Full Scale Range for A/D Converter

[Thunder]

Hi, I am using the HC9S08LG32 (SPI connection) to interface with the ADS1241, a 24-bit A/D converter.  The Vcc supply is 5V, while the Vref is 1.25V (Vref- is grounded).  My problem is that there's a 24-bit register in the A/D called FSR (Full Scale Register).  I am unable to figure out what 24-bit value to insert here.  The maximum voltage that the A/D would ready is about 50mV, so I'm programming in a gain of 16 to the A/D (which has an internal pre-amp).  I've been trying different values of FSR, but no luck.  Any inputs welcome, thanks.

[Thunder]

Hi, Range is set to 0, such that Vref+ (1.25V) - Vref- (GND) = 1.25V.  The expected input voltage range (fr a thermocouple output) is 0..50mV. 

According to the datasheet, FSR (with PGA=16) is 312.5mV.  This, however, assumes a 2.5V ref.  The formula referenced is (2*Vref/PGA), so using 1.25 as a ref gives an FSR of 156mV.  Using this equation, I could go with a gain of 32, giving a FSR of 78mV. 

Reading your comments would indicate that the FSR would then read all 1's with an input voltage of 78mV (range=0 and 1.25Vref).  Is that correct ?  There is also the 24-bit Data Output Register (which is read-only) which provides the A/D computation.  I really still do not understand what the utility of the FSR register would be.  Thanks.

[bigmac]

Hello,

The datasheet is quite unclear whether the FSR value is used for calculation of the data output register result, or is available purely for external full scale calibration calculation, i.e. the A/D reading associated with a specific input voltage.  If it is the latter, I guess its use is optional.  I do not know the significance of the rather odd power on reset FSR value.

Regards,

Mac

[bigmac]

Hello,

If my understanding of the ADS1241 data sheet is correct, the full scale register is automatically updated during the SELFCAL and SELFGCAL commands.

Is your input voltage range bipolar or unipolar, i.e. -50mV to +50mV, or 0 to +50mV?  And what is the setting of the RANGE bit?  These will influence the optimum gain setting.

Regards,

Mac