In the DRL circuit show in the figure
why do we need the voltage follower ( can't we connect directly to inverting amplifier) ?
why do we use the capacitor 68pF, how do we choose the value?
why is 220pF used in the high pass filter after IN-amp ?
The follower is needed to avoid loading on the Rg pins, which are low-impedance. Any current sourced or sunk by the inverting amp to maintain its virtual ground would have to flow through the 24.9k resistors as well as the internal 24.7k resistors, causing large errors.
The 68pF is needed to maintain stability for the loop. Here is an EETimes article with a nice discussion and some sims on this.
As for the 220pF, I'm not completely sure why it was done this way. In the forward direction it doesn't really do anything except a very slight capacitive divider beyond the cutoff frequency, but from the 2.5V reference's point of view, it introduces an extra zero at about 700Hz. My guess is there may have been some overshoot when the reference was turned on, so this zero was added to increase phase margin and improve the step response. In a real-world system, it may be more practical to switch in a smaller resistor in parallel with the large resistor for some time after power-on to reduce turn-on time. As shown, it could take almost 25 seconds after power-up for the HPF to get within 1% of its final DC value.
I hope this helps.
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