Cremat's charge sensitive preamplifiers (CSPs) are relatively sensitive and low noise. To obtain the best possible results with them and to take full advantage of their capabilities, precautions should be made to avoid certain commonly made mistakes when configuring them for use. The purpose of this guide is to assist the user integrate Cremat charge sensitive preamplifier modules into his or her instrumentation by discussing the factors that limit performance and by suggesting methods for the user to optimize the performance of these modules in the application.
Detector Coupling: AC coupling vs. DC coupling
The charge sensitive preamplifier can be connected to the detector using one of two methods. Using DC coupling, one leg of the detector is connected to the preamplifier input while the other is connected to a bias supply (most detectors require the application of a bias voltage, which creates the electric field inside the detector - necessary in most cases to generate the signal current). A diagram of a DC coupled circuit is shown here:
One of the drawbacks of the DC coupled detector is that the detector current (both the dark current and signal current) must be sourced or sunk by the preamplifier input. This results in a DC offset of the preamplifier output signal. In many cases the detector current produces an offset in the preamplifier output that is impractically large. To avoid this problem, most designers use AC coupling to connect the detector to the preamplifier. In this configuration, the DC detector current bypasses the preamplifier input and is instead supplied by a bias resistor. An AC-coupled circuit is shown below:
Cremat's CR-150-R5 evaluation board uses an AC-coupling scheme in part because of the improved performance with large detector currents. More information can be found in the specifications for the CR-150-R5.
Besides bypassing the detector dark current, there is a second advantage using AC coupling: the counting rate capability is increased. This is true if we use the simplifying assumption that the detector signal current is made of very many relatively small pulses. Using AC coupling, the DC (or average) detector current no longer passes into the preamplifier input. As a result, the preamplifier output saturates only when the output fluctuations become so large that they exceed the rail-to-rail output range of the preamplifier. The size of these fluctuations depend on the size distribution and rate of the detected pulses, and it is difficult to quantitatively describe the count rate limitations of the preamplifiers under these conditions. To generalize, however, we can say that the AC coupled detector has improved count rate performance over the DC coupled version.
There are two possible disadvantages of using AC coupling. The first is that the decay of the preamplifier signal has two different decay times when using AC coupling. The first is the decay time of the preamplifier (for the CR-110 this figure is 140 microseconds). The second decay time is the slower decay of the coupling capacitor times the bias resistor. This decay time is on the order of one second. So if the shape of the signal at very long times is important, then you must consider the extra decay of due to the coupling capacitor. In almost all applications this is not important, however.
The second possible disadvantage of using AC coupling is that the 'bias resistor' (on the CR-150-R5 board this resistor is valued 200 megohms) is a source of thermal electronic noise. Careful choice of the value of this bias resistor, however, can reduce the magnitude of this noise so that it becomes negligible. If the detector current produces a voltage drop of at least 0.5 volts across the bias resistor, then the shot noise from the detector leakage current dominates the noise and the thermal noise of the bias resistor is comparatively small. Choosing the bias resistor value does, however, require some approximate knowledge of the detector current. The specification sheet for the CR-150 evaluation board contains a table which may assist in selecting bias resistor values.
Because radiation detection usually requires high sensitivity amplification, the 'front-end' instrumentation (detector and preamplifier) should be operated in an electrically shielded environment. The connection between the detector and preamplifier should be shielded as well
One of the most commonly-encountered pitfalls is the creation of ground loops in the front-end instrumentation. The presence of ground loops can often be detected by observing a 60 Hz signal or a series of fast pulses (voltage spikes) in the output in sync with the local AC power line. Keep in mind that detection of these line-synched signals may be caused by other problems, however, and they do not necessarily indicate the presence of a ground loop. Ground loops are formed by grounding the front-end electronics at more than one point. Providing this extra ground connection allows current to flow into one grounding point and out of another, traveling through the detector/preamplifier ground connection. Current can flow around a ground loop for a number of reasons: There may be a radio frequency signal nearby inducing a current in the loop (the loop is effectively a magnetic dipole antenna). Also, there may be electrical equipment nearby drawing current from (and thereby producing small voltage fluctuations in) the local grounds. When these 'ground currents' flow through the grounds of the front-end instrumentation, they can be large enough to produce a small but significant voltage drop across the detector ground connection. In this way the unwanted ground current 'signal' is mixed in with the detector signal. To avoid a ground loop problem, avoid making any extra (perhaps inadvertent) connections to the detector ground. In fact it may be advisable to shield the detector ground if the system is placed in a particularly noisy environment. The problem and solution are illustrated here:
Caution should be exercised when placing a charge sensitive preamplifier (CSP), or anything connected to the input of the CSP near (within a few inches) to any subsequent amplification stages (e.g. shaping amplifiers). This may result in oscillation or distortion of the output waveform due to feedback between the input of the CSP and the output of the subsequent amplifier. It is best to electrically shield the CSP from any subsequent amplifiers to prevent this unwanted feedback. A problem with feedback may be indicated if the observed output waveform distortions or oscillations diminish upon reducing the gain of the subsequent amplification stages.
Using switching power supplies to power CSPs may present a problem because these power supplies often introduce a small but significant amount of radio frequency (RF) power into the front end environment which can be easily detected by the preamplifier. For this reason the use of switching power supplies should be implemented only when there is effective shielding and filtering of the switching power supply from the preamplifier input or power leads.
