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ADS1261: Questions(external clock, unused input, refout, single ended)

Part Number: ADS1261
Other Parts Discussed in Thread: ADS1256

Hello team,

I have questions about ADS1261 as following. Could you please support below?

1. External clock and data rate
We can apply 1MHz to 8MHz into the device as external clock, according to datasheet. Can I think data rate changes linearly? In other words, what data rate is expected when using for example, 3.6914MHz(half of nom. frequency) with 1200SPS register setting? We can get 600SPS then?

2. Unused analog inputs
Datasheet recommends connecting to mid-supply voltage or AVDD. If we connect it to GND, is what we face only large leackage current? or also any performance degradation?

3.  REFOUT drive capability
Can we use REFOUT for ohter device's reference voltage? 

4. Single ended application.
When input signal range to the device is 0 to 2.5V, what configuration do you recommend for best SNR performance? Although I think additional 1.25V can help for the situation, can AINCOM be used for the purpose?

Best regards,

  • Hi Taketo-san,

    I can help answer your questions...

    Taketo Sato said:
    1. External clock and data rate
    We can apply 1MHz to 8MHz into the device as external clock, according to datasheet. Can I think data rate changes linearly? In other words, what data rate is expected when using for example, 3.6914MHz(half of nom. frequency) with 1200SPS register setting? We can get 600SPS then?

    Correct, data rate will scale linearly with the external clock. Using 1/2th the nominal clock rate will change the "1200 SPS" data rate setting to result in a 600 SPS data rate.
     

    Taketo Sato said:
    2. Unused analog inputs
    Datasheet recommends connecting to mid-supply voltage or AVDD. If we connect it to GND, is what we face only large leackage current? or also any performance degradation?

    Only leakage current increases slightly. Minimum leakage occurs when the pins are floating, but typically it is best practice to tie these pins in case of EMI.
     

    Taketo Sato said:
    3.  REFOUT drive capability
    Can we use REFOUT for ohter device's reference voltage? 

    Yes, it is okay to output this voltage to other device. Just don't use this voltage to drive a large load (output current is limited to 10 mA, but reference voltage will shift as output current increases).
     

    Taketo Sato said:
    4. Single ended application.
    When input signal range to the device is 0 to 2.5V, what configuration do you recommend for best SNR performance? Although I think additional 1.25V can help for the situation, can AINCOM be used for the purpose?

    To make full use of the ADC's resolution, you typically want to try to utilize as much of the input range as possible. For a 0 to 2.5V signal, this is really difficult without some front-end circuit to level-shift the signal, which I generally discourage as this in-itself usually degrades performance. Some possible approaches might be:

    1. You could try to measure the 0-2.5V signal directly (and enable VBIAS on the AINCOM pin), but you'll find that the PGA's input common-mode range does not extend all the way down to 0V, so there will be linearity errors for small (<0.3V) input signals. The pGA could be bypassed, but your signal would need to be buffered.

    2. Power the analog supply from a bipolar supply, such that 1.25V becomes the mid-supply voltage (AVDD = 3.75V, AVSS = -1.25V). This way you can measure the input signal directly with the internal PGA in a gain of 1 V/V. This only utilizes the positive full-scale, so you would lose a bit of resolution
    3. Alternatively, using an ADC such as the ADS1256 would allow you to input a 0 - 2.5V signal directly, but here again you wouldn't make fully use of the ADC's input range.

    4. If you do decide to go with a front-end amplifier, you could do a single-ended to differential conversion, as shown below, but there are a lot of caveats to this circuit... If you gain up the signal by 2 V/V, you must provide a 5V external reference to the ADC, but the ADC's REFOUT can be used for VCM in the signal conditioning circuit. However, if you use the internal 2.5V reference, then you still need to provide an accurate 1.25V common-mode voltage to this circuit. In both of these cases, you will need to provide a slightly negative supply voltage for the amplifiers, and the additional noise and offset from the amplifiers degrade the performance of the ADS1261, so overall this is not an ideal use case for these devices.

       

    Best regards,
    Chris