Electronics > Beginners

Virtual ground resistor choice

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TimNJ:
A voltage divider is a simple way to create a split supply for a circuit.

My question is, what do you have to consider when choosing the two resistors? What are the pros and cons of using high/medium/low value resistors?

Paul Price:
Using low value resistors increases power drain, something you don't want to do if you are running on a battery. Use the highest value resistors as possible and they both should be identical in value.  Too high a resistor and the offset currents of the opamp or the inputs of the virtual ground generating device could cause a small offset in virtual ground or cause your circuit to be more likely to be upset by electrical interference.

c4757p:

--- Quote from: Paul Price on May 20, 2013, 11:52:57 pm ---Using low value resistors increases power drain, something you don't want to do if you are running on a battery. Use the highest value resistors as possible and they both should be identical in value.

--- End quote ---

An interesting, semi-related note on this - one of the option boards for the Tek 24x5B series of scopes has a number of very low impedance voltage dividers, for level shifting around an ECL chip. Seeing 70 ohms across the 5V rail on a digital board was enough to make me think it was faulty...  |O The board as a whole takes 1A just to sit and twiddle its silicon thumbs. I am guessing there was a speed-related reason for this, though (it's the counter/timer board). Not familiar with ECL.

Paul Price:
C4757P:  ECL operates at GHz frequencies and the lower the impedance, the closer all wiring approaches a transmission line low impedance, the better the rise and fall times and the fewer the reflections from point to point. ECL is power hungry and performance poor and so it is no more.

c4757p:
The really fun part is the level shifting back from ECL levels to TTL. In the high speed locations, they've accomplished that with discrete long-tailed pairs made out of 2N3906 (not sure if they're supposed to be matched or anything). The slow signals get a 339 comparator. Were there really no affordable ECL-TTL level shift chips in the late 80s when ECL was common?

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