It turned out that the VDD switch off speed plays some important role. On our VME board, we have a linear regulator, then a 4.7 uF capacitor, then the DRS4 chip (DVDD and AVDD). When switching off the VME power, it takes quite some time to discharge the 4.7 uF capacitor, since the DRS4 chip goes into a high impedance mode if VDD < ~1V. This gives following VDD trace:
Rising edge is power on, falling edge is power off. Note the horizontal time scale of 2 s/div. So to get below 0.3 V or so, it takes up to 30 seconds. If the power is switched back on when AVDD is above 0.3V, the DRS4 chip can get into a weird state, where probably many domino waves are started and the chip draws an enormous amount of current. Typically the linear regulator limits the current, so the 2.5V drops to ~1.5V, and the board is not working. If people are aware of this and always wait >30sec. before turning the power on again, this is fine, but people might forget.
So the solution is to put a resistor (typically 100 Ohm to 1 kOhm) parallel to the 4.7 uF capacitor in order to have some resistive current load of a few mA. The discharge then looks like this:
Note the horizontal scale of 10ms/div. So after 30 ms AVDD is discharged and powering on the chip again does not do any harm. The same should be done to DVDD.
It has turned out that the stability of the AVDD and DVDD power supplies for the DRS4 are very critical. On the evaluation board I use a REG1117-2.5, on our VME board I use a ADP3338-2.5 for the DVDD power supply. When the domino wave is started, the power consumption of the DRS4 chip jumps up by ~40 mA, which has to be compensated by the linear regulator. Following screen shot shows what happens:
The blue trace is the DWRITE signal indicating the sampling phase when high. The yellow is the SRCLK showing when the readout takes place. The pink is now the DVDD power. It can be clearly seen that there is a dip of ~50 mV when the domino wave starts, a positive dip when it stops and another smaller dip when the readout starts. This causes strange effects: If the trigger arrives during the first dip, the actual sampling takes place when the DVDD is ~50 mV smaller, which leads to a baseline shift of a sampled 0V DC input voltage of about the same amount (-50 mV).
The obvious improvement is to put a huge capacitor on the power supply, but that does not help much:
The dip gets a bit smaller, but it's still there. So a better solution would be to use a faster LDO regulator. Please take care of this if you plan a new design.
Furthermore, I believe that the chip internally has some "warmup" phase, where the die heats up a bit when the additional 40 mA are drawn. So a "good" solution is to wait some time after starting the domino wave until one allows for triggers. Tests showed that a few milliseconds are necessary to keep the baseline shifts below a few millivolts. This of course decreases the dead time of the system significantly, so one has to choose the proper balance between increase dead time and increased base line shift. On some applications where a baseline shift is not an issue, one could opt for the minimum dead time.
The current version of the DRS readout example program drs_exam.cpp has two problems:
Both problems have been fixed and the fix will be contained in an upcoming software release.
Maybe some of you have experienced that the DRS4 chip can get pretty hot after power up. After it's initialized the first time, the power consumption goes back to normal. I finally found the cause of this problem and have a remedy. Here is the new paragraph from the updated data sheet:
During power-up, care has to be taken that the DENABLE and DWRITE signals are low. If not, the domino wave can get started before the power supply voltages are stable, which brings the DRS4 chip into a state where it draws a considerable amount of current and heats up significantly. This can be problematic if the signals are directly generated by a FPGA, since most FPGAs have internal pull-up resistors which get activated during the configuration phase of the FPGA. In such a case, the DENABLE and DWRITE signals should be connected to GND with a pull down resistor. This resistor should be much smaller than the FPGA pull-up resistor in order to keep the signals close to GND during the FPGA configuration. A typical value is 4.7 kOhm.
The attached schematics shows the location of the two required resistors.
A new software verison for the DRS4 Evaluation Board has been has been released. Version 2.1.3 adds a switch for the input range of the DRS4 board. Once can choose between -0.5V...0.5V and 0V...1V:
A board firmware update is not necessary for this. It was originally planned to have even a negative range -1V...0V, but this is not possible with the current board design. People who want to record negative pulses have to use an inverter to produce positive pulses. In a future version of the board it might be possible to include this functionality since this is determined by the analog front-end and not the DRS4 chip.
Several people asked for s simple application to guide them in writing their own application to read out a DRS board. Such an application has been added in software revions 2.1.1 and is attached to this message. This example program drs_exam.cpp written in C++ does the following necessary steps to access a DRS board:
I know that we are still missing a good documentation for the DRS API, but I have not yet found the time to do that. I hope the example program is enough for most people to start writing own programs. For Windows users (MS Visual C++ 8.0) there is a drs.sln project file, and for linux users there is a Makefile which can be used to compile this example program.
