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ADS1298: ADS1298

Suman B
Prodigy 110 points

Part Number: ADS1298
Other Parts Discussed in Thread: ADS1294,

HI,

I am using ads1298 and ads1294 together in daisy chain configuration,how to we write these two IC'S registers since din is common to both.

How do we configure registers in daisy configuration?

  • 0
    TI__Guru 69446 points

    Hello Suman,

    Thanks for your post.

    The register addresses for both devices are exactly the same, so the only difference between the two is that registers 09h - 0Ch are disabled in the ADS1294. You can still write values to these registers, but nothing will happen. As long as the remaining register settings are equivalent, you can write to both the ADS1294 and the ADS1298 at the same time in a daisy-chain configuration.

    However, if any of the register settings need to be different from one device to the next, then you must use a separate /CS signal for each device. During data collection, you can bring both /CS pins low simultaneously and read all the data through the first device in the chain.

    Best Regards,

  • 0 in reply to Ryan Andrews
    Prodigy 110 points

    Hi,

    Thanks for reply i have another doubt,

    1) if we want to operate with internal clock of ads1298,as i understand we need to individually configure ads1298(internal clock operation),ads1294(external clock input) from ads1298 after this we can go daisy chain operation with single chip select for read operation .please confirm if this is correct

    2)should we use some external reference ic for supplying for both ads1298 and ads1294? because when each ads is independently operating with its internal reference then there can be mismatch in the reference due to its accuracy +-0.2%

  • 0 in reply to Suman B
    TI__Guru 69446 points

    Hi Suman,

    You are correct about both points.

    1. You must share a single master clock signal between the two devices in order for them to be synchronized. You can achieve this by configuring the CLKSEL pin. When CLKSEL is pulled high, the device will use the internal master clock. You can output the internal master clock on the CLK pin by setting the CLK_EN bit = 1 (CONFIG1[5]). When CLKSEL is pulled low, the device will expect an external clock input and the CLK_EN bit (CONFIG1[5]) will be ignored.

    2. For highest accuracy, you will have to use an external reference IC with a buffer to drive each VREFP pin. If both devices are synchronized properly, I believe a single buffer will be sufficient to drive them. Each ADC should have local 10uF capacitors directly across their VREFP and VREFN pins as well. Be sure to power down the internal reference buffers in your register settings.

    Best Regards,

  • 0 in reply to Ryan Andrews
    Prodigy 110 points

    Hi,

    Should we take care of any other register setting when using daisy chain configuration as explained above question for clock.

    since data is same for both ads

  • 0 in reply to Suman B
    Prodigy 110 points

    Hi,

    Which configuration is better to use cascaded or daisy chain configuration

  • 0 in reply to Suman B
    TI__Guru 69446 points
    Hi Suman,

    Sorry for the delay, I was out of the office.

    For daisy-chain operation, /DAISY_EN (CONFIG1[6]) must be set to 0. You can write this value to both devices. The last device in the daisy-chain must have the DAISY_IN pin tied to ground.

    If the register settings are exactly the same for both devices, then the Daisy-Chain configuration requires fewer connections to route and may be a simpler solution. However, there is a requirement for an extra "dead bit" to be inserted between the data sets from each device. See Figure 67.

    The Cascaded Configuration requires an extra GPIO pin on the MCU in order to allow multiple devices to share the same SPI bus.

    If you can meet the relevant GPIO and timing requirements, then either configuration will work fine.

    Best Regards,
  • 0 in reply to Ryan Andrews
    Prodigy 110 points

    Thanks for reply,

    i have one more doubt,when we use external oscillator should we use two seperate oscillators for two ads or one oscillator is enough for both ads shorted,will it give some loading effect?

    generally what is advised

  • 0 in reply to Suman B
    TI__Guru 69446 points

    Hi Suman,

    If you are using an external master clock source (i.e. a crystal oscillator), then you can use one device to drive both ADS129x CLK pins. There should not be any negative loading effect.

    I hope my explanation earlier was clear - you can achieve synchronous sampling of the ADS1298 and ADS1294 in a daisy-chain configuration without using an external crystal oscillator. All you need to do is:

    1. For ADS1298, tie CLKSEL to DVDD (use internal oscillator)
    2. For ADS1294, tie CLKSEL to GND (use external oscillator)
    3. Short the CLK pins from each device
    4. Write CONFIG1[5] = 1 to both devices (output internal oscillator; ignored when CLKSEL = 0)

    Best Regards,