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Kai Klaas
01/13/06 06:53
Read: 1329 times
Germany


 
Msg Score: +1
 +1 Good Answer/Helpful
#107279 - Answers II
Responding to: Farshid Jafari Harandi's previous message
Farshid said:
About the Offset Shifting, My 20 MHz standalone Scope have a ground adjust dial. Sometimes i fed a 12VDC to my scope with V/Div placed at the 10mV section .Then I lower the signal till it gets visible on the screen. This gives me a nice way to see how much noise exists in this signal. I like to have this feature in This DSO , I know i could do this by using the AC Input but I think maybe this feature could be useful somewhere, but I can't think of anything right now ! :)
This should be hard to implement I think ?

To implement this is very easy. You just have to add a big offset. On the other hand, then your signal will clip somewhere in your circuit. So, you need a circuit, which can easily be overdriven without corrupting the signal in the seen window. This means some intensive designing and testing, but is possible.


Farshid said:
I have connected Both the input and output unused pins of the 4052 to the Analog ground , is it alright ?

Yes, grounding is always good in HF circuits...


Farshid said:
By removing the 'additional cap between inverting input and signal ground' then we have lower bandwidth .I would like to have the possibility to increase the bandwidth , So could i do this later by soldering a simple Cap ?

Take care, this is not a standard procedure to increase bandwidth with HF OPamps, but will very probably result in wild oscillations and heavy instability! Connecting caps in the feedback loop is a big no no, when working with HF OPamp. Datasheet of AD844 only recommends this for high gains, at about 100 or so. Then this cap can help a bit. But if you connect it at gains lower than 100, then instability will occur.
There's no need for this capacitor at gains like 2 and 10. By the way, if you have a look at figure 11, 16 and 21, then you will notice, that there's already a peak in the gain versus frequency plot. This is an indication that the additional cap would result in a disaster. So, please omit it!


Farshid said:
Offset section is incomplete also , please verify if the resistor/capacitor values are correct.

Anyhow you produce the offset voltage there, you should have caps connected to ground as it's shown at the bitscope's schematic.
Maybe it's not a good idea to use this transistor arangement for AD844, although it makes sense with the MAX477. AD844 and MAX477 need not to have the same temperature dependency of offset voltage. So, just connect the output of a LM741 (or similar) to superimpose the offset. But, again, don't forget the caps to ground.


Farshid said:
FET buffer is incomplete ,cause I don't know how to build a FET amplifier. What N-channel FET i have to look for ?

BF245B seems to fit well. Do select them in pairs, though, as mentioned at bitscope's schematic.


Farshid said:
Are the Resistors' value In feedback of the Op-Amps correct ?

Use two 300R resistors for the gain +2 section. Use a 470R resistor and a 25R plus 27R resistor (=52Ohm) for the gain +10 section.

