Signal pulses from an avalanche photodiode (APD) can be read out with high precision using a charge sensitive preamplifier (CSP). This is due to the low noise characteristics of CSPs, as well as the integrating nature of the output signal which provides an output proportional to the total charge flowing from the APD detector during the pulse event.
Just a couple examples of pulse-detection applications using avalanche photodiodes are the detection of radiation events from scintillators, or the detection of reflected laser pulses.
Connecting an APD to the CSP:
The usual connection scheme of an APD to the preamplifier is shown below, and is known as an 'AC coupled' circuit:
The charge sensitive preamplifier (CSP) shown in the figure above is shown as an 8 pin module. Cremat offers four different CSP modules (CR-110, CR-111, CR-112, and CR-113) which differ primarily by their gain. APD detectors will likely be best served by the CR-110, which has the lowest noise and greatest gain of the available models.
An APD based detection setup
An overview of the instrumentation for an APD detection setup is shown below:
Given this overview, there are a number of choices and refinements still to be made. These are described below.
Do you need bias voltages in excess of 500V?
Cremat's CR-Z-110 and CR-Z-111 CSP instruments use BNC connectors at the 'bias in' and 'input' connections. BNC connectors are rated at 500V maximum. If your APD requires a bias greater than 500V you should use the '-HV' option which uses SHV connectors at these positions. The CR-Z-11X-HV instrument is rated at 2000V.
Do you need the CR-Z-110 or the CR-Z-111 instrument?
CR-Z-110 and CR-Z-110-HV use the CR-110 CSP module within the instrument. This CSP module has the highest gain and lowest noise and is usually the best choice for most APD applications. In some instances where the signal pulses may be very large (in excess of approximately 107 electrons, or 2 pico-Coulombs) you may consider using the CR-Z-111 instead. Because the CR-Z-111 has somewhat faster rise time response, you may also be interested in using the CR-Z-111 in situations where you will use a fast shaping time (<100ns). Shaping time is discussed in more detail below.
Choice of bias and filter resistors in the CR-Z-11X
Cremat's CR-Z series of charge sensitive preamplifier instruments use 'AC-coupling' to connect the detector to the input of the preamplifier's amplification stage, as was discussed previously. The detector dark current flows into the preamplifier instrument through the 'bias in' connector, is filtered in a simple RC low pass filter (10M/0.01μF), flows through the bias resistor (200M ohms), and finally flows through the APD to ground. The bias resistor serves as a high impedance to the detector signals, so the APD signal current passes instead through the relatively low impedance of the 0.01μF blocking capacitor to the preamplifier input.
In the event that the APD dark current exceeds approximately 10nA, you should consider reducing the value of the bias resistor in order to prevent the resulting voltage drop across the resistor from becoming too large. The voltage drop across the bias resistor (as well as the filter resistor) reduces the APD bias voltage by that same amount because they are in series. So if precise control of the detector bias is required and the detector current exceeds approximately 10nA (2 volt drop across the 200 Mohm bias resistor), you should consider reducing the value of the bias and filter resistors to keep these voltage drops small. It is for this reason that there are a couple unpopulated resistor positions on the main circuit board of the CR-Z instrument, labeled R-A and R-B, which can be used to shunt the filter and bias resistors respectively.
A table of recommended values of R-A and R-B is shown below, based on the average detector current. Because most voltmeters cannot accurately measure voltage drops across very large resistances, the best method is to use your knowledge of the approximate detector current to calculate this voltage drop. Refer to the APD specifications to determine the APD dark leakage current if no other information is available.
|leakage current range||R-A||R-B|
|0 to 10nA||(Left open)||(Left open)|
|10nA to 30nA||(Left open)||22M|
|30nA to 100nA||(Left open)||10M|
|100nA to 300nA||3.3M||3.3M|
|300nA to 1μA||1M||1M|
|1μA to 3μA||330k||330k|
|3μA to 10μA||100k||100k|
|10μA to 30μA||33k||33k|
Choice of shaping time in the CR-S-X shaping amplifier
In the diagram above, the CR-S-1us shaping amplifier is used. This shaping amplifier uses a 1 microsecond shaping time, and is a 'middle of the road' choice for many applications. There are, however, some guidelines to help you determine the best shaping time for your application:
1) Chose the shaping time that will give you a minimum of electronic noise. As was mentioned above, shaping amplifiers are electronic (band pass) filters - the shaping time is related to the band pass frequency. There are a few sources electronic noise in a typical detection system, and some of them differ in their power spectra. While it is outside the scope of this guide to quantify the total electronic noise or provide complicated formulae for calculating the noise minimum, there generally is a noise minimum within the range of available shaping times. Changing the shaping time of the CR-S-X instrument is a simple matter of swapping out the installed CR-200-X shaping amplifier module for that of another shaping time, so determining the noise minimum could be done experimentally.
2) Choose a shaping time that is long enough to collect all the charge from the APD. While APDs can be fast detectors, the photon signal may have a duration lasting microseconds. For example, detection of a light signal from a CsI(Tl) scintillator occurs over a 2 microsecond duration. In this case and in other applications involving relatively slow signals, use a shaping time that is at least as long as the photon signal duration.
3) Choose a shaping time that is short enough to accommodate the expected counting rate of the detection system.