update notes

git-svn-id: https://bucket.mit.edu/svn/nilm/zoom@6583 ddd99763-3ecb-0310-9145-efcb8ce7c51f

notes-a6302.txt

@@ -17,7 +17,27 @@ to scale the probe output.  Scale by (4.32) so that
   1mA at probe = 43.2mV at PIC
   37.5μA at probe = 1.62mV at PIC
 
-(PIC input) = clamp(0, 1.25V - (probe output * 4.32), 2.5V)
+(PIC input) = clamp(0, 1.25V - (probe current * 10 * 4.32), 2.5V)
 
 Clamp by putting a 1K in series with diodes to VSS/VDD.
 
+----------
+
+DAC range is ± 10 A.
+Probe range should be about 2^-14 as big, so ±610 μA
+which is 0.000610*10*4.32 = 26.35 mV.
+
+PIC measurements:
+  (1.25V + 26.35mV) / (2.50 / 2^12) = 2091
+  (1.25V - 26.35mV) / (2.50 / 2^12) = 2004
+
+--
+
+1 bit change at DAC is 1.2mA.
+At PIC, the voltage will change by (1.2mA * 10mV/mA * 4.32) = 0.518 mV
+This means changing 1 bit at the DAC should cause the PIC count to change by
+0.518 mV / (2.50 / 2^12) = 84.935
+
+To reset the PIC input to 2048, we therefore need to step the DAC by 
+((2048 - count) / 84.935)
+