The charge sensitive preamplifier can be connected to the detector using one of two methods: 'AC' or 'DC' coupling.
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:
In an AC-coupled configuration, a coupling capacitor is inserted between the detector and preamplifier input, together with a high-value bias resistor. This arrangement blocks detector leakage (dark) current from flowing directly into the preamplifier input.
As a result:
Detector dark current does not create a large DC output offset
The preamplifier baseline remains more stable
Sustained operation at higher average count rates becomes possible
For a single large pulse, the saturation limitation is essentially the same as with DC coupling because the instantaneous signal amplitude entering the preamplifier is unchanged. The output pulse still decays with the normal preamplifier decay time constant, and sufficiently large pulses can temporarily drive the output into saturation.
Besides bypassing the detector dark current, there is a second important advantage to AC coupling: the counting rate capability is increased. This is particularly true if we make the simplifying assumption that the detector signal current consists of very many relatively small pulses. In a DC-coupled system, the average detector current passes directly into the preamplifier input, causing a progressive output offset as count rate increases. In the AC-coupled configuration, however, the DC (average) detector current no longer flows into the preamplifier input. As a result, the preamplifier output tends to saturate only when the statistical output fluctuations themselves become large enough to exceed the available output voltage swing of the preamplifier.
The magnitude of these fluctuations depends on many interacting factors, including:
Pulse amplitude distribution
Average count rate
Statistical spacing between events
Detector leakage current
Preamplifier decay time constant
Bias resistor value and coupling capacitance
Because these factors vary substantially between applications, it is difficult to define a single quantitative “maximum count rate” for an AC-coupled detector system. In practice, occasional large pulses may still produce temporary saturation even when the average count rate is acceptable, while a continuous stream of smaller pulses may be tolerated at substantially higher rates. For this reason, count rate limitations are generally application-dependent and are best evaluated under actual operating conditions.
To generalize, however, AC-coupled detector systems typically provide substantially improved count rate performance and better baseline stability compared with DC-coupled configurations, particularly when detector leakage current or sustained high average count rates are present.
Cremat's CR-150-R6 evaluation board uses an AC-coupling scheme in part because of the improved performance with large detector currents and improved count rate limitations.
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-R6 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 product page for the CR-150-R6 evaluation board contains a table which may assist in selecting bias resistor values.