Originally Posted by ozshadow
I am using the 4.7k resistor to set a gain of just under 6
I do not understand the idea of optimizing the resistor values. What do you suggest and what differences would it make ?
I will be running the tread at about 26.6 which by the time it comes out of the charging circuit, it should be about 26.0 flat.
Bandwidth is the operating range of your amplifier, with the values chosen previously, you would be running a functional equivalent of a low pass filter. This happens because the amplifier effectively doesn't amplify frequencies as much above the bandwidth. The higher above the cutoff, the lower the amplification. Your cutoff would have been somewhere in the audible range, probably 12-16kHz somewhere, depending on the tolerances of the opamp.
Basically, in a typical voltage feedback opamp circuit, gain and bandwidth share a generally linear relationship. I.E. if the gain is changed by a factor of 5, the bandwidth will change by a factor of 5. What we are doing in a multiloop is intentionally lower the overal bandwidth by using 2 loops. The inner loop sets the local gain and limits the bandwidth of the amp. The outer loop sets fairly close to the effective gain of the amplifier. The benefit of doing this is we can limit the frequencies outside of a preset range to gain stability, resistance to electrical noise, and improve phase margin. Basically you are going to set inner loop gain as (R5/R6)+1. This should be high such that GBP/ILG = somewhere from 60-100 kHz; this will be the operation bandwidth of the whole amplifier, the "cutoff" if you will. There are a couple guidelines in determining the whole set of values - general consensus is that R3 should be set somewhere above 220R, R5 should be 5-10x higher than R3, and lastly, you should avoid picking any of the values greater than 1M. Outer loop gain, which has a greater influence on the total gain of the amp is then adjusted using R4. Also, when picking values, remember the lower resistance will equal a lower noise floor and higher current use; the higher the resistance, the lower the current use but the higher the noise floor. From here, you want R1 = R3+R5. You are going to, of course, want to pick values that are available for purchase rather than arbitrary values.
Alright, I'm out of time. I can help you out or suggest some values for you in the morning if you want. PM me or respond here, I'll be around.
Edit - Wow, that was in a rush, fixed a bunch of spelling/grammar errors