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Chapter 5—Operating Instructions

The polarity of the voltage at the I MONITOR connector is that of the current (linear output
mode). For example, a current level of -1 mA will give -1 V at the I MONITOR connector (1 mA)
Current Range, representing an anodic or reducing current.

There are two selection modes, MANUAL and AUTO. In the MANUAL Mode, the range is
selected with associated arrow keys. The

key moves the selection upwards through the

available ranges. The

key moves it downwards.

In the AUTO mode, the Model 273A automatically seeks the current range that causes the I
MONITOR Output to be between 15% and 190% of full scale, that is, between 150 mV and
1.9 V. If the current level is less than 15 nA, the 100 nA range will be selected. If it is more than
1 A, the 1 A range will be selected. This process occurs after each point is acquired, with the
constraint that it can only change one range at a time. Data can be lost if the data amplitude is
changing faster than it can be tracked by the auto-ranging function. Auto-ranging is restricted to
CONTROL E operation. Should the user attempt to select Auto Ranging in CONTROL I
operation, an error message will be generated.

Automatic current ranging is additionally affected by the LOW PASS filter, if selected. If LOW
PASS is selected, there is a 150 ms dead time after each range change. No further current
ranging can occur during this dead time, which is inserted to allow the input to settle before
making a new ranging decision.

Note, however, that an overload, even if it occurs during a dead-time interval, will always force a
range shift to the next higher current range, and that the overload detection circuit is not affected
by the presence of a filter.

5.2.14. OUTPUT Group

There are three keys, three indicators, and a BNC connector in this group. A voltage dependent
on the cell current or transferred charge is available at the OUTPUT connector (identically the
rear-panel Recorder Y-Axis Output). The other items in the group either set or indicate the nature
of the dependency.

1. OUTPUT: An analog of the cell current is available at this connector. This output tracks the

current (linear or log) or transferred charge (coulombs). The output impedance is 1 k

. In the

linear mode, the output range is ± 2 V (± 1 V full scale). In the other modes it is ± 10 V. Note
that the voltage at this connector is derived from a 12 bit digital-to-analog converter that is
updated each time the applied potential or current changes. These updates may be marked
by the presence of moderate digital switching transients at this output. For best results, use
the I MONITOR output to track current unless use of the low-pass filter is desired.

2. COULOMBS, LINEAR, LOG: These three lights indicate the output mode as selected by the

associated key. There is additionally a fourth mode, OFF, established when the other three
indicators are extinguished. The output in each mode is as follows.

OFF: 0 V is present at the OUTPUT connector.

LINEAR: The voltage varies linearly with the current. A full-scale current gives 1 V out.
The maximum output, as dictated by the maximum current, is 2 V, corresponding to a
current of two times the selected current range (1 V maximum on 1 A range). Low
currents on a fixed current range may show the stepped response of a digitally
reconstructed signal.

LOG: The voltage varies with the log of the current. A current equal to the Log
Reference Range (Function 12, Section 5.2.4) gives 0 V out. The transfer function is 1 V
per decade, consistent with the constraint that the output cannot exceed ±10 V and
currents larger than two times full scale will cause an overload condition. For example,