One note: The program drs_exam.cpp published in the previous message needs the current version of the DRS library in DRS.cpp and DRS.h. They are contained in the software release 2.1.1 which has to be downloaded. For simplicity, I attached the two files to this message.
This is a quick notification to all users of the current DRS4 evaluation board.
As you all know, the DRS4 chip needs some calibration for each individual cell which corrects the offset and the non-equidistant width in time. While the first evaluation boards have been shipped without this calibration, the current version of the software implements a full amplitude and timing calibration. The offset correction reduces the noise of the board by almost an order of magnitude to below 1 mV RMS. The timing calibration using an on-board reference clock allows a timing accuracy in the order of 10 ps. To illustrate that the following two pictures show a reference clock signal before and after timing calibration:
The integral temporal nonlineairy at 5 GSPS before timing calibration is about 600 ps as can be seen by the jitter of the overlaid waveforms.
In order to do a timing calibration, the firmware revison 13297 or later is required. The current software package 2.1 contains an updated firmware, but unfortunately one needs a Xilinx download cable to flash this new firmware (see http://drs.web.psi.ch/download/ under "Software Versions"). If some people want an update but do not want to buy such a cable, we offer a free update at our institute (just the postage has to be paid). The old evaluation board (Rev. 1.0, plastic housing) can unfortunately not be updated.
After the offset calibration is made, there are small (~20mV) short spikes left. They probably come from some cross-talk between the USB interface and the analog part of the board. This is currently under investigation. If new updates become available, they will be announced in this forum.
April 27th, 2009,
Many applications using the DRS4 need to measure fast rising signals, like for PMTs or MCPs. This short note shows the minimal rise-times which can be measured with different input signal conditioning.
The evaluation board contains four passive transformers ADT1-1WT from Mini-Circuits to convert the single-ended input signal into a differential signal. Although these parts are rated 800 MHz bandwidth (-3dB), they have hard time to drive the DRS4 inputs. This is because at high frequencies the input impedance of DRS4 becomes pretty small (~20 Ohm at 500 MHz) due to its capacitive nature. Furthermore, each transformer drives two DRS4 inputs (channel cascading) which enhances this problem by a factor of two. We made a quick test sending a signal to the evaluation board with a rise time of 277ps and a fall time of 280ps. The result measured with the evaluation board is seen here:
The measured rise-time (10%-90%) is only about 2ns. Disconnecting the second channel from each transformer improves this situation a bit:
so the rise-time comes down to ~1.6ns.
We tested the behavior using an active buffer ADA4937 to replace the passive transformer. Without the DRS4 connected to this buffer, we measured with the oscilloscope a rise time of 408ps and a fall time of 644ps. When we connect the DRS4 (single channel), this values increase to 702ps (rise) and 1400ps (fall), all measured with a differential oscilloscope probe (WL300 4 GHz Bandwidth, LeCroy 7300A, 3 GHz Bandwidth). In this case the rise time seen by the DRS4 is wieth ~700ps accordingly shorter:
(The signal was not properly terminated and therefore we have a small overswing).
To obtain an optimal rise-time measurement, the design of the input stage is rather important. A fast active driver seems to do a better job than a passive transformer (which was used on the evaluation board for power reasons). Connecting only one DRS4 channel to the input improves the rise-time measurement significantly. If channel cascading is still needed, a design should use one driver for each channel, and not driver two or more DRS4 inputs from a single buffer.
If anybody comes up with an even better input driver, I'm happy to publish the results here.
Please note the new datasheet Rev. 0.8 available from the DRS web site. It fixes the label of pin #76, which was AGND but is actualy AVDD. The input IN8+ is located at pin #20 and not at pin #19 as described in the old table 2.
Another tricky issue comes from the fact that the external TTL trigger and the comparator are in a logical OR. So if the comparator level is set such that the signal is always over the threshold, the trigger is always "on" and the TTL trigger does not have any effect. It is therefore necessary to set the analog trigger level to a very high value in order to make the TTL trigger work.
Several people mentioned that the external trigger input (TTL) does not work on the DRS4 Evaluation Board Rev. 1.1. This is not true. The requirement however is that the input signal must exceed approximately 1.8V. Since the input is terminated with 50 Ohms, not all TTL drivers may deliver enough current to exceed this threshold. To verify this, the trigger signal can be monitored with an oscilloscope at test point J24. Only if the input signal exceeds 1.8V, the signal will be seen at J24 and correctly trigger the FPGA. If the TTL driver is too weak, the termination resistor R9 can be optionally removed, but care should then be taken that reflections in the trigger input do not cause double triggers. The locations of the tap point for the input signal, the termination resistor R9 and the tap point J24 after the input level converter U5 are shown in this image:
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