diff -ruN arduino-1.8.3.orig/hardware/arduino/avr/cores/arduino/wiring_analog.c arduino-1.8.3/hardware/arduino/avr/cores/arduino/wiring_analog.c
--- arduino-1.8.3.orig/hardware/arduino/avr/cores/arduino/wiring_analog.c	2017-05-31 18:58:40.000000000 +0200
+++ arduino-1.8.3/hardware/arduino/avr/cores/arduino/wiring_analog.c	2017-08-21 13:47:27.118658770 +0200
@@ -69,6 +69,9 @@
 #else
 	ADMUX = (analog_reference << 6) | (pin & 0x07);
 #endif
+#elif defined(ADMUXA)
+	ADMUXA = (pin & 0x07);
+	ADMUXB = (analog_reference << 5);
 #endif
 
 	// without a delay, we seem to read from the wrong channel