| ??? 01/14/06 14:22 Read: times Msg Score: +1 +1 Good Answer/Helpful |
#107392 - Answers IV Responding to: ???'s previous message |
Farshid said:
LM741's output could swing from -3 to -1 v. Still way too high! Remember, output voltage of 741 is mulitplied by factor 470/(27+25)=9 by the AD844! Again, only -0.177V is needed at C20 to provide the proper shifting. Farshid said:
Would it be better if i move the ADC to Digital PCB ? As the ADC must be read out very fast, the ADC must sit near the digital section. As also the AD844 must not be separated too far from ADC, it's best to have the analog and digital section on same board. Farshid said:
Would it be better to provide different regulators for the Analog and Digital supply (like the Bitscope) ? If a higher resolution than 8bit was needed, then the use of separate regulators was a must. In your application you can try to use only one supply. But then you will need really proper filtering of the supply lines. In any case, use separate ground planes! Farshid said:
LM7805 provides 3 mV ripple rejection , Is this ripple acceptable or do I have to go for a better regulator ? Should be good enough. Farshid said:
Could I Use BL02RN02 ferrite beads instead of the 4.7R resistors which are serried with the Op-Amp supplies (If yes, Do I have to use a serried resistor also ,considering that I'll use a .22u Tantalum for decoupling) Use the ferrites additionally to the resistors, means in series to them. 0.22µF tantals are not adequate! Use 10µF tantals in parallel to 100n X7R. Farshid said:
Trigger Circuit of Bitscope uses 74AC86 , this chip provides exactly Vil,Vih=Vcc/2 at it's inputs where HC provides Vih=3.15,Vil=1.35 (Vcc=4.5V). Could I solve this problem by chaing the 2 resistor's at the input circuit ? There does not exist such a difference between ACMOS, AHCMOS and HCMOS. The one data is a typical, the other is min/max data. Above chip families do all toggle at about Vcc/2 at input. By the way, Bitscope uses 74AHC86, not ACMOS variant. Farshid said:
Woudln't it be better to use a fast Op-Amp like LM319 to pick up the trigger signal and have the ability to move up/down the trigger point ? I cannot help here, because I do not exactly know, how Bitscope uses this trigger. Farshid said:
I corrected some mistakes , now i can't find anymore , Please let me know if you see one. You must take into account, that R22 and R24 see R1 in parallel to them. So, with your choosen values you will not get proper 10:1 and 100:1 dampening. Please use my values. More, relays K7 and K8 do also need the ground connection. Otherwise stray capacitance of open contacts will violate the dampening perfomance. Connect a cap from pin3 of IC3 to ground. Kai |
