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Tera Ohm Meter |
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A
client had been manufacturing various versions of this instrument
for over 20 years. The design in production had been designed
several years before, but still incorporated manual wiring,
with critical components soldered directly to switches to
minimize parasitic electrical leakage. The client produced
two similar instruments, but the electronics for each was
unique. |
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System Overview
Bolton Engineering took the basic instrument concept,
re-engineered it using modern components, and implemented
it on a single standard FR-4 circuit board. The new design
incorporated a microcontroller that generated the programmable
power supply reference voltage, and set the input transimpedance
amplifier gain using high-isolation reed relays. The design
eliminated a high-cost precision analog multiplier required
to normalize the meter output to the applied programmable
voltage by performing the multiplication in the time domain
using a microcomputer generating PWM signal and an inexpensive
analog switch.
A variety of circuit board guard techniques were used to
isolate the pA-sensitive input amplifier from the on-board
1100V power supply. The guards surrounding the high-voltage
power supply were implemented as inter-digitated plated
slots to prevent leakage current through the bulk circuit
board material. Other guard rings were placed surrounding
the input transimpedance amplifier and range switches on
both sides of the board.
The new design allowed the two instruments to be manufactured
using a single board design, by adding a strap to select
different microcontroller programs and by selectively populating
two of the three applied voltage range switches:
- A lower cost instrument with five preset applied voltages.
- A premium instrument with a programmable applied voltage
in 10V steps.
Results
- Reduced instrument cost by two-thirds.
- Implemented 7 full scale ranges ranging from 20 MegOhms
(2x107 Ohms) to 200 TeraOhms (2x1014 Ohms)
- Implemented programmable power supply, from 10V to
1100V, in 10V steps.
- Reduced part count by half.
- Simplified system wiring.
- Microcontroller and firmware eliminated many precision,
single-sourced components.
- Implemented design so that two different instruments
were produced off a single board design.
Project Scope
Bolton Engineering was responsible for designing the schematics,
prototyping and testing the circuitry, writing microcontroller
firmware, designing the circuit board, and writing documentation. |
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Consumer and Computer
