diff --git a/hardware/sam/system/libsam/include/adc.h b/hardware/sam/system/libsam/include/adc.h
index 2d189b7ee..31534753c 100644
--- a/hardware/sam/system/libsam/include/adc.h
+++ b/hardware/sam/system/libsam/include/adc.h
@@ -43,11 +43,14 @@
 
 #include "../chip.h"
 
-#define ADC_FREQ_MAX   20000000
-#define ADC_FREQ_MIN    1000000
+#define ADC_FREQ_MAX      5000000
+#define ADC_STARTUP            20
 
-#define ADC_STARTUP_NORM     40
-#define ADC_STARTUP_FAST     12
+#define ADC12_FREQ_MAX   20000000
+#define ADC12_FREQ_MIN    1000000
+
+#define ADC12_STARTUP_NORM     40
+#define ADC12_STARTUP_FAST     12
 
 /* Definitions for ADC resolution */
 typedef enum _adc_resolution_t {
diff --git a/hardware/sam/system/libsam/source/adc10_sam3u.c b/hardware/sam/system/libsam/source/adc10_sam3u.c
index 1b73691d1..9531651df 100644
--- a/hardware/sam/system/libsam/source/adc10_sam3u.c
+++ b/hardware/sam/system/libsam/source/adc10_sam3u.c
@@ -64,7 +64,6 @@ extern "C" {
  */
 uint32_t adc_init(Adc *p_adc, uint32_t ul_mck, uint32_t ul_adc_clock, uint32_t ul_startuptime)
 {
-    uint32_t ul_prescal,ul_startup;
     p_adc->ADC_CR = ADC_CR_SWRST;
 	
     /* Reset Mode Register */
@@ -76,9 +75,14 @@ uint32_t adc_init(Adc *p_adc, uint32_t ul_mck, uint32_t ul_adc_clock, uint32_t u
     p_adc->ADC_RNCR = 0;
     p_adc->ADC_TCR = 0;
     p_adc->ADC_TNCR = 0;		
-    ul_prescal = ul_mck/(2 * ul_adc_clock) - 1;
-    ul_startup = ((ul_adc_clock/1000000) * ul_startuptime / 8) - 1;
-    p_adc->ADC_MR |= ADC_MR_PRESCAL( ul_prescal ) | ( (ul_startup<<ADC_MR_STARTUP_Pos) & ADC_MR_STARTUP_Msk);
+    uint32_t prescal = ul_mck/(2 * ul_adc_clock) - 1;
+    // check for rounding errors
+    if ( (ul_mck/((prescal+1)*2)) > ul_adc_clock ) {
+    	prescal++;
+    	ul_adc_clock = ul_mck/((prescal+1)*2);
+    }
+    uint32_t startup = ((ul_adc_clock/1000000) * ul_startuptime / 8) - 1;
+    p_adc->ADC_MR |= ADC_MR_PRESCAL(prescal) | ADC_MR_STARTUP(startup);
     return 0;	
 }
 
diff --git a/hardware/sam/system/libsam/source/adc12_sam3u.c b/hardware/sam/system/libsam/source/adc12_sam3u.c
index 8da95532c..2ca25bf7d 100644
--- a/hardware/sam/system/libsam/source/adc12_sam3u.c
+++ b/hardware/sam/system/libsam/source/adc12_sam3u.c
@@ -64,7 +64,6 @@ extern "C" {
  */
 uint32_t adc12_init(Adc12b *p_adc, uint32_t ul_mck, uint32_t ul_adc_clock, uint32_t ul_startuptime, uint32_t ul_offmode_startuptime)
 {
-    uint32_t ul_prescal,ul_startup,ul_offmode;
     p_adc->ADC12B_CR = ADC12B_CR_SWRST;	
 	
      /* Reset Mode Register */
@@ -76,11 +75,17 @@ uint32_t adc12_init(Adc12b *p_adc, uint32_t ul_mck, uint32_t ul_adc_clock, uint3
     p_adc->ADC12B_RNCR = 0;
     p_adc->ADC12B_TCR = 0;
     p_adc->ADC12B_TNCR = 0;		
-    ul_prescal = ul_mck/(2 * ul_adc_clock) - 1;
-    ul_startup = ((ul_adc_clock/1000000) * ul_startuptime / 8) - 1;
-    ul_offmode = ((ul_adc_clock/1000000) * ul_offmode_startuptime / 8) - 1;	
-    p_adc->ADC12B_MR |= ADC12B_MR_PRESCAL( ul_prescal ) | ( (ul_startup<<ADC12B_MR_STARTUP_Pos) & ADC12B_MR_STARTUP_Msk);
-    p_adc->ADC12B_EMR |=  (ul_offmode<<16) & (0xffu << 16);	
+    uint32_t prescal = ul_mck/(2 * ul_adc_clock) - 1;
+    // check for rounding errors
+    if ( (ul_mck/((prescal+1)*2)) > ul_adc_clock ) {
+    	prescal++;
+    	ul_adc_clock = ul_mck/((prescal+1)*2);
+    }
+    uint32_t startup = ((ul_adc_clock/1000000) * ul_startuptime / 8) - 1;
+    p_adc->ADC12B_MR |= ADC12B_MR_PRESCAL(prescal) | ADC12B_MR_STARTUP(startup);
+
+    uint32_t offmode = ((ul_adc_clock/1000000) * ul_offmode_startuptime / 8) - 1;
+    p_adc->ADC12B_EMR |= ADC12B_EMR_OFF_MODE_STARTUP_TIME(offmode);
     return 0;	
 }
 /**
diff --git a/hardware/sam/variants/arduino_due/variant.cpp b/hardware/sam/variants/arduino_due/variant.cpp
index e1133db3e..2e4276e62 100644
--- a/hardware/sam/variants/arduino_due/variant.cpp
+++ b/hardware/sam/variants/arduino_due/variant.cpp
@@ -350,8 +350,8 @@ extern void init( void )
    *     prescal: ADCClock = MCK / ( (PRESCAL+1) * 2 ) => 64MHz / ((4+1)*2) = 6.4MHz
    *     ADC clock = 6.4 MHz
    */
-  adc_init( ADC, SystemCoreClock, 6400000, 10 ) ;
-  adc_configure_timing( ADC, 1200 ) ;
+  adc_init( ADC, SystemCoreClock, ADC_FREQ_MAX, ADC_STARTUP ) ;
+  adc_configure_timing( ADC, 15 ) ;
   adc_configure_trigger( ADC, ADC_TRIG_SW ) ;	
   adc_disable_interrupt( ADC, 0xFFFFFFFF ) ; /* Disable all adc interrupt. */	
   adc_disable_channel( ADC, ADC_ALL_CHANNEL ) ;
@@ -366,8 +366,8 @@ extern void init( void )
    *     prescal: ADCClock = MCK / ( (PRESCAL+1) * 2 ) => 64MHz / ((4+1)*2) = 6.4MHz
    *     ADC clock = 6.4 MHz
    */
-  adc12_init( ADC12B, SystemCoreClock, 6400000, 10, 1200 ) ;
-  adc12_configure_timing( ADC12B, 1200 ) ;
+  adc12_init( ADC12B, SystemCoreClock, ADC12_FREQ_MAX, ADC12_STARTUP_FAST, 1 ) ;
+  adc12_configure_timing( ADC12B, 15 ) ;
   adc12_configure_trigger( ADC12B, ADC_TRIG_SW ) ;	
   adc12_disable_interrupt( ADC12B, 0xFFFFFFFF ) ; /* Disable all adc interrupt. */
   adc12_disable_channel( ADC12B, ADC_ALL_CHANNEL ) ;