Measurementconsiderations - Keithley 485 Instruction Manual

verted analog output voltage is developed by the operational
amplifier. The feedback (Rangel resistors are selected such
that voltage for an on-scale reading on the 2nA range will be
between zero and k200mV. The output voltage for the rest
of the ranges will be between zero and f2V. The relationship
between the analog output signal and the digital display is as
follows:
2nA Range: 10000 counts = -1OOmV. or -1OFVIcount
20nA-2mA Ranges: 10000 counts = -IV or -lOO~V/count
The analog ouput is input protected up to 20V RMS and dur-
ing an input current overload the analog output is clamped to
a maximum voltage swing of approximately *4V. Although
protected up to 20V to prevent instrument damage, a voltage
applied to the analog output could cause an erroneous
display, rather than the "OL" message.
Since the output of the current-to-voltage converter is
bipolar, the inverting analog output can also be used to
measure low frequency AC current which falls within the
specified analog rise time. The inverting analog output
resistance is 1 .Ol kD; 9lOD in series with the signal leads and
1OOD in series with the ground lead.
Typical Analog Output Accuracy:
Linearity: < 0.1% full scsle
Gain Accuracy: k 1%
Output Resistance: +5%
Ff+ 'Efif
Figure 2-2. Model 485 Inverting Analog Output
2.7.7 Mm/Max and 100 Point Data Store
The data store can save up to 100 readings and store the
minimum and maximum readings recorded during the period
that the data store is active. The 100 points of data are stored
at one of six selectable rates. Manuel data entry (r = 6) is also
available. Readings for minimum and maximum are sampled
at the rate of three per second regardless of the selected rate.
The minimum and maximum readings are continuously up-
dated while in the data store mode. The procedure for
operating the data store is as follows:
1. Connect the desired measurement configuration to the
Model 465. Make sure that the controls of the Model 485
are set appropriately.
2. Storing Data:
A. Press and hold the STO/CLR pushbutton. The follow-
ing reading rates will scroll on the display:
r = 0 I3 rds/sec)
r = 1 (1 rdglsecl
r = 2 (1 rdg/lO secj
r = 3 II rdglmin)
r = 4 (I rdg/lO mini
r = 5 (I rdglhr)
r = 6 (I rdg every time ST0 button is pressed)
NOTE
There is no need to select a rate if just
minimum/maximum readings are desired.
Momentarily press the STO/CLR button to start
the store.
6. Release the STO/CLR pushbutton when the desired
reading rate is displayed. The ST0 annunciator will
turn on and data will be stored at the selected rate.
NOTE
The date store cycle can be terminated at any
time by doing the following:
A. If in r=6 mode, press the RCL button and
the STO/CLR button.
6. If in any other r mode, only press the STO/
CLR button.
With the data store off, data is retained and can be recalled at
any time as long as the instrument remains on.
3. Data Retrieval: Data can be retrieved at any time, but a
flashing RCL annunciator indicates that the maximum
number of readings (100) have been stored.
A. Press and hold in the RCL pushbutton. The display will
scroll through the date points and MlNlMAX (LO/HI).
The first data point displayed will be the last stored
reading. The next two data points will be the HI and LO
readings made during that store cycle. Notice that the
longer the RCL pushbutton is held in the faster the data
points will scroll on the display.
B. Release the RCL pushbutton at the desired data point
and note the reading (data) on the display. The data
pointer can be incremented by steps of one by momen-
tarily holding in the RCL pushbutton.
4. Shut off the data store by pressing the STO/CLR
pushbutton. All stored data will be retained until a new
store cycle has commenced.
2.6 MEASUREMENT CONSIDERATIONS
This section describes the effects of source resistance,
capacitance, noise and leakage resistance on the measure-
ment. Other measurement considerations are also discussed.
Minimum and maximum values of these source parameters
are defined, at which point a significant degradation of the
measurement can begin to occur. Compensation methods
are described to minimize these effects. Refer to Figure 2-3
for the following discussion.
2-6