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
The
charge sensitive
preamplifier can be
connected to the detector using one of two commonly used
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 evaluation board uses an AC-coupling scheme
because of the improved performance with large detector
currents. More information can be found in the
specifications for the
CR-150.
Electrical Shielding
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
Ground Loops
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:
Feedback
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.
Power Supply
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 is discouraged.
Rise time
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
article:
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.
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.
