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ADS8686S: How to implement the function of PGA?

Part Number: ADS8686S

Hi,

I'm using ADS8686S to get some sensers signals and there is problem I'm a little confused.

I don't know how to use the function of PGA refered in the datasheet.

I choose hardware mode, and set HW_RANGESEL[1:0]=01 [-2.5v, 2.5v].

Does the different ranges(2.5v, 5v, 10v) of input mean programmable?

By the way, if I set the input range as [-2.5v, 2.5v], the range of output data[15:0] is from 0x0000 to 0xffff.
0x0000~0x0fff: 0 ~ +2.5v
0x8000~0xffff:  0 ~ -2.5v
Is this right?

Waiting for your reply,thank you very much!

Best regards,
Sarah

  • Hi Sarah,

    When you select a different input range, the ADS8686S ADC will configure the internal PGA automatically. You just need to select the input range by either HW_RANGSELx pins or Range_Ax registers.

    Your understanding about code vs. voltage is correct, please see the Transfer Function Figure 54 in ADS8686S datasheet.

    Best regards,

    Dale

  • Hi Dale,

    Thank you for your reply.

    I want to know how many times amplified by PGA or can I control the times PGA amplified? Following is my understanding about the PGA function in ADS8686S.

    For example, my sensor output range is [-5mv, 5mv].

    If I set the HW_RANGESEL[1:0] = 01, the input range is [-2.5v, 2.5v], then the amplified time is 500, the ADC output code range based on my sensor's signal is [0, 65] and [65480, 65535].

    If I set the HW_RANGESEL[1:0] = 11, the input range is [-10v, 10v], then the amplified time is 2000, the ADC output code range based on my sensor's signal is [0, 15] and [65520, 65535].

    Is above understanding right?

    If it is right, I think even if I set HW_RANGESEL[1:0] = 01, the output range is a little small.

    Best regards,

    Sarah

  • Hi Sarah,

    Your understanding is incorrect. As I said, the ADS8686S ADC will configure the internal PGA automatically. The ADC's PGA is not used to amplify your signal, it's only used to configure the input range according to your selection through either HW_RANGSELx pins or Range_Ax registers. When +/-2.5V range is selected, you will have to amplify your sensor's signal externally to meet the +/-2.5V range. 

    Thanks&Best regards,

    Dale

  • Hi Dale,

    Thank you very much for your detail explaination.

    Now I understand what's the function of ADS8686S's PGA. 

    Best regrads,

    Sarah