Advanced Measurement Technology 142AH 운영 및 서비스 매뉴얼 - 페이지 14

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4.6. INPUT PROTECTION

A provision is built into the preamplifier to protect

the input FET stage from damage when high-

voltage transients are applied to its input. These

transients can result from any one or more of

many causes, including detector breakdown,

moisture condensation on the input connector,

short circuits or uncharged capacitance

connected across the input while bias is being

applied through the preamplifier, or disconnection

of a bias voltage without first reducing it gradually

to zero.

The protection circuit is installed in the

preamplifier when the unit is shipped from the

factory. Although it offers protection to the FET, it

also causes some degradation of the noise

performance of the preamplifier, which increases

as detector capacity increases.

With the protection circuit in, the collector lead of

Q11 is attached to the center tap of the two series

input resistors to the FET, R39 and R5. Transistor

Q11 is connected as a diode, with both the base

and the emitter tied through R8 to ground. This

prevents the voltage in the input circuit from

increasing beyond the safe limit for the FET input.

To take the protection circuit out, simply remove

the collector lead of Q11 from its circuit

connection and install a wire jumper across R39

and R5.

Inherent to all high-speed charge-sensitive

preamplifiers is the problem of keeping the total

system stable while interconnecting the detector

and preamplifier with long cables. To help ease

this problem and permit more flexibility for

installations, the protection circuit is designed to

serve not only as a protection for the FET input

but also to terminate the input cable in a nominal

100

. So it is recommended that the protection

circuit be left in the preamplifier circuits whenever

possible.

In order to take full advantage of the rise time

capabilities of the 142AH for timing experiments

(typical rise times of 4 to 10 ns for detector

capacities of 1 to 100 pF), the total cable length

should be kept as short as possible even though it

is terminated. Due to vagaries in the scattering

chamber system - ground loops, stray

inductances, etc. - and since the maximum cable

length is a factor in the input capacitance to the

preamplifier, it is not possible to give absolute

numbers. Generally, two feet is a typical

maximum length to obtain fastest rise times for

low-detector capacities. Of course, the system

can be compensated for cable lengths of up to 5

or 6 feet but slower rise times will be obtained.

The screwdriver adjustment inside the case can

be used for this type of system. The rise times

under these conditions will be in the range of 10

to 25 ns for capacitances from 1 to 100 pF.

Typical results are shown in Fig. 4.4.

Fig. 4.4. 142AH Typical Rise Time.