Metal Samples MS2500L, Руководство оператора

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Polarization Current Control and Measurement
The circuitry for the polarization current control and measurement resides on the A1 Board and consists
of:
•
A 10 millivolt polarization reference made up from the reference portion of AR3.
•
Current drive and scaling amplifier using the power amp portion of AR3.
•
Polarization balancing amplifier; AR1-D.
•
Current sensing differential amplifier; AR4-A.
•
Solid state current control switching; U1-A and U1-B.
Upon the end of conversion signal generated by the potential sample and hold, the current control is
enabled via SW1-A. U1-B and U1-A are enabled allowing a current path through the cell between the
auxiliary and test electrodes. As current is allowed to flow, the potential of the cell (voltage between the
reference and test electrodes, changes. AR1-D monitors this change, compares it with the 10 millivolt
reference (generated by AR3, R3, R7and R10), and controls the current drive by AR3 until a balance
point is reached where the potential of the cell has changed by 10 millivolts. The 1.82K resistor, R11, in
the feedback loop of AR3 gives a 0.0 - l.0 volt drop for a span of 0 - 100 MPY using the conversion
factor of 5.5 µ/MPY. AR4-A is used as a differential amplifier which converts the 0 - 1 volt drop across
R11 to a 0.0 - 1.0 volt signal which is sent to the output circuitry.
Output Sample and Hold
The output sample and hold circuits are contained on the A2 Board and consist of:
• An infinite term digital sample and hold consisting of a successive approximation ADC
using U13, U14, U18, AR9 and AR8.
• A short term sample and hold using U15-A, AR7 and C13.
When operating in the SAMPLE mode, there is a possibility of 3.5 hour holds between reading up-
dates. This long hold time is accomplished by a second infinite term digital sample and hold.
At the end of a polarization cycle, a conversion is initiated to sample the current amplifier output in the
LPR and SAMPLE modes. During this conversion process, a conversion in process signal is generated
by the successive approximation registers. This signal is used to disable U15-A thus maintaining a short
term sample and hold using C13 and AR7. Once completed, the end of conversion signal reenables
U15-A and triggers the relaxation portion of the LPR cycle. The gain of AR7 rescales the signal to a 1.0
to 5.0 volt signal (with reference to true ground), which is used by the current loop control circuitry.