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The end stations


The ammeters 

The current measurements (for instance, reflectivity with diodes or total electron yield from the sample) require the use of Keithley model 6517A.
 

Setting the BIAS
The ammeter has inside a VOLTAGE SOURCE, that can be used to bias (positive or negative) the samples respect to GROUND according to the Diagram 2 (negative bias). The Bias to the sample is given by the  'Bias with respect to GND' control. When the 'Bias with respect to GND' control is different from 0, the VOLTAGE SOURCE is ON, when the BIAS is 0 V the VOLTAGE SOURCE is off. When  the VOLTAGE SOURCE is ON (see the picture BELOW and diagram 2), the LOW of the VSOURCE is internally shortcircuited to the LOW of the ammeter, and the HIGH of the VSOURCE (RED PLUG) should be GROUNDED by connecting the RED PLUG to the GREEN PLUG of the BACK of the Keithley. In this way, the sample is BIASED to - the VSOURCE of the ammeter (for example, to bias the sample at 100 V, the Real Bias (V) control should be 100, corresponding to a Keithley Vsource=-100V). When the VOLTAGE SOURCE is OFF (Real Bias (V)=0),  see Figure UPPER PART and diagram 1. In this case, the sample is grounded (for XPS!!!!!) by connecting the LOW of the ammeter (BLACK PLUG) to GROUND (GREEN PLUG). In the case that the measurement is not ground referenced (for example, when measuring reflectivity with the photo current from a diode), this connection can be removed only in the case that a triaxial cable to the diode is used.    
 

INTEGRATION AND WAITING TIME
 The INTEGRATION TIME is the time in which the ammeter acquires data. It is given by the Number of Power Line Cycles NPLC (1 NPLC = 20 msec) times the number of readings N (N=1-100). The default value of integration time (basic mode)  is 0.4 sec (i.e. NPLC = 2, N = 10). The WAITING TIME is the time (to be set) in order the ammeter to stabilize. The default value of the waiting time is set to  0 msec. The time required to stabilize depends on the ammeter time constant according to the following table
 
range/gain       time constant
20 - 200 pA             1 s
2 - 200    nA           20 ms
2 - 200    uA             1 ms
2 - 20      mA         < 1 ms
 
During the waiting time, the ammeter measures but the data are not acquired. The following diagram shows the sequence of measurement:
Set of scan parameter accomplished (for example, the monochromator energy);
 Start of ammeter reading (but not sending data to PC) for a time interval equal to the waiting time;
 Start of ammeter acquisition (data filed);
 Stop of data acqisition.
 //<--parameter set--> <--waiting time--> <-- integration time-->//<--parameter set--> < --waiting time--> <-- integration time--> //<--param..
      
                         
>>>>>>>>>>>>>>>>>>>>>>time>>>>>>>>>>>>>>>>>>>>>>>>>>>>>time>>>>>>>>>>>>>>>>>>>


Scheme for current measurement: Diagram 1: No bias; Diagram 2: Bias to the sample 

 

Last Updated on Thursday, 31 August 2017 10:44