1,004
edits
Changes
Jump to navigation
Jump to search
Line 1:
Line 1: − + − + Line 17:
Line 17: − + − + + + + + + + − + + + + + + +
SiPM Amplifier Optimization (view source)
Revision as of 04:40, 4 February 2009
, 04:40, 4 February 2009no edit summary
The SiPM amplifier currently in use for SiPM characterization ([http://www.photonique.ch/ Photonique] item "AMP_0604") must be adapted for tagger microscope use. The following is a brief outline of the design requirements. They are discussed in detail in the following sections
The SiPM amplifier currently in use for SiPM characterization ([http://www.photonique.ch/ Photonique] item "AMP_0604") must be adapted for tagger microscope use. The following is a brief outline of the design requirements. They are discussed in detail in the following sections
# adjustable gain, ranging from readout of hundreds of pixels to calibration with single-photon counting
# adjustable gain, ranging from readout of hundreds of pixels to calibration with single-photon counting [#Gain]
# less than 15% gain variability on transistor <math>\beta</math> (<math>h_{FE}</math>) parameter
# less than 15% gain variability on transistor <math>\beta</math> (<math>h_{FE}</math>) parameter
# summing circuit to pool SiPM signals in groups of 5 (readout of individual channels must not affect readout of the some regardless of termination used)
# summing circuit to pool SiPM signals in groups of 5 (readout of individual channels must not affect readout of the some regardless of termination used)
# minimized pulse duration for higher running rates
# minimized pulse duration for higher running rates
The amplifier provided by Photonique with a gain of roughly 3 kΩ was well suited for single photon counting. However, for typical signals ranging in the hundreds of SiPM pixels, this gain excessive. However, the option of switching back to single photon detection for the purposes of calibration would be a nice feature.
The amplifier provided by Photonique with a gain of roughly 3 kΩ was well suited for single photon counting. However, for typical signals ranging in the hundreds of SiPM pixels, this gain excessive. However, the option of switching back to single photon detection for the purposes of calibration would be a nice feature.
From the perspective of expected signal amplitudes (taking into account optical and SiPM's quantum efficiencies) signals around 300 pixels (px) are expected. With a SiPM gain of about 2e5 ~ 9.6 pC are expected to be deposited. Design uncertainties that go into the full calculation summarized here can easily allow variation in this result by a factor of two or more. Roughly estimating this charge to be contained in a triangular pulse with 5 ns FWHM (after all the broadening inherent in the amplifier) yields a total signal peak of 0.5 mA. With this figure and the full range of the ADC (2V) it seems that 3 kΩ is still appropriate. However this does not leave room for variation discussed above. Instead, a goal of sub-1 kΩ gain was adopted.
From the perspective of expected signal amplitudes (taking into account optical and SiPM's quantum efficiencies) signals around 300 pixels (px) are expected. With a SiPM gain of about 2 × 10<sup>5</sup> ~ 9.6 pC are expected to be deposited. Design uncertainties that go into the full calculation summarized here can easily allow variation in this result by a factor of two or more. Roughly estimating this charge to be contained in a triangular pulse with 5 ns FWHM (after all the broadening inherent in the amplifier) yields a total signal peak of 0.5 mA. With this figure and the full range of the ADC (2V) it seems that 3 kΩ is still appropriate. However this does not leave room for variation discussed above. Instead, a goal of sub-1 kΩ gain was adopted.
For the high gain setting, the issue is mainly the vertical resolution of the ADC. For most of the duration of this project, the 8-bit version of the ADC was planned to be allocated for microscope readout, imposing a stringent requirement on gain in order to avoid the digitization noise inherent in signals only a few adc voltage steps. The 12-bit ADC makes clean readout of single pixel wavefunctions more realistic: at the most sensitive scale of 0.5 V, the resolution is 0.12 mV. However, we must also take into account noise and a possible factor of two loss in the split of the signal between the ADC and the CFD (constant fraction discriminator to prepare for time pick-off.) This time, it is appropriate to take a pessimistic scenario of the pulse shape: taking a triangular pulse with 30 ns FWHM, leading to a single pixel current peak of 0.27*nbsp;μA. Under these conditions, gain of 7 kΩ is enough, giving 15 adc steps per pixel.
For the high gain setting, the issue is mainly the vertical resolution of the ADC. For most of the duration of this project, the 8-bit version of the ADC was planned to be allocated for microscope readout, imposing a stringent requirement on gain in order to avoid the digitization noise inherent in signals only a few adc voltage steps. The 12-bit ADC makes clean readout of single pixel wavefunctions more realistic: at the most sensitive scale of 0.5 V, the resolution is 0.12 mV. However, we must also take into account noise and a possible factor of two loss in the split of the signal between the ADC and the CFD (constant fraction discriminator to prepare for time pick-off.) This time, it is appropriate to take a pessimistic scenario of the pulse shape: taking a triangular pulse with 30 ns FWHM, leading to a single pixel current peak of 0.27 μA. Under these conditions, gain of 7 kΩ is enough, giving 15 adc steps per pixel.
== Minimal Dependence on <math>\beta</math> ==
Dependence of a design on a value β of a transistor is never a good idea, given its variability as a function of temperature. The value also varies between one transistor and the next. The high speed transistors necessary in this design are already at a disadvantage with respect to this design goal, due to their low beta with the result of greater significance of the parameter's variation. This issue will be discussed at length below.
While it is true that this variation can be compensated by adjusting the bias voltage on the SiPM to alter the gain, this sort of tweaking is not desirable. The detection efficiency changes right along with the gain. (Detection of the maximimum number of photons is critical for good time resolution.) With these concerns in mind, a design requirement of gain variation no greater than 15% variation was set.
== Summing Circuit ==
While the microscope contains 500 optical channels resulting from the two-dimensional segmentation of the focal plane, only the energy bin information is important. The vertical column of 5 scintillating fibers in the focal plane can then have its signals summed, reducing the count of channels requiring readout to 100. However, the design of the microscope calls for individual readout in 5 columns spread around the focal plane in order to retain the knowledge of the two-dimensional orientation of the electron stripe.
Thus a summing circuit is necessary to tie together 5 amplifiers. Drivers for single channel readout must still be available on the board and their output must be available on the board's interface pins. (The choice of which group of channels can be accessed is up to the layout of the backplane board.) The individual channel and summing circuit outputs must remain independent regardless of termination. This means that the input to the summing circuit must have its own current source so that the load on the individual channel is not relevant. Additionally, as has been discussed above under gain requirements, the summing circuit must offer additional gain when system is set to high gain mode.
Going from this integrates charge figure to actual signal heights requires knowledge of the pulse duration due to pulse shaping in the amplifier. However, gain and pulse shape are coupled in the amplifier design - one resetting the goals for the other.
Going from this integrated charge figure to actual signal heights requires knowledge of the pulse duration due to pulse shaping in the amplifier. However, gain and pulse shape are coupled in the amplifier design - one resetting the goals for the other.