AD5700-1 Supply Voltage

I am not achieving the 1.71V minimum supply voltage specified for the AD5700-1.

The lowest I can achieve is 1.83V while the device is still outputting a modulated signal. Any lower (1.827V) and the signal disappears completely. This is measured at the Vcc/IOVcc pin of the device with a maximum ripple of 6mVp-p.

I have the device configured to use the internal oscillator and reference, and there is a series 470ohm resistor suppling Vcc/IOVcc.

For reference, I noticed that the silicon was changed for date codes 1303 and later to allow for this lower 1.71V supply voltage (down from 2V previously) but this particular IC under test is dated 1310.

There is concern that the voltage drop across the 470ohm series resistor brings the supply voltage dangerously close to the 1.83V figure tested above, when the device operates at the worst case current listed in the datasheet (285uA+270uA).

I am not aware of any damage this IC could have sustained to bring it out of spec but it is the only IC I currently have available.

Any suggestions would be appreciated?

Adrian

  • Hi Adrian

    What is the purpose of the 470Ohm resistance? Just to confirm that even with the voltage drop, the voltage on the VCC pin is above 1.83V?

    Could you share a schematic of the circuit?

    Kind Regards,

    David

  • Hi David,

    The purpose of the 470ohm resistor is to reduce noise caused by variations in current draw (as detailed under 'Supply Decoupling' in the datasheet p17), and the 1.83V I reference above is AFTER any voltage drop across this resistance (measured at the input pin).

    I have included a subsection of the schematic below. The CD, TXD, RXD and RTS pins connect directly to a MCU.

    Adrian

  • Hi,

    My apologies for the delay,

    If you could try bypassing the 470Ohm resistor. After discussions with the AD5700 team, they had the following comments:

    The 470ohm resistor is to filter noise coming from the AD5700 internal voltage regulator back to the power supply. However, at 1.8V power supply voltage the AD5700 internal regulator should be saturated, and therefore no resistor should be required…

    Kind Regards,

    David