Darek,

The ADC1286 chip has not pure SPI. Your reading algorithm is wrong. Did you read the datasheet?

Please have a look at example from http://www.nomad.ee/micros/ads1286.html below

10 REM ADS1286 demo v1

15 GOSUB 1050
20 GOSUB 1200
30 PRINT AV
40 GOTO 20

1000 REM start conversion
1010 PORT1=0FFH
1015 PORT1=07DH
1020 RETURN

1050 REM stop ADC
1060 PORT1=0FFH
1070 RETURN

1100 REM one clock, leaves clock high
1110 PORT1=07DH
1120 PORT1=07FH
1130 RETURN

1200 REM take ADC reading
1210 GOSUB 1000
1215 AV=0
1220 GOSUB 1100 : GOSUB 1100 : GOSUB 1100
1230 FOR I=1 TO 12
1240 GOSUB 1100
1250 BT=PORT1.AND.1
1260 AV=AV+AV+BT
1270 NEXT I
1280 GOSUB 1050
1290 RETURN

Want to get voltage reading. No problem. Just measure the VREF voltage, and modify the program to read (set V to your VREF voltage on line 16):

10 REM ADS1286 demo v1

15 GOSUB 1050
16 V=5.1
20 GOSUB 1200
30 PRINT V/4096*AV
40 GOTO 20

...

Seems it can help to undestand the problem.

Regards,
George