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Common mode range for ADuC7060 when PGA enabled

Question asked by mpcengineer on Feb 9, 2011
Latest reply on Feb 20, 2011 by MMA

Hi,

 

We have the ADuC7060 design working, code running and ADC values being output ok.

 

    One problem.
   
    We are using the primary ADC. External reference 2.5V with Vref+/- connected to the analog ground and analog vcc

 

    We have the HIGHEXTREF0 set.

 

    We have 244mV on V- and 311mV on V+ ie 67mV input signal.

 


    For gain of 4 we get 0x361828 which is 66mV
    For gain of 8 we get 0x6C2EDD which is 66mV

 

    For gain of 16 we get 0x78D5B3 which is 37mV ???????

 

 

     By increasing the input common mode voltage so V- is 785mV and V+ is 836mV we get the correct value for x16 gain

 

 

     This means the input common mode range is limited as you use the PGA. I could not find any limits on the data sheet highlighting this. Can these limits be added?

 

Thanks, mpc

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