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ADA4254 - DVSS and AVSS: same voltage, different supplies

Category: Hardware
Product Number: ADA4254

Hi everyone,

On page 26 of the ADA4254's datasheet (Power Supplies chapter), we can see that both the low voltage analog stage and the digital side must share the same voltage. The few applications I could find directly connected AVSS and DVSS together.

However, our entire system sits 2.5V below the ground level and we provide our analog side (-2.5 to 2.5V) and digital side (-2.5 to 0.8V) with two separate power supplies. Both are referenced to the same ground plane but the -2.5V levels are generated separetely. Here are our three questions:

  1. We are guessing that everything should be fine once steady state on both supplies is reached and the IC would be correctly biased, even with no direct connection between the two -2.5V. Is that a correct assumption?
  2. How much leeway between AVSS and DVSS 's voltage levels do we have?
  3. Can we expect issues during the transitory stage wherein AVSS and DVSS don't reach -2.5V at the same time? Do we need to cascade our power supplies? Do we need AVSS or DVSS to reach a lower voltage first?

Let me know if you need additional information to answer the questions above. 

Thank you for your answers!

  • Hi Lucy,

    Few things to take note with using a -2.5V to 2.5V output amplifier supply:

    1. AVSS must be within VSSH − 0.3 V to VSSH + 30 V and VDDH – 30 V to VDDH + 0.3 V. This affects the specified diamond plot tool in the datasheet. I suggest you try out a certain configuration in LTSpice first before applying it to your board. ADA4254's spice model is pretty accurate, you should have no problem using it.

    2. AVDD-AVSS is minimum at 2.7V and max at 5V. In your case, -2.5V to 2.5V should be okay as long as other configuration is in linear region.

    3. The datasheet table is specified performance at output amplifier's 0V-5V supply. If you opt to use -2.5V to 2.5V, (although will work) we cannot guarantee other specs of the device.

    4. Vocm should be output amplifier supply mid-range. If you're using our eval board, make sure to make the correct modification since the board connects Vocm to a AVDD/2 divider.

    5. It would be advisable to power up the analog and digital supplies at the same time.

    Hope this helps!

    Regards,

    Kristine