The rise time of the output signal from a charge sensitive preamplifier may be limited by either the speed of the detector or the speed of the preamplifier, depending on which is slower. In the cases where the pulse rise time is limited by the speed of the preamplifier, the rise time has a linear relationship with the input capacitance. The rate at which the pulse rise time slows with increasing input capacitance is closely related to the input impedance of the CSP, which varies among the various models offered by Cremat. To calculate the expected pulse rise time for a particular preamplifier model, use the equation found in the preamplifier specification sheet.
In the interest of obtaining the fastest possible rise time for your detector, you should strive to minimize any stray capacitance at the preamplifier input. We caution against the use any lengthy coaxial cable connecting the detector to the preamplifier, in part for this reason.
Theoretical Sources of Electronic Noise
In typical detection systems using charge sensitive preamplifiers, the equivalent noise charge (ENC) is due to a combination of these factors:
1). The series thermal noise of the input JFET in the preamplifier (which is proportional to the total capacitance to ground at the input node),
2). The parallel thermal noise of the feedback resistor and any 'biasing' resistor attached to the detector,
3). The shot noise of the detector leakage current,
4). The series 1/f noise, which is produced by the electrical contacts of the detector and preamplifier input JFET,
5). The parallel f noise caused by the proximity of lossy dielectric material near the preamplifier input node.
These noise sources can often be individually quantified in an operating detection system by measuring the dependence of the ENC on the "shaping time" of the shaping amplifier which usually follows the preamplifier stage. This method is described in more detail in the
Bertuccio G; Pullia A; "A Method for the Determination of the Noise Parameters in Preamplifying Systems for Semiconductor Radiation Detectors", Rev. Sci. Instrum., 64, p.3294, (1993).
Other articles which describe typical noise sources and signal processing techniques when using charge sensitive preamplifiers are:
Radeka V; "Low-Noise Techniques in Detectors", Ann. Rev. Nucl. Part. Sci., 38, p.217, (1988).
Goulding FS; Landis DA; "Signal Processing for Semiconductor Detectors", IEEE Trans. Nuc. Sci., NS-29, p.1125, (1982).
Practical Tips on Reducing Electronic Noise
Given the constraints imposed by the detector and operating environment, there remain a few aspects of the detector/preamplifier connection which, if neglected, may introduce unnecessary noise into the detection system. Attention to the following factors will help avoid these possible problems:
1) Keep the detector/preamplifier input nodes on the board clean. After soldering the detector connections to your circuit board, carefully clean the residue (mostly solder flux) off the board. This also applies to the bias resistor (resistor placed between the detector and the filtered detector bias supply), bias filtering components, as well as the preamplifier input pin connections. Keeping the board clean and dry will prevent stray surface currents from introducing unnecessary noise.
2) Choose the bias resistor value carefully. If AC coupling is used, an important decision to make is the value of the bias resistor. This resistor is a source of parallel thermal noise. The magnitude of this noise is proportional to the reciprocal of the square root of the resistor value. A good approach to preventing the noise from this resistor from becoming a dominant noise component is to choose a resistor value that produces less thermal noise than the detector shot noise. This of course requires that you have at least an approximate knowledge of the detector leakage current. The point at which the thermal noise of the bias resistor equals the detector shot noise is when the bias resistor voltage drop is =2kT/q, or approximately 50 mV. If the voltage drop is significantly greater than this, then you can be certain that the thermal noise of the resistor is not limiting the performance of the circuit. To be safe, you can use a bias resistor that will drop approximately half a volt.
It may be tempting to choose an extremely large valued resistor to avoid this thermal noise. Keep in mind, however, that another consideration should be that a very large voltage drop across the bias resistor (in excess of several volts) may significantly subtract from the voltage drop across the detector. Furthermore, there is no need in choosing a bias resistor in excess of 200 megohms because other noise sources in the preamplifier will dominate at that point. More specific advice on choosing the value of the bias resistor can be found in the specifications for the CR-150-R5.
3) Minimize the capacitance at the preamplifier input The 'series noise' component of the electronic noise is proportional to the capacitance-to-ground at the preamplifier input. Of course the detector capacitance is included in this figure, which often accounts for the majority of the total input capacitance. There may be other sources of input capacitance, however, and the user should try to minimize these as much as possible when designing the detection system. Some applications require the detector and preamplifier to be in separate housing connected by coaxial cable. Keep in mind that the use of coaxial cable between the detector and preamplifier adds noise for a couple of reasons, one of which is because of the added input capacitance.
4) Avoid 'lossy' dielectric materials as much as possible Another noise concern in the design of your detection system is the introduction of 'parallel f noise', which is introduced by the proximity of lossy dielectric materials at the preamplifier input. To minimize this source of noise, which in some situations can be quite significant in magnitude, detector circuit designs should keep the input traces on the circuit board as short as possible. This is because the circuit board itself is often the lossy dielectric material introducing this form of noise. Epoxy and glass, which are usually considered to be good dielectrics (and circuit board materials) in most circuit applications, are actually too lossy to be used in the usual manner when designing detector circuits. Better construction materials are Teflon and to a lesser extent alumina. These materials, however, are more unusual and expensive than standard FR-4. To avoid the expense of Teflon boards, consider lifting the input lines off the circuit board in some fashion, perhaps by suspending the input lines above the board using Teflon standoffs. If electronic noise is not a primary consideration, however, it may suffice to use short traces on an epoxy-based circuit board. The use of coaxial cable to couple the detector to the preamplifier may introduce noise, not only by adding capacitance (as mentioned previously), but also because of the lossiness of the cable's dielectric layer. If coaxial cable absolutely must be used between the detector and preamplifier, its length should be as short as possible.