Kai

List of 87 messages in thread
TopicAuthorDate
20Mhz Analog Input of a DSO      Farshid Jafari Harandi      01/05/06 15:26      
   Link      Kai Klaas      01/06/06 05:43      
      why go for 20MHz      Erik Malund      01/06/06 07:07      
         No 4 layer PCB & faster ADC available      Farshid Jafari Harandi      01/07/06 03:56      
      Initial look ...      Farshid Jafari Harandi      01/07/06 03:51      
         Trigger Circuit      Farshid Jafari Harandi      01/07/06 07:55      
            Try of explaination      Kai Klaas      01/07/06 14:21      
               Badly drawn      Steve M. Taylor      01/07/06 14:47      
               Trigger and Amp      Farshid Jafari Harandi      01/07/06 17:18      
                  Take care...      Kai Klaas      01/07/06 18:26      
                     Speed      Steve M. Taylor      01/08/06 05:32      
                        Like This ?      Farshid Jafari Harandi      01/08/06 07:52      
                           Just wait...      Steve M. Taylor      01/08/06 09:12      
                              so what do they do there ?      Farshid Jafari Harandi      01/08/06 10:29      
                                 Direct Digital synthesis      Steve M. Taylor      01/08/06 10:51      
                                    I just can think about one benefit      Farshid Jafari Harandi      01/08/06 13:09      
                     Searching...      Farshid Jafari Harandi      01/08/06 07:44      
                        Misinterpretation      Kai Klaas      01/09/06 06:20      
                           Proper Op-Amp      Farshid Jafari Harandi      01/09/06 16:49      
                              AD844 has 50 Ohm input resistance on -      Frieder Ferlemann      01/10/06 02:04      
                                 inverting input resistance      Farshid Jafari Harandi      01/10/06 09:05      
                              specifications      Kai Klaas      01/10/06 07:24      
                                 sampling      Farshid Jafari Harandi      01/10/06 09:14      
                                    Of course      Kai Klaas      01/10/06 09:27      
                                       Yes but      Steve M. Taylor      01/10/06 14:53      
                                          Exactly!      Kai Klaas      01/11/06 06:15      
                                 Preamp Initial Schematic      Farshid Jafari Harandi      01/10/06 21:18      
                                    please resubmit the schematic in a forma      Erik Malund      01/11/06 06:24      
                                       Real size button      Kai Klaas      01/11/06 06:39      
                                          not quite good enough      Erik Malund      01/11/06 06:53      
                                             Higher Resolution      Farshid Jafari Harandi      01/11/06 07:17      
                                       Resize      Farshid Jafari Harandi      01/11/06 06:56      
                                          I do not care if it fits, I can move it      Erik Malund      01/11/06 07:21      
                                             readable with gthumb      Frieder Ferlemann      01/11/06 08:17      
                                             They are1N4148s      Farshid Jafari Harandi      01/11/06 08:18      
                                             Problem is at your end      Ian Bell      01/11/06 12:04      
                                                do you refer to the first submit      Erik Malund      01/11/06 12:09      
                                                   Web Browser      Steve M. Taylor      01/11/06 13:28      
                                                   Both      Ian Bell      01/11/06 14:17      
                                    Some changes needed      Kai Klaas      01/11/06 19:05      
                                       have to sleep a little ,will post later      Farshid Jafari Harandi      01/11/06 22:19      
                                       I would try this      Kai Klaas      01/12/06 07:20      
                                          not an analog eagle      Erik Malund      01/12/06 07:34      
                                             Why not      Kai Klaas      01/12/06 07:58      
                                                not objecting, just wondering      Erik Malund      01/12/06 08:31      
                                          Updated Schematic      Farshid Jafari Harandi      01/12/06 13:21      
                                             FETs      Farshid Jafari Harandi      01/12/06 13:39      
                                                I remember going to the races on Thursda      Erik Malund      01/12/06 13:45      
                                                   Yes, Nowadays they're held at friday      Farshid Jafari Harandi      01/12/06 14:13      
                                             Answers      Kai Klaas      01/12/06 19:01      
                                                JFET Input good      Farshid Jafari Harandi      01/12/06 21:30      
                                                   Deviations      Kai Klaas      01/13/06 07:06      
                                             Answers II        Kai Klaas      01/13/06 06:53      
                                                Corrected mistakes      Farshid Jafari Harandi      01/13/06 10:31      
                                                   Answers III      Kai Klaas      01/13/06 17:44      
                                                      RE : Answers III      Farshid Jafari Harandi      01/13/06 19:05      
                                                      new questions about PCB/Supply      Farshid Jafari Harandi      01/13/06 20:48      
                                                         Answers IV        Kai Klaas      01/14/06 07:22      
                                                            RE : Answers IV      Farshid Jafari Harandi      01/14/06 12:29      
                                          How do adjust the cap trimmers?      Kai Klaas      01/15/06 18:06      
                                             Thanks for everything      Farshid Jafari Harandi      01/16/06 02:29      
                     Found these      Farshid Jafari Harandi      01/08/06 13:06      
            inductance      Kai Klaas      01/10/06 09:39      
               Relays      Farshid Jafari Harandi      01/10/06 21:24      
                  Reed-relay        Kai Klaas      01/11/06 07:57      
                     Reed Relays      Farshid Jafari Harandi      01/11/06 08:19      
         Plays no significant role      Kai Klaas      01/07/06 13:59      
            but they are sensitive      Erik Malund      01/09/06 10:36      
               Analog Mux      Farshid Jafari Harandi      01/09/06 16:42      
                  today Philips announced this MUX      Erik Malund      01/10/06 07:21      
                  Strac capacitance make the worse      Kai Klaas      01/10/06 09:22      
                     Sorry, stray not strac      Kai Klaas      01/10/06 09:23      
   a better aproach maybe      Jez Smith      01/12/06 08:27      
      PLD question      Farshid Jafari Harandi      01/12/06 14:15      
         Re PLD questions      Jez Smith      01/12/06 15:57      
            link      Jez Smith      01/12/06 16:05      
            Go for CPLD or not !      Farshid Jafari Harandi      01/12/06 20:28      
               Not in that timescale      Jez Smith      01/13/06 00:22      
                  so about 10K gates are needed ?      Farshid Jafari Harandi      01/13/06 12:46      
                     part numbers      Jez Smith      01/14/06 01:10      
                        Wow pretty expensive !      Farshid Jafari Harandi      01/14/06 02:18      
                  I'll have to wait till tommorow      Farshid Jafari Harandi      01/13/06 13:09      
   is this helpful?      Erik Malund      01/12/06 14:25      
      good link      Farshid Jafari Harandi      01/12/06 20:22      
   Heres a start      Jez Smith      01/13/06 00:59      
   'must read'        Rob Klein      01/14/06 07:25      
      What a great reference !      Farshid Jafari Harandi      01/14/06 12:33      

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