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PGA308: PGA308 setup & command order

Scott Metzger
Prodigy 70 points
Part Number: PGA308


Hello, 

I'm troubleshooting a design using a PGA308. I have been able establish bidirectional communication but it appears that I'm not doing something correctly with the internal device setup.

For instance when attempting to set the output offset using either the course or fine offset registers the only outcome is that the offset moves either to V+ or GND. 

Can you provide me with a set of register setup values based on my design?

 Analog Connections:

Current RAM writes:

-        Write SFTC Register – Command = 0x07, Data = 0x0050

-        Write CFG2 Register – Command = 0x04, Data = 0x0C00

-        Write ZDAC Register – Command = 0x00, Data = (fine offset unsigned 16 bit value)

-        Write GDAC Register – Command = 0x01, Data = (fine gain unsigned 16 bit, 0.333=0x0000, 0.999=0xFFFF)

-        Write CFG0 Register – Command = 0x02, Data = xxx1yyyyzzzzzzzz

 Where:  xxx = Final Gain Code (0x0=2, 0x1=2.4,…0x6=6)

               yyyy=Front End Gain Code (0x0=4,…0xD=1600)

               zzzzzzzz=Coarse Gain (2’s compliment, -63 to +63, 0x00=0, 0xff= -1, 0x80=-FS, 0x7f=+FS)  

Scott

  • 0
    TI__Genius 15470 points
    Hi Scott,
    I can't think of a likely cause for this issue off hand. I'd expect a roughly mid-scale output with Vin1=Vin2 if CFG0 = 0x1000, ZDAC = 0x0000, and GDAC = 0xFFFF. I'd expect to see the output climb ~600uV for each ZDAC LSB increase. For changes to the coarse offset, I'd expect to see output changes of ~8mV for each LSB.

    If this doesn't work, it might be worth checking if this persists across multiple devices. If so, it's likely a programming issue, but if not, the PGA308 was likely damaged.
  • 0 in reply to Alex Davis
    Prodigy 70 points

    At power up I'm seeing a mid-scale output (as expected). However if I try to make a offset change using the ZDAC I get no output movement.  I can elicit a change if I full scale the ZDAC. After I do this I can't return to mid-scale. I'm a bit confused as to if I need to complete a checksum with each RAM command (software lock mode, 101). Figure 5-14 seems to indicate that it's not necessary, however the notes under CK[15:0] (pg. 96 of user manual) state that a checksum must be computed and entered for the PGA308 to function properly. 

    Scott Metzger

  • 0 in reply to Scott Metzger
    TI__Genius 15470 points
    Hi Scott,
    The software lock should override the checksum calculation, but if the checksum was incorrect I'd expect the output to be disabled and floating rather than pulled to mid-supply.

    Can you confirm if this issue persists with a replacement PGA308?
  • 0 in reply to Alex Davis
    Prodigy 70 points

    I have 4 PGA308's on the PCB I'm currently working on now. So far I have checked out two of the PGA308's with the same result. I also ordered a PGA308 eval PCB and I plan on instrumenting the 1W pin to see if the bit patterns are same. 

    Scott Metzger