HP 3468A - Page 30

=
g-EMENTS (Cont'd)
ESISTANCE ME,
MELO
P-L
.
iL
| Command
F7
Selects
the
Extended
Ohms
function
(also
H7)
{2- wire mode)
Other Considerations
1. There are two situations in which the 3468A may indicate a negative
(minus) resistance. One is when small negative voltages exist on the circuit under
test. The second is where the inputs to the 4-WIRE SENSE and the input leads are
inverted from each other in the 4-wire ohms function.
2. Always use the shortest possible test leads. This is important on the lower
ranges to minimize errors due to lead resistance (2-wire ohms mode), and on the
higher ranges to minimize noise pick up.
3. Additional settling time may be required when using the higher ohms ranges
under program control. This is important if there is more than F shunt
capacitance connected externally as might be the case if you were using the
3468A with a scanner. Theoretically, the settling time necessary is:
—RC*In(P/100)
Where R is the resistance being measured, C is equal to $56pr plus any external
Capacitance, and P is the desired percentage of step accuracy. For example, lets
say we want to measure a 3.0 Mohm resistor through a scanner with 1200pF
capacitance (High-to-Lo terminals). If a short was previously applied (short to 3.0
Mohms = step) and a .001% reading is desired, the settling time necessary is:
o
~ (3108) +(1 200 + $88} «(10-12)«{In(.001/100)) = £3mns
Singe on the 3 Mohm range there is an internal delay of 20mS, an additional delay
of to a8 mS should be allowed. The 30 Mohm range has an internal delay of
200 mS.
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