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Farshid Jafari Harandi
01/12/06 13:21
Read: 2857 times
Sweden
Gothenburg


 
#107198 - Updated Schematic
Responding to: Kai Klaas's previous message
I know you have spend lots of time and efforts, but unfortunately the circuit needs some changes:

Thanks Dear Kai,
But definitely I'll have to sped a lot more and I could not get here without your help.

But the buffer following R5 and R6 needs to have a much higher input impedance than what an AD844 can offer. So, please use the FET-buffer at this place, shown in the schematic of bitscope. There's hardly an alternative to such a FET-buffer at this place.

Non-Inverting Input resistance of AD844 is specified at 7M ohm min. and 10M ohm typical ,Problem here that you are talking about should be that we are supplying this 10M ohm input by a High Impedance signal ?

In the Bitscope website a JFET buffer has been used. I remember that the author said A very High impedance input(~10^12 M ohm) OP-Amp have to be used after this JFET buffer since they are not good at providing enough output current, So i omitted the idea and fed the input signal directly to the Op-Amp input like what DSOA mk3 and Bitscope BT300 did.
My hesitate about using the FET input buffers is because they may need a lot of knowledge to design and their performance may degrade drastically over the time and temperature range. What do say about these problem ?

f you want to handle a signal range of 0.01V to 5V in 1:2:5 steps, as it common practice with standard scopes, then you need three separate dampening paths in parallel at input of FET-buffer, which provides a dampening of 1, 10 and 100. This also, because the AD844 being used later in the signal chain can only handle signals smaller than +-1V at input

So yo mean , we divide the input signal So we would try to have not more than 0.5 Vp-p at the first Op-Amp's input ?
By the X2 gain of the Op-Amp then We will have Max. 1Vp-p at It's output Then this would be divided by 1 or 2 or 5 and fed to the second Op-Amp. This should give us these Voltage swings at input :
+/- 50mv (at max x20 gain) ,+/- 100mv, +/- 250mv, +/- 250mv, +/-500mv, +/-1v, +/-2.5v, +/-5v, +/-10v, +/-25v (at Gain X 2/50)
Thanks for your great suggestion. This will give me up to .3mV precision (in X20 Gain).

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 ?

Here is the new Design :
http://www.8052.com/users/far...pdated.GIF

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

- 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 ? (After this much effort i got interested into 40MSPS TDA8703)

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

- FET buffer is incomplete ,cause I don't know how to build a FET amplifier. What N-channel FET i have to look for ? I'll take a look at some providers website to see what I could find available.

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

Again I appreciate all your help,

Farshid


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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