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DAC8775: output current calibration

Jake Liang
Intellectual 310 points
Part Number: DAC8775

We are using DAC8775 in our new product that is under development. We have the following questions with this device:

1. With the range of 0 ~20mA of this device, is there any offset at the low end ( 0mA ) ? if it might have some offset at there ( 0mA), is it possible to calibrate it out? 

2. With the range of 0 ~20mA of this device, is there any room to extend a little bit over 20mA if  we need to if we do calibration at the point of 20mA ?

3. if we do not have room to extend a little bit over 20mA in item2, I guess we can use 0 ~24mA range of this device. In this way we can do the calibration at 20mA. The question is : can we do the calibration at 0mA with this range (0mA ~ 24mA )?

This product is designed to support 0 ~ 20mA and 4 ~ 20mA applications in current mode and +/-10V applications in voltage mode, we need to run calibration at  two points within the range.  we are not sure if we can do the calibration at 0mA and 20mA with 0 ~20mA range of this device. If we are using 0 ~ 24mA range of this device for our application, we should be ok to do the calibration at 20mA, but not sure if we can do the calibration at 0mA as well. 

  • 0
    TI__Expert 4280 points

    Hi,

    1. Based on my lab testing with the EVM, the device cannot use a negative offset to extend lower than 0 mA.

    2. The device also can't go past the range's maximum value by increasing a channel's gain. I believe the OFFSET and GAIN internally calculate the appropriate DAC code, not actually modifying the output of the DAC. This means the code itself is gained and reaches its limit at 0xFFFF.

    3. You should be able to make the 0 - 24 mA fit a 0 - 20 mA range with some over-range, though you will not be able to lower the minimum value unless you use the bipolar 24 mA range.

    Please let me know if you have any more concerns.

    Thanks,
    Lucas

  • 0 in reply to Lucas Burnette
    Intellectual 310 points

    I still do not understand how to have 0mA with 0 - 24mA range if it has some offset at 0mA. for example, if I make 0 -24mA range for an 0 ~ 20mA application, when I send a command for 0mA, and the actual output current I get is 0.05mA, how can I adjust it to be 0mA?  We do not need to have a negative output current, but we do need to have a "true" 0mA for 0mA output.

  • 0 in reply to Jake Liang
    TI__Expert 4280 points

    Hi,

    Since the calibration is purely digital (as in the device adjusts the value of the code being used), you likely won't be able to calibrate lower than what you are getting at the 0x0000 code. The last paragraph in the DAC Calibration section of the datasheet, page 47, mentions that the calibration only makes sense for endpoints inside of the true device end points, which does not include near zero-code or full-scale code.

    You may not need a negative output current, but by using a bipolar range, you may be able to get closer to zero by sacrificing some resolution.

    Thanks,
    Lucas