AD5222: Absolute maximum ratings, latchup, zero code resistance and distortion

Q 

1.In DS of AD5222 absolute max rating of Va,Vb,Vw to GND equals  0V,Vdd. Is it
correct? I think it should be equals Vss,Vdd.

2.Must I cut off input voltage of pins Va,Vb,Vw before switching power off?
If I understand correctly, pins Va,Vb,Vw and digital inputs of AD5222  should
be powered after Vdd,Vss  because otherwise the internal protection diodes may
be damaged.
But I can to power these input pins via external serial resistors.
  What values of input voltage or current via input terminals of AD5222 are
really  dangerous in that case?

If latch up can occur when the prescribed power-up sequence is not followed,
can I preserve from such by connection to Vdd,Vss pins external serial
resistors?

3.What is B-W resistanse and max B-W current if counters code equal  zero?

4.What is Total Harmonic Distortion  vs. code (i.e. resistanse value)?

 

A 

1. Yes you are correct. In the specification page of the datasheet, V_a,b,w can
go from Vss to Vdd.

2. If there is a risk that input voltages may be present at AD5222 without the
power supplies being present, you need to take some measures to limit the
current that can flow in the ESD protection diodes - by placing series
resistors at R, W and B. We recommend choosing resistor values to keep the
current below 5mA in these protection diodes.

The FAQs mention the danger of the latchup if the power sequencing is not
respected. By limiting the current to low values you prevent any latchup
phenomenon from taking place.


3. The B-W resistance at zero code is simply the wiper resistance (45 Ohms typ,
100 Ohms max). The current should be limited to 20mA max to avoid damaging the
device.

4. The THD should remain essentially constant with code. The point is that the
only source for non-linearity is the variation of Ron of the analog swtich
connecting the W terminal to the resistor string (see figure 16 Incremental
Wiper Contact Resistance vs. VDD/VSS).

There is only ever one switch connecting the resistor string to the W terminal
so the source of non-linearity is always the same independent of code.