LTC4316 XORL translation voltage


I'm using the LTC4316 I2C address and voltage translator for the first time in a product re-design. For system reasons, the incoming I2C bus is 5V TTL, and the 4316 Vcc is 3.2V.  XORL is defined as per the data sheet by a 1% resistor pair from Vcc to GND, decoupled with a 1nF ceramic capacitor. XORH is grounded.

I am finding that the addresses do not translate correctly, An example is a divider of 28K0 and 100K, giving a ratio of 0.78125, exactly the ratio shown in the data sheet to generate an XOR on address bits 3 and 2. However, the translated I2C output has bits 3,1,0 flipped rather than 3,2. When I measure the XORL voltage as a fraction of the supply using a 7-digit DVM in ratiometric mode, it shows 0.78124. I checked that there was no difference in I2C translation whether the DVM measurement leads were attached or not.

I attach a scope shot showing the I2C clock at the top, input I2C data in the centre and output I2c data at the bottom. The upper two traces are at 5V/div and the bottom one 2V/div. The timebase is 20us/div. Other data-sheet resistor ratios give similar but not identical errors. I noted that all observed translations imply a lower voltage at the XORL pin than is measured.

Can you suggest what might be the problem?