TDC-GPX acam messelectronic gmbh, TDC-GPX Datasheet - Page 25

• Manufacturer Part Number: TDC-GPX
• Description: Precision Time Interval Measurement
• Manufacturer: acam messelectronic gmbh

Start-Offset
For several reasons a mathematical offset is added to
the stop time. One reason is to allow handling Start-
Stop intervals down to 0 and even less. This ‘StartOff1’
is set in register 5 in multiples of BIN and it is 18 Bit
wide. Internally the start offset is added to the time
measurement result and has to be subtracted from
the value read from the TDC. The other reason is to
allo the ALU to look "into the past" without handling
negative values (which the ALU could not do).
Figure 20
The real time interval Start to Stop is calculated from
the output data as (Start# >0):
Adjusting the Start-Offset
The Start-Offset register of the TDC-GPX allows the
compensation for the offset due to the different inter-
nal delays. It allows to do measurements down to 0 ns
time intervals between Start and Stop. The correct
setting should be done by experiment.
Procedure for single start applications:
1.
2.
3.
4.
5.
Time = (hittime - StartOff1 + Start01) * resolution +
Asynchronous
Start
Set StartOff1 = 0
Apply Start and Stop signals with a short delay
(e.g. 12ns)
Step down the interval and look at the output
data. They are getting smaller and smaller until
you pass the internal Start time stamp. The out-
put data then jumps to a very high value.
Take the time interval t
where this happens
Calculate StartOff1 = t
value into register 5, StartOff1. In case you ex-
pect negative values add an additional amount X
to StartOff1 and substract this value later on
from your output data
(Start# - 1) * (StartTimer +1) * Tref
acam-messelectronic gmbh - Am Hasenbiel 27 - D-76297 Stutensee-Blankenloch - Germany - www.acam.de
Start01
Start# 1
Time
SyncPeriod
cross
cross
Start#
/ BIN and write this
(from your generator)
Start# 2
Output data
hit time
Time data
StartOff1
Start# 3
25
With start retrigger the value StartOff1 should be set
to
The reason is that the ALU is internally stopped for
about 50 ns during a start retrigger to add the new
start time stamp. If there is a hit on each channel
during this period the ALU will need
4 x 25 ns + 50 ns = 150 ns to transfer those data to
the IFIFO. The ALU cannot handle negative values.
start period and reducing the start number by 1.
By default the Start and Stop inputs of the TDC-GPX
are open immediately after a reset.
The consequence for the Stop channels is that even
hits coming before a Start pulse will be measured. The
bit StopDisStart in register 5 disables the Stop chan-
nels until there is a Start pulse. A consequence is that
there is a minimum delay of 7 ns after the Start before
hits are accepted on the Stop channels.
With single-start and internal start-retrigger only the
first Start pulse is used for the measurement. In cases
where more than one Start pulse is expected this
might cause an overflow of the Hit FIFO of the Start
channel. This will produce garbage data. There we
recommend setting the StartDisStart bit in register 5.
A ‘1’ disables the Start channel after the first Start
pulse.
Internal Data Processing
The raw values of the stop events are stored in 32-
stage Hit FIFOs. One bit is added indicating the slope of
the signal. This Hit FIFO can be filled with data at a
peak rate of 182 MHz.
The following pipelined post-processing unit is respon-
sible for compression, Start selection, correct Stop-
Start subtraction and adding the start number to the
output data. Subsequently a collection unit transfers
StopDisStart & StartDisStop
time
Therefore the start offset is added so that
the ALU can handle the data being collected
during the break. In case the input data rate
is higher (bursts) it will be necessary to in-
crease also the offset value , e.g. to 10,000.
The start retrigger adds some indeterminacy
– due to the 25ns reference – to whether a
stop refers to the old start or the new one.
This is not an uncertainty. Each time stamp
that is negative after offset substraction can
be remapped to the old start by adding the
StartOff1 = 2,000 ( 162 ns).
TDC-GPX