BEAM DIGITIZER Diagram & comment
The BEAMDIGITIZER is a fore channel current to frequency converter. The first diagram consist of the channel select circuit and the second diagram is the current to frequency converter. The dynamic range goes from 10pA to 200µA.
Diagram part 1 :
Download diagram-1 (15k) in PostScript or diagram-1 (35k) in Protel format.
Diagram part 2 :
Download diagram-2
( ) in PostScript or diagram-2 (74k) in Protel
format.
Comment :
Block diagram (of diagram part 2) :
The current (pos.) is runs to C11 which results in a negative voltage on IC6,pin6. For positive current IC7 will invert the voltage so that we have a pos. voltage on R15. For negative current the voltage on IC6,pin6 is all ready positive and the inverting is therefore not necessary. R15 and C13 combine to a RC-circuits which the off time is depending (se figure). IC8 is comparing VC13 with VR18 (370mV) and for VC13 > VR18 IC8,9 is going low and is trigging the monostable multivibrator IC9 which output is trigging the monostable multivibrator IC10. The time IC10 is active is pulse with of the Digital output positive pulse (ln2C17R22=150nS).
The hole circuit is based on a feedback loop and until now we have seen that the accumulated voltage over C11 is resulting in a 125nS pulse on IC9,pin6. With this pulse we either activate the circuit +Vref Q4-6 (neg. current in) or -Vref Q7-9 (pos. current in) so the charge [1] in C11 decrease which results in a reduced voltage over C11. With a reduced voltage it will take longer time before VC13 reach the trigger voltage on VR18. The discharging of C11 is resulting in a negative feedback and for higher input current will frequency rice to optician the balance between added charge and removed charge.
Footnotes
[1] The charge there has been removed from C11 is a constant amount for every pulse (125nS).
Last Modified 11 January 2019