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Model 6517B Electrometer Reference Manual
Section 2: Getting started
Range messages
The following display messages may occur when making measurements:
OVERFLOW: This message is displayed when the integrated (average) input signal level (voltage,
current, or charge) exceeds 105 % of full scale for the selected measurement range. For example, on
the 20 nA measurement range, the OVERFLOW message occurs when the integrated input level
exceeds 21 nA.
An OVERFLOW condition can be resolved by selecting a higher measurement range, using AUTO
range, or reducing the magnitude of the input signal.
The OVERFLOW message does NOT occur during resistance or resistivity measurements.
UNDERFLOW: This condition is similar to OVERFLOW but pertains to resistance and resistivity
measurements. An ohms measurement is performed by sourcing voltage and measuring current. An
ohms measurement that is too low causes the current to exceed full scale. The message
UNDERFLOW is used to indicate that the measured resistance or resistivity is lower than the lower
limit of the selected range.
The UNDERFLOW condition can usually be resolved by selecting a lower ohms range or by using
AUTO range.
OUT OF LIMITS: This message indicates that a momentary or transient out-of-range condition
appeared at the input, even though the integrated (or average) signal was within the full scale range
of the analog to digital converter (A/D). It usually indicates that there is too much noise on the input
signal for a valid measurement or that the signal exceeds 120 % of the range.
Generally, the OUT OF LIMITS condition can be eliminated by better shielding of the signal source or
by using other noise reduction methods. Another solution is to select the next higher range (or lower
R range) to keep the transients less than full scale.
The 2 nA, 20 nA, 200 nA ranges, and the resistance (R) measurements that use these current ranges
are particularly susceptible to this condition because of the combination of speed and sensitivity.
A sine wave signal riding on a DC bias level is used to demonstrate an OUT OF LIMIT reading. The
"Input signal" figure below shows a sine wave riding on a 20 nA bias level. If this signal is measured
on the 200 nA range at normal speed, it would read 20 nA (which is the DC average). If however, you
use the 20 nA range, the positive peaks of the sine wave is clipped as shown in the "Measurement on
20 nA range" figure. Clipping occurs at 110 % of full range (22 nA on the 20 nA range). Because of
clipping, the measurement of the input signal is significantly less than 20 nA.
To avoid these bad readings, the Model 6517B displays the OUT OF LIMITS message instead of the
inaccurate reading.
Note that the positive peaks of the input signal (which exceed full scale) does not cause an
OVERFLOW condition on the 20 nA range since the average reading over the 16.67 ms integration
period is less than full scale.
The A/D hardware limit detection circuit can be disabled, which in turn disables the OUT OF LIMITS
message. However, the presence of OUT OF LIMIT readings may result in measurements that are
slightly, moderately, or completely inaccurate. When both OVERFLOW and OUT OF LIMITS
conditions occur, the OUT OF LIMIT message is displayed.
6517B-901-01 Rev. C / August 2015
2-9
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