Download LTgamma System Descr..

APD Long Term Gamma Test Description
Apparatus
There are two identical test PCBs, which each hold 20 APDs. 16 of the APDs on each PCB are
arranged in a rectangle in the center of the board, and have the Integrator/Sample/Hold amplifiers
supplied by A. Dolgopolov. The other four are outside the rectangle. One of the test boards is in a
steel box bolted to the source assembly, which will expose the center 16 APDs to the gamma
flux, and the other is in an aluminum box of the same dimensions, which will act as a nonirradiated control.
A single 0-500v bias supply for both boxes is controlled through a DAC output on the Agilent
34970A. Each test PCB contains a voltage divider for monitoring the actual bias value. Only the
Value from PCB1 is used in the calculations, with the second displayed and recorded mainly as a
telltale for problems.
We designed a sequencer/interface PCB which accepts TTL signals from the DIO port of the
Agilent 34970A for control of the pulser. The sequencer PCB takes care of the timing of the
nanosecond Gate/Start/Hold sequence to the Sample/Hold amps (see Figure 1) and the Minsk
LED Pulser. Between measurements, a 200hz astable multivibrator to maintain pulser stability
continuously triggers this sequence.
Temperature of the APDs is maintained within +/-0.1 degC. In each box, a spring loaded
aluminum plate is held in thermal contact with the APDs using gap filler. Two 100 ohm platinum
resistance thermometers (RTDs) are mounted on each plate, one monitored by the data acq
system, the other used as feedback for a proportional (PID) temperature controller. The
controllers, one for each box, vary 24VDC power to foil heater elements on the plates as needed
to hold the temperature at the setpoint, 35degC.
Pulse measurements are taken at a fixed bias voltage by the Labview application on a time
interval in seconds. While either the red or blue LED can be used, red is being used due to its
higher output. When a pulse measurement needs to be made, the astable is stopped, and the last
held values are immediately read from pin 6 OUT of the sample/hold amps by the 34970a using a
low resolution, fast scan. All 32 pulse channels are read in about 1 sec. Bias and temp readings
are also taken after the pulse channels.
The DC gain and breakdown measurements are taken on a longer time interval, and include four
additional APDs (117-120, 217-220) on each circuit board, which have no pulse amplifiers, but
identical series resistances. This measurement sequence takes about 20 minutes to complete,
since the high resolution (10 power line cycle integration) scan must be used to obtain the gain
curves. Again, the astable LED trigger is stopped during this procedure. For each supply bias
value, current is calculated from the voltage drop across the 10kohm resistor in the anode leg of
each APD, and then the individual biases are factored from the supply value using the known
values of the series resistors. First the dark current curves are measured, then the Constant (blue
only) LED output is turned on, and the total current curves are measured. Then Vr and Id are
calculated for gain 50.
Immediately after the DC Gain measurements are finished, the bias is ramped up to a value
above breakdown for all APDs and the zener breakdown voltage is read using a fast scan. The
series resistors in each APD circuit limit the breakdown currents. Again, actual APD voltage is
factored from the common supply voltage using APD current and the known series resistances.
The entire Breakdown step takes about 5 seconds, after which the APD bias is ramped back
down and astable pulsing is resumed until the next measurement.
Figure 1 LTgamma input circuit (1 of 32) showing APD and Integrator/Sample/Hold Amplifier
Data Description
Four data files are created and written to during operation of the tester. One file is maintained for
the calculated results of each of the Pulse, Vr, and Breakdown tests, with a new line appended
for each repeat. An additional file for the raw data used in each Vr test is kept with a more
complicated structure consisting of a multi-line table of data per repeat test.
All files are tab delimited ASCII, with two header lines. The top line contains instrument settings
and the second line contains column headers for the data. APD numbering convention is:
1xx=non-irradiated, 2xx=irradiated. New data files are opened each time the Labview program is
started. Filenames have the date/time of the start of execution embedded.
Truncated examples of all four files have been imported into an Excel workbook file,
LTGamma_Datafile_Examples.xls, imported as text so the data appears exactly as written to the
files.
Pulse Response Data File
Pulse response data is written to the yymmdd_hhmm_Pulse_Tests.txt file. The top line headers
document voltmeter range, integration time, and the number of repeat measurements being taken
and averaged. At this writing, the range and integration headers say “Gain xxx” and actually
indicate the integer value of an enumerated type control where the string selections should have
been**. The decoded selections are:
Range (full scale):
0=100mV, 1=1V, 2=10V, 3=100V, 4=300V
Integration time (number of power line cycles):
0=0.02NPLC, 1=0.2NPLC, 2=1NPLC, 3=2NPLC, 4=10NPLC, 5=20NPLC
Column headers form the second header line. They are as follows with a description of column
data:
Date/Time
Timestamp in mm/dd/yyyy hh:mm:ss format
Bias1
Bias volts from divider on non-irradiated PCB
Bias2
Bias
“
“
“
irradiated
PCB
PCB1 degC
RTD temp on heater plate non-irradiated PCB
PCB2 degC
RTD temp
“
“
“ irradiated PCB
Amb degC
Type J TC in air near 19” rack
APD101
Sample/Hold volts from amplifier on APD101
……….
APD216
Sample/Hold volts from amplifier on APD216
DC Gain Data File
Data calculated from the total current and dark current curves are written to the
yymmdd_hhmm_DC_Gain_.txt file. The first line headers document voltmeter range, integration
time, voltmeter impedance, and number of averaged repeats (in plain English). Impedance is
unfortunately labeled #Scan/step**. Second line column headers and data descriptions follow:
Date/Time
Timestamp in mm/dd/yyyy hh:mm:ss format
PCB1 degC
RTD temp on heater plate non-irradiated PCB
PCB2 degC
RTD temp
“
“
“ irradiated PCB
Amb degC
Type J TC in air near 19” rack
Vr101
Gain=50 operating voltage for APD101
………
Vr220
Gain=50 operating voltage for APD220
Id(uA)101
Gain=50 dark current for APD101
………
Id(uA)220
Gain50 dark current for APD220
M1uA101
Calculated gain=1 photocurrent in microamps for APD101
……….
M1uA220
Calculated gain=1 photocurrent in microamps for APD220
M1StDev101 Standard deviation of Gain=1 calculation APD101
……….
M1StDev220 Standard deviation of Gain=1 calculation APD220
** These mistakes(mine, -BG) caused by copying and pasting Labview front panel controls and
forgetting to set new values as defaults.
DC Raw Data File
Raw data used in Gain 50 Vr calculations is saved in the yymmdd_hhmm_DC_Raw.txt file. The
first line headers document voltmeter range, integration time, voltmeter impedance, and number
of averaged repeats. Each time the Vr test is run, a single time stamp is written alone on a line
followed by a line of data for each bias step, first for the dark current curve, then the total current
curve. Second line column headers and data descriptions follow:
Date/Time
PCB1 Bias
PCB2 Bias
PCB1 degC
PCB2 degC
Amb degC
Bias101
…..
Bias220
uA101
…….
uA220
Timestamp in mm/dd/yyyy hh:mm:ss format
Bias volts from divider on non-irradiated PCB
Bias
“
“
“
irradiated
PCB
RTD temp on heater plate non-irradiated PCB
RTD temp
“
“
“ irradiated PCB
Type J TC in air near 19” rack
Actual APD101 bias factored from current and resistance
Actual APD220 bias factored from current and resistance
APD101 current in microamps at this step
APD220 current in microamps at this step
DC Breakdown Voltage, Vb
Data from the breakdown test is written to the yymmdd_hhmm_DC_Brkdn_.txt file. The first line
headers document voltmeter range, integration time, voltmeter impedance, and number of
averaged repeats. The second line headers are as follows:
Date/Time
PCB1 Bias
PCB2 Bias
PCB1 degC
PCB2 degC
Amb degC
Vb101
……….
Vb220
uA101
……….
uA220
Timestamp in mm/dd/yyyy hh:mm:ss format
Bias volts from divider on non-irradiated PCB
Bias
“
“
“
irradiated
PCB
RTD temp on heater plate non-irradiated PCB
RTD temp
“
“
“ irradiated PCB
Type J TC in air near 19” rack
APD101 calculated breakdown voltage
APD220 calculated breakdown voltage
APD101 breakdown current, microamps
APD220 breakdown current, microamps