are driven with +15V and -15V logic levels, but the ADG442s are clamping the
low logic level to about -1V. The data sheet (rev. A, 2005) gives the maximum
rating for the analogue and the digital inputs as Vss-2V to Vdd+2V, or 30mA. I
took this to mean that the logic levels could extend over the full supply
voltage but this does not appear to be the case.
As far as I can see from our support history on ADG444, the ADG44x digital
inputs can only be driven from VDD to GND. The ADG44x parts have an