How to sample negative voltages in msp430fg4618?

[iVenky 0]

I am using msp430fg4618 experimenter's board. I am able to sample the positive voltages using the adc with 0000 for 0V. But when I send negative voltages I am getting 0000 only in output of the adc. How to sample negative voltages?

 

Thanks in advance.

[RobG 1,892]

You will need to use op amp to either bias or invert negative input voltage.

[gwdeveloper 275]

The on-chip op-amp could be easily used in an Inverting mode. TI has examples for the configuration.

[nobody 47]

Back to reality.

 

The operational amplifier is not working above its power rails. To use an operational amplifier for processing negative input voltage - it is necessary to use a negative supply voltage.

 

There is another way to handling the negative input voltage:

1) allow buffered output reference voltage at the output pin.

2) use a voltage divider (two resistors) - one end of the reference voltage, the other end to the negative input voltage and the center of the analog input.

3) select resistors so that the analog input voltage was always greater than zero.

[zeke 693]

I agree. There is no way to do this with a single supply op amp. With a dual supply (+/-) then yes you can do it.

 

Another way to do it is to use a bridge rectifier. Yes, you will have a diode drop in the way but it will make every signal be above ground.

 

How accurate do you have to be?

[RobG 1,892]

How about something like this:

BTW, there is a misprint in this schematic, R1 and R2 values are switched.

 

 

Reference: sloa030a, page 13, case 3 & 4

[jim940 28]

Cheapest easiest method would be something like this:

 

 

http://cq.cx/interface.pl

 

Jim