Amptek CoolFET A250CF Handbuch - Seite 5

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4.3

AC and DC Coupling

The A250CF is shipped from the factory AC coupled with a

300 MOhm Bias resistor and filter installed. The Bias

resistor is connected through a teflon post to the input BNC.

If the user connects a detector with high leakage current,

this resistor should be adjusted (reduced) appropriately.

Three 100 MOhm resistors and one 10 MOhm resistor have

been put in series to facilitate reducing the resistance. Use

the vias next to the resistors to short the resistors as needed.

To DC couple the A250CF the user must remove the H.V.

connection from the teflon post to input BNC and solder a

jumper wire across the input capacitor or remove it and

make a direct connection to the input node. See "H.V.

Bias" location in Figure 5 below. DC coupling is often used

when the lowest noise is required and therefore no input

protection is used. The user must exercise extreme caution

in this configuration because of the increased risk of

blowing the FET. Damaged FETs are not covered under

warranty.

4.4

Input Protection

The A250CF is shipped with the protection network (Rp

and D1) enabled (as shown in figure 5 and 6). The best

noise performance is obtained without any protection, but

the risk of damage is much higher. Follow the precautions

outlined in Section 1 when disabling the protection. The

diode protection degrades the noise performance more the

higher the detector capacitance. To enable the protection

circuit, RP must be enabled (not shorted) and D1 connected

to JP3. To disable to protection circuit, short RP using the

jumper wire and disconnect D1 from JP3.

Figure X. Shows an AC coupled configuration (H.V.

connected) with the protection resistor disabled (Rp

shorted).

Amptek A250CF CoolFET User Manual –Rev. A5

Charge Sensitive Preamplifier

A250CF CoolFET

Figure 5. Shows an AC coupled configuration with the

protection network enabled. The Rp resistor is enabled (not

shorted).

Figure 6. Shows the protection diode D1 enabled. D1 is

connected to TP3.

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