Max1272 in Bipolar mode

mike.blankenship

I am using the the MAX1272 (12 bit AD internal 4.096 ref) This part is able to run in both Uni-polar and Bi-polar mode, however I am having issues in Bi-polar mode.
I can read the data with no problem with the device set to unipolar mode (Thanks to some code from Tracy Allen I found on this site)

The command byte is as follows for Bi-polar mode :

BIT7 BIT6 BIT5 BIT4 BIT3 BIT2 BIT1 BIT0
START RNG BIP PD MODE1 MODE0 RESERVED REF
1 0 1 1 1 1 0 1

BIT7 = 1 START Bit
BIT6 = 0 These next 2 bits sets the device in bi-polar mode Neg FUll scale -Vrf 1.2207 FUll scale Vref 1.2207
BIT5 = 1
BIT4 = 1 These next 3 bits set for normal operation no power down mode
BIT3 = 1
BIT2 = 1
BIT1 = 0 Spec says to write a logic 0
BIT0 = 1 Sets internal 4.096 reference

With the following commands even though Im setup in Bi-polar mode I am still able to get FUll scale resolution from 0 to 5 volts
However in this same mode I am not able to get the converted 2's compliment result (even after converting it back to unsigned decimal)

SHIFTOUT Serdata_out, Sclk, MSBFIRST, [111101] ' Set for BI-POLAR, Internal ref
SHIFTIN Serdata_in,SCLK,MSBPOST,[ADC_RESULT\12]

sign = ADC_RESULT.BIT11 ' Look at the signed bit to see if negative voltage was applied

IF (sign = 1) THEN VALUE = (~ADC_RESULT + 1) ' Ok this is 2's complement now convert back to a value I can read for the negative rail
IF (sign = 1) THEN VALUE = (VALUE ** 14464) * 2 ' Tracy Allens code (I added the * 2 because it made my value come out +- 2 mv from what my scope was saying)

IF (sign = 0) THEN VALUE = (ADC_RESULT + (ADC_RESULT ** 14464)) * 2

DEBUG "ADC Vout ... SBIN = ", BIN VALUE , 13
DEBUG "ADC Vout ... SDEC = ", DEC value , 13

Even though I have the part selected for Bi-polar mode and apply a positive signal I am able to measure all the
way from 0V to 5V. The Bi-polar transfer function shows Full scale between -2.048 and +2.048. so Im thinking in Bi-polar
mode I should only be able to measure up to 2048 steps

When I do place a negative voltage on the input my full scale is ~ 2048 but the volatges I read are way off but the range is ~ 2048.

If anyone has had experience with A/D in bi-polar mode I would appreciate some help.

Thanks

mike

Tracy Allen

Hi Mike, welcome to the forums!

It can help us look at the code blocks if you can attach the program, or follow the advice...



A couple of things look strange in this section:

[FONT=courier new][SIZE=1]IF (sign = 1) THEN VALUE =  (~ADC_RESULT + 1)  ' Ok this is 2's  complement now convert back to a value I can read for the negative rail
IF (sign = 1) THEN VALUE =  (VALUE ** 14464) * 2  ' Tracy Allens code (I  added the * 2 because it made my value come out +- 2 mv from what my  scope was saying)[/SIZE][/FONT]

The value returned is already twos complement, and it only needs to have the sign extended from the 12th bit out to all bits to the left, namely nibD.

[FONT=courier new][SIZE=1]ADC_result.nibD = -ADC_result.bit11[/SIZE][/FONT]

The ** operator doesn't work properly on negative numbers (sign=1), so you have to extract the absolute value, then apply the ** scaling operator, then restore the sign. My memory of that chip is hazy, but the scaling operator does depend on the range selection by a factor of 2.

mike.blankenship

Tracy,

Thanks for your help, it worked like a charm. I am ~ +- 5mv from rail to rail +5 to -5. This will be more then enough resolution
for my project.

If anyone will be using this ADC make sure to put a 1uf cap on pin6 reference,

Here is the code :

' {$STAMP BS2p}
' {$PBASIC 2.5}
' ADC CONSTANTS
' MAX1272 - BIPOLAR
' Forum thread http://forums.parallax.com/showthread.php/148478-Max1272-in-Bipolar-mode

SCLK             PIN  15      'serial clock pin
CS               PIN  14      'chip select pin
serData_in       PIN  13      'serial data into the Stamp
serData_out      PIN  12

ADC_RESULT       VAR   Word
VALUE            VAR   Word
sign             VAR   Bit

main:

Convert:
PAUSE 250
VALUE = 0

LOW CS '  Enable ADC
  SHIFTOUT Serdata_out, Sclk, MSBFIRST, [%00000000]
  SHIFTOUT Serdata_out, Sclk, MSBFIRST, [%00000000]
  SHIFTOUT Serdata_out, Sclk, MSBFIRST, [%10111101] ' Set for BI-POLAR
  SHIFTIN Serdata_in,SCLK,MSBPOST,[ADC_RESULT\12]
HIGH CS ' DISABLE ADC

sign = ADC_RESULT.BIT11

IF (sign = 1) THEN DEBUG  "SIGN = 1",13
IF (sign = 1) THEN GOTO NEGATIVE
IF (sign = 0) THEN DEBUG  "SIGN = 0",13
IF (sign = 0) THEN GOTO POSATIVE

NEGATIVE:
 ADC_RESULT.NIB3 = -ADC_result.BIT11
 ADC_RESULT = ABS ADC_RESULT
 VALUE = (ADC_RESULT + (ADC_RESULT ** 14464)) * 2
 DEBUG "ADC Vout ...  SDEC = ",SDEC -VALUE , 13
 ADC_RESULT = %0000000000000000
 VALUE = %0000000000000000
GOTO convert

POSATIVE:
 VALUE =  (ADC_RESULT + (ADC_RESULT ** 14464)) * 2
 DEBUG "ADC Vout ...  SDEC = ",SDEC VALUE , 13
 ADC_RESULT = %0000000000000000
 VALUE = %0000000000000000
GOTO convert

GOTO MAIN