SLAC National Accelerator Laboratory
BL4-2 Biological Small Angle Scattering/Diffraction

Photodiode Intensity Monitor / Beamstop

The preamplifier is a 2-channel ultra-low drift current-to-voltage converter. The schematics were designed using ultra-low current noise JFET input operational amplifiers; thus they do not contribute significantly to the total system current noise and, in addition, do not adversely affect the system resolution. The preamplifier is located in a separate aluminum enclosure and is connected to the controller chassis by a 50ft twisted pair shielded cable. This configuration provides high ambient noise cancellation.  The bandwidth (BW) of the preamplifier depends on the gain setting and is tabulated below for the AXUV-12 photodiode:
 

Gain (V/A) Bandwidth (Hz)
104 10000
105 7000
106 4300
107 3500
108 1000

The gain match between channels is better than 0.2% due to the use of precision 0.1% 5ppm feedback resistors.

The controller is located in a double width NIM module. Besides providing the remote preamplifier gain control this unit also has 2-channel synchronously controlled low pass filters.
 

Front Panel Control Function
POWER  Power switch with LED power indicator
TEST Panel meter test switch
Digital Panel Voltmeter DPM
Four saturation indicators 
(A-channel on the left and B-channel on the right side)		 When the input signal exceeds the positive or negative linear range of the preamplifier one or both LED indicators on the correspondent channel light up
DPM mode  This switch provides 3 DPM modes:
A - channel A 
A-B - the difference between 
channel A and channel B 
B - channel B 
GAIN 4, 5, 6, 7, 8 (corresponding to 104 through 108 amplification)
FILTER  1, 2, 3, 4 (corresponding to 10 Hz, 100 Hz, 1000 Hz, and 10000 Hz, respectively)
OFFSET (Channel A and B) When these switches are ON the user may add DC offset to A and B outputs 
COARSE (Channel A and B output offset coarse regulators) 10-turn precision potentiometers that can offset the channel output to ±10VDC
FINE (Channel A and B output offset fine regulators) 10-turn precision potentiometers that can offset the channel output to ±0.25VDC
OUT A and OUT B  Channel A and B BNC outputs; 
A-B BNC output is located back panel of the NIM module 

The photoamplifier system is ready immediately after the power is turned ON. However, it is recommended that the user let the system warm up for ~45 minutes in order to minimize the temperature drift of the electronics. The offset option may then be used to compensate the DC output shift.  However, the user should be careful as the OFFSET is implemented in the controller schematic and therefore will have no affect on the input preamplifier stage. Thus, if the input signal is about the preamplifier full scale range (±10V) and some input DC offset is coming form the photodiode, the usage of OFFSET will not prevent output saturation. In this case the user should first reduce the gain and then try to compensate the total output DC shift with the OFFSET option.