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ADS1248: Basic question about CMRR/IDACs

Takumi Suzuki1
Mastermind 8630 points

Part Number: ADS1248
Other Parts Discussed in Thread: ADS124S08

Hi,

Let me ask you basic questions about CMRR and IDACs as below, datasheet page9.

Q1) CMRR

Why is the CMRR bigger in case PGA is larger?

That is to say, CMRR is 90dB(TYP) @Gain=1 and CMRR is 125dB(TYP) @Gain=32.

The CMRR @Gain=32 is bigger than that one of Gain=1.

Why is the CMRR different, depends on Gain settings?

Q2) IDACs (1)

What is the definition(meaning) of "Compliance voltage"?

Q3) IDACs (2)

Could you tell me the "max" value about following items? (reference value is OK)

"Absolute mismatch", "Temperature drift", Temperature drift matching"

Thank you,

Best Regards,

  • 0
    TI__Guru* 99465 points
    Takumi-san,


    1. CMRR will be larger in gain. The measurement is a ratio of a change in common mode voltage, vs the change of measurement. I believe that similar to the noise performance, the CMRR does not affect the PGA as much as the delta-sigma measurement. Modulator allows for the same amount of common-mode rejection, against a smaller input range. Because the measurement change is smaller with larger gain, the common-mode rejection is larger.

    2. The compliance voltage is the operating voltage range of the IDAC current that allows for normal operation. The output voltage of the IDAC requires some voltage to the positive supply so that can operate.

    The IDAC is constructed from a buffered PMOS device sourcing a current. Imagine that the current comes out of the drain of the PMOS device and the source is connected to the supply. As the output voltage gets higher, VDS becomes smaller. Because of the buffering the, current generally doesn't change much as the output voltage goes from AVSS. However as the output voltage gets closer to AVDD, the current starts to drop quickly.

    If you look at Figures 41 and 42. These plots show how much the current drops as you get closer to AVDD (which is set at 5V. In figure 1, the 50uA IDAC will be fine within 1V of AVDD. At the same time, the 1.5mA IDAC wil have dropped 10% of the current when the IDAC output voltage gets to 1V of AVDD.

    3. I don't have data on the IDAC absolute match, drift, and drift match. Typical values are listed and I would consider them to be close the the standard deviation of the specification. If the IDAC current specifications are very important to the application, I would consider using the ADS124S0x device. The ADS124S0x guarantees these specifications and are generally superior in performance.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Mastermind 8630 points
    Hello Jpseph-san,

    I appreciate your kind answer very well !
    Thanks so much, I understood.

    Let me ask you one more question.
    ADS124S08 is upgrade of ADS1248, correct?

    That is to say, the customer using ADS1248 can replace it to ADS124S08, right?

    Best Regards,
  • 0 in reply to Takumi Suzuki1
    TI__Guru* 99465 points
    Takumi-san,


    The ADS124S08 is considered the next generation of the ADS1248. It has several more input channels, some extra features, and generally better specifications. Note that this device is not a drop-in replacement. The command set and pinout for this device is different.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Mastermind 8630 points
    Joseph-san,

    Thank you for your reply.
    I will ask the customer to consider ADS124S08.
    Thanks and Best Regards,