??? 01/06/06 12:43 Read: times |
#106690 - Link Responding to: ???'s previous message |
A working circuit can be found here:
http://www.bitscope.com/design/ Especially, have a look at this: http://www.bitscope.com/design/hardware/ Take care, designing circuits with these special HF-OPamps is tricky, because the least amount of stray capacitance seen from inverting input to signal ground will make the Opamp oscillate. So, all designs, which connect more components to the inverting input than the obligatory needed feedback resistors will not work, especially if they relevantly increase the stray capacitance there. Also, interfacing a high impedant probe is difficult for these HF-OPamps, because they are built to interface 50...75Ohm lines. That's the reason, why in the above link the first buffer is made of discretes. Take also care, that every capacitive load will also make these HF-OPamps to oscillate. This means, that even connecting a scope directly to the output will result in oscillation. If load capacitance increases, a well choosen resistance at output is needed to prevent instability. The correct value can be found in datasheet. This is the more true the faster the HF-OPamp is! Kai |