Advanced Measurement Technology 273A, User Manual

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Chapter 3—Characteristics 15
The differences between metric and American Standard hardware are not always easily
recognized. As an aid, metric hardware is marked with a yellow chromate finish or, in the
instance of metric screws, with a yellow locking pad applied to the threads.
3.2. Internal Organization
3.2.1. Introduction
Although the Model 273A is a complex instrument, its basic organization is straightforward as
depicted in Fig. 3. The following paragraphs discuss the Model 273A, as represented in the
figure, in some detail. The symbols used in Fig. 3 are, for the most part, conventional, and
require no description. However, it should be noted that the many switches drawn inside
rectangular boxes are digitally controlled switches, and not under direct mechanical control. The
gains and scaling of internal signals are omitted for the sake of clarity.
3.2.2. Bus
Referring to Fig. 3, notice the bus that interconnects all of the major circuit blocks. All of these
blocks communicate with each other and with the controlling microprocessor via this bus. This is
true both for control via the front panel and for control from an external computer via the GPIB or
RS-232C port.
3.2.3. Applied Potentials and Currents
Note the Control Amplifier, located near the center of the figure. As shown, many different
signals are applied to the input of this amplifier. However, only three of these "inputs" are
sources of applied (control) potential or current, that is, user-controlled sources that are summed
together and that directly set the current or potential applied to the cell. Those three are:
1. BIAS DAC (Digital-to-Analog Converter), the output of which is scaled to provide either ±8 V
in applied E Mode, or ±2X full scale current in APPLIED I mode.
2. MOD DAC, the output of which, also ± 10 V, is scaled to ±2 V, ±20 mV, ±20 mV in APPLIED
E mode operation, or ±2 × full scale in APPLIED I mode operation.
3. EXT INPUT. The voltage applied to the input causes an equal voltage to be applied to the
cell in CONTROL E mode. In CONTROL 1 operation, 1 V applied to this input causes one
full scale of current to be applied to the cell.
As previously mentioned, these sources are summed. However, the sum must not exceed ±10 V
in APPLIED E operation or ±2 times full scale current up to 1 A maximum in APPLIED I
operation. Each of these three inputs can be disabled without changing the values at the DAC's
by means of the associated CMOS switches (switches depicted in rectangular boxes). The
switches for the two DAC's are shown to the right of the Scaling Circuits. The switch for the
External Input is shown to the right of the Buffer Amplifier. Note that the switches for the DAC's
cannot be accessed from the front panel or via the command set. The External Input switch, on
the other hand, is controllable via the command set (EXT command). The Model 273A
Remote-Programming Command Handbook describes the computer command set.
3.2.4. Analog Circuits
The analog section of the Model 273A comprises three operational amplifiers. They are: (1) the
Control Amplifier, (2) the Electrometer, and (3) the Buffer Amplifier that drives the I/E MONITOR
connector. Note that this potentiostat does NOT incorporate a grounded-potential working
electrode; the working electrode can be off ground by as much as 2 V. The 2 V occurs because
of the potential drop across the current sensing resistor.
Buffer Amplifier; I Measurement Circuit:
Referring to Fig. 3, note the Current Range Relays in the box to the right of the Buffer Amplifier
that drives the I/E MONITOR connector. These computer-controlled relays select any one of the