6517A
Question: What does OUT OF LIMIT message on the display of 6517A mean?
The presence of this message indicates that there is too much noise riding on the input signal for a valid measurement. The OUT OF LIMIT message occurs when out of range transients appear at the input. The 2nA, 20nA and 200nA ranges are particularly susceptible to this condition. The integrated or average signal is within the selected full scale range.
A sine wave noise signal riding on a 20nA DC bias level demonstrates a condition for the OUT OF LIMIT reading. If the signal is measured on the 200nA range, it would read 20nA, which is the average DC value. But if downranged to the 20nA range, the positive peaks of the sine wave would be clipped. Because of the clipping, the average signal will be significantly less than 20nA. To avoid these inaccurate readings, the Model 6517A displays the OUT OF LIMIT message (and transmitts 10e37 over the bus) instead of the inaccurate reading.
The out of limit message implies the signal should be better shielded to reduce the noise. If proper shielding techniques are not possible, the A/D hardware limit detection circuit can be disabled. OUT OF LIMIT feature can be disabled from the front panel Menu / General / A/D controls / OOL control / off. However, with this configuration, the integrity of the measurement is compromised.