tweak resid dac leakage. patch by leandro nini

git-svn-id: https://svn.code.sf.net/p/vice-emu/code/trunk@45175 379a1393-f5fb-40a0-bcee-ef074d9b53f7

vice/src/resid/dac.cc

@@ -40,6 +40,12 @@ static union MSVC_EVIL_FLOAT_HACK INFINITY_HACK = {{0x00, 0x00, 0x80, 0x7F}};
 namespace reSID
 {
 
+// "Even in standard transistors a small amount of current leaks
+//  even when they are technically switched off."
+// https://en.wikipedia.org/wiki/Subthreshold_conduction
+static const double MOSFET_LEAKAGE_6581 = 0.0075;
+static const double MOSFET_LEAKAGE_8580 = 0.0035;
+
 // ----------------------------------------------------------------------------
 // Calculation of lookup tables for SID DACs.
 // ----------------------------------------------------------------------------
@@ -73,6 +79,8 @@ void build_dac_table(unsigned short* dac, int bits, double _2R_div_R, bool term)
   // double vbit[bits];
   double vbit[12];
 
+  const double leakage = term ? MOSFET_LEAKAGE_8580 : MOSFET_LEAKAGE_6581;
+
   // Calculate voltage contribution by each individual bit in the R-2R ladder.
   for (int set_bit = 0; set_bit < bits; set_bit++) {
     int bit;
@@ -119,10 +127,7 @@ void build_dac_table(unsigned short* dac, int bits, double _2R_div_R, bool term)
     int x = i;
     double Vo = 0;
     for (int j = 0; j < bits; j++) {
-      // "Even in standard transistors a small amount of current leaks
-      //  even when they are technically switched off."
-      // https://en.wikipedia.org/wiki/Subthreshold_conduction
-      Vo += ((x & 0x1) ? 1. : 0.01)*vbit[j];
+      Vo += ((x & 0x1) ? 1. : leakage)*vbit[j];
       x >>= 1;
     }