Re: peak detector design
In this circuit the input offset voltage of IC2 is multiplied by 10 (or R2/R1). This may be the cause of the error you´ve reported. If this is right then if you measure using different input voltages they will show the same error (about 0.1V). If this is the case I would like suggesting your using a low offset voltage op-amp in replacement of the (old) LM358 or using a potentiometer to null the offset. Also, to achieve faster results, try to decrease the value of C1 (1uF).At high frequencies it can be very small.
Furthermore, if you´re using 5V supplies I recomend you´re replacing the LM358 by a rail-to-rail output op-amp, since the output voltage span of the LM358 is very limited (please see it´s manual). 3V is it´s limit and this can be the cause of the error you´ve seen. So, make some tests at a lower input voltage to confirm.
You may also find some interesting tips in AppNotes AN1097 and AN1309.0 from Intersil.
Regards