Metal Samples MS2500L, Руководство оператора

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count down, a pulse is generated to reload the counters and to initiate either a polarization cycle or a
relaxation cycle, (initially a polarization cycle), depending upon the previous state. Pulses from Q14
output of U12 are generated upon 1/10 minute intervals, (0.166 Hz), which are divided down from the
2730 Hz signal developed by the U16 clock oscillator. A 5.332 Hz signal generated by the Q9 output
of U12 is also used to clock the sample and hold circuits.
Potential Sample and Hold Circuits
The potential sample and hold and measurement circuits are contained on the A1 Board and are made
up of:
• An infinite term digital sample and hold consisting of:
A bipolar successive approximation ADC using U7, U4, U5 and AR4-B-C-D.
A 200 millivolt ADC reference using part of AR3.
• A low power instrumentation amplifier made up from AR1-A-B, AR2, R4, R5, R8 and
R9.
• An inverting amplifier using AR1-C, R1 and R2.
As explained earlier, the first part of the LPR measurement process is the sampling and storing of the
potential value. This is accomplished by a digital sample and hold technique with an indefinite holding
capability. On the initiation of a polarization cycle, a pulse triggers the start of a successive approxima-
tion conversion performed by U4 and U5. U4 and U5 switch the output of the 12 bit D/A converter
circuit, (U7, AR4-C and AR4-B), using the comparison generated by AR4-D between common and
the output of the instrumentation amplifier, (AR1-A, AR1-B and AR2), unto the output of the instru-
mentation amp is zero. This is by setting a bias on the reference of the instrumentation amp, SW1-C,
using the output of the D/A converter circuit.
This scheme nulls the instrumentation output to zero so an offset shift of 10 millivolts can be easily read
by the next process step. Upon the completion of this process, an end of conversion signal is generated
by the successive approximation registers. This signal is used to trigger the remaining portion of the
polarization cycle.
If the or OFFSET- function is selected, the above scheme is bypassed with the reference
point of the instrumentation amplifier, SW1-C, set to common and the output of the instrumentation
amplifier, which reflects the potential difference, is sent directly to the output circuits or, if OFFSET- is
selected, the output is inverted and amplified by AR1-C.