CR-112 charge sensitive preamplifier

Cremat's CR-112 is a single channel charge sensitive preamplifier module intended for use with various types of radiation detectors. The CR-112 is one of a series of four charge sensitive preamplifiers offered by Cremat, which differ from each other most notably by their gain. A guide to selecting the best charge sensitive preamplifier for your application can be found at our web site. As with all Cremat's preamplifier modules, the CR-112 is small (less than one square inch in area), allowing for compact multichannel detection systems to be constructed using a modular design.

Detector coupling
The CR-112 can be used either in a direct coupled (DC) mode, or an AC coupled mode. If the detector current exceeds 3 A, it is recommended that an AC coupled mode be used to prevent the resulting DC offset of the preamplifier output from saturating. Low frequency detector current (e.g. 'dark' current, or leakage current) produces an offset in the preamplifier output voltage at a rate of 0.7 V per A. The use of AC coupling also is useful in improving the counting rate capability of the preamplifier. For further discussion of this topic and a schematic diagram of an AC-coupled detection circuit utiltizing the CR-112, see the specification sheet for the CR-150-AC evaluation board at CR-150.pdf.

Package Specifications
The CR-112 circuit is contacted via an 8-pin SIP connection (0.100" spacing). Leads are 0.020 inches wide. Pin 1 is marked with a white dot for identification.

Figure 2 above shows a simplified equivalent circuit diagram of the CR-112, which is a single stage amplifier. Pin numbers corresponding with the CR-112 preamplifier are shown. Rf (680 k ) and Cf (75 pF) are the feedback resistor and capacitor respectively.

Charge sensitive preamplifiers are used when radiation is detected as a series of pulses, resulting in brief bursts of current flowing into or out of the preamplifier input. Depending on the type of detector, this burst of current may be very brief (<1ns) or as long as a few s. For an idealized detection current pulse taking the form of a delta function, the detected charge (time integral of the input current) will ideally take the form of a step function.
The output waveform of an actual charge sensitive preamplifier will of course have a non-zero rise time: for the CR-112 this figure is approximately 6 ns. Furthermore, capacitance at the preamplifier input (i.e. detector capacitance) will further slow the rise time at a rate of 0.25 ns / pF.
Keep in mind the output rise time will also be limited by the speed of the detector. For example, the detection current pulse from a CsI(Tl)/PMT scintillation detector has a duration of approximately a couple s, so the expected rise time of the charge sensitive preamplifier output will be at least that long.
The output waveform of the CR-112 using a capacitively-coupled fast square wave pulser at the input is shown below to the left. At long time domains, the output decays due to the discharge of the feedback capacitor through the feedback resistor, with an RC time constant of 50 s. This decay of the output waveform is also shown below, to the right.