| ??? 01/14/06 00:44 Modified: 01/14/06 00:49 Read: times |
#107367 - Answers III Responding to: ???'s previous message |
Farshid said:
Since you have a lot of experience using Scopes did you used this feature so far ? Yes, very often. In the old days of analog scope I turned-off all the lights, sat directly in front of the scope with my nose nearly hitting the screen and looked at the screen like the mouse looks at the snake. There were lots of situations where I had to use all these subtil features the scope offered... Farshid said:
I have a question in my mind, Consider we want to fed a signal from our MCU to an analog cihp like 4052. Here Current will flow from Digital VDD to the Analog Ground. Is this alright or we have to use a buffer here ? No buffer, but a low pass filter. Use separate ground planes for analog circuitry and microcontroller stuff. The 4052 is a typical example for a circuit, where both worlds meet again. Here, feed the signal from microcontroller to 4052 over the barrier (gap between the ground planes) via a low pass filter: Locate a low pass formed by 4k7 resistor and 100pF cap directly at the barrier, but on digital side. Now, feed the filtered signal via 4k7 resistor across the barrier, where this resistor traverses the gap like a bridge does over a river. Directly at gap, but on analog side connect an additional 100pF cap from signal to ground. So, this arrangement looks like RCRC. As the rise time of this signal is drastically lowered, sometimes a Schmitt trigger might be needed to restore the edges of signal again. But here, with the 4052, this is not needed. The advantage of this arrangement is, that no relevant digital currents are flowing along the analog section and by this contaminating the anlog ground! Farshid said:
Is the offset circuit alrigt ? The range of adjustment is much much too high. The sense of this offset shifting is to match the signal to input voltage range of ADC. For this, only about -0.177V must be produced by LM741. Also, take care that C20 doesn't become reverse polarized! A diode connected in parallel to C20 can help. Additionally, insert a resistor of 220R between the output of LM741 and the caps. Farshid said:
I placed those Caps between the Inverting input to the ground. Did i do what you meant ? Yes. By the way, C11 should be 100nF. Farshid said:
I'll try to find the best ones , and as Bitscope author says I'll try to measure the Ids with Vgs=-0.6 to find a matched pair. I guess, at Vgs = -0.6V Ids will be about 5mA for the BF245B. Take care, FETs show drastical manufacturing tolerances, much much higher than bipolar transistors. So, you will need a handful of them to get a properly matched pair. In the Pre-Gain Division Stage there are still some errors! Kai |
