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ADS1298: About ADS1194 External-Reference-Range

Part Number: ADS1298
Other Parts Discussed in Thread: ADS1198,

Hello.

I have a question about the external reference voltage range of the ADS1198.

The specifications(Recommended Operating Conditions) are as follows.

3V Supply: VREF = 2.5V (NOM)

5V Supply: VREF = 4.V (NOM)

(When 5V Supply(AVDD=+2.5V, AVSS=-2.5V), GAIN = 1)

・ There is no problem in inputting VREF = 2.5V, right?

・ When VREF = 2.5V, the input near VIN = -2.5V or + 2.5V is saturated, does ADC-CODE not reach 8000h and 7FFFh?

(FSR=±2.5V, VIN-Range: -2.2V ~+2.2V)

・ When VREF = 5V is input, does the ADC-CODE with VIN = -2.5V to + 2.5V output only up to about C001h and 3FFFh?

(FSR=±5V, VIN-Range: -2.2V ~+2.2V)

  • Hello and thank you for your post.

    Keep in mind that the ADC is using the differential reference voltage (VREF) to determine the full-scale range. VREFN must be tied to AVSS, while VREFP is nominally 2.5 V or 4 V above AVSS. 

    For a 5-V bipolar supply (+/- 2.5 V), VREFP should be connected to +1.5 V.

    Anytime that you apply an input voltage equal to one-half the ADC reference voltage, you will get approximately C001h and 3FFFh. However, for the ADS1198, we do not recommend increasing VREF beyond the nominal voltage levels in section 7.3 of the data sheet.

    Best regards,

  • Thank you for your reply.

    >Keep in mind that the ADC is using the differential reference voltage (VREF) to determine the full-scale range.

    ・The VREF is only used to determine the FSR.

    Is the above recognition correct?

    Is the VIN range limited by AVDD and AVSS?

    And does ADC-CODE saturate when VIN> +VREF/GAIN or VIN <-VREF/GAIN?

    >VREFN must be tied to AVSS, while VREFP is nominally 2.5 V or 4 V above AVSS. 

    When external reference voltage, is it recommended to do as follows?

    ・±1.5V power supply → VREF = VREFP-VREFN = +2.5V (NOM)

    ・±2.5V power supply → REF = VREFP-VREFN = +4V (NOM)

    When 5V Supply(AVDD=+2.5V, AVSS=-2.5V, GAIN = 1), is VREF=2.5V Bad Condition?

    >For a 5-V bipolar supply (+/- 2.5 V), VREFP should be connected to +1.5 V.

    VREFP = VREF + VREFN = 4V + (-2.5V) =+1.5V

    Is it as above?

    >Anytime that you apply an input voltage equal to one-half the ADC reference voltage, you will get approximately C001h and 3FFFh.

    ⇒I understood.

    >However, for the ADS1198, we do not recommend increasing VREF beyond the nominal voltage levels in section 7.3 of the data sheet.

    Is section 7.3 the section of the ADS129x datasheet?

    I couldn't find it on the data sheet of ADS119x.

  • Hello,

    1. Yes - you can read more in the Data Format section. ADC output code will saturate when abs(VIN_DIFF x Gain) > VREF.
    2. Yes - absolute voltage on each analog input pin is limited by the supplies. See Absolute Maximum Ratings table.
    3. Yes - those are the nominal reference voltages for 3-V and 5-V analog supplies. It is perfectly acceptable to use a 2.5-V VREF and 5-V analog supply.
    4. Sorry you're right - I was looking at the ADS1298 datasheet. The information presented in the Electrical Characteristics table is the same. The nominal reference voltages are 2.4 V and 4 V for 3-V and 5-V supplies, respectively.

    Best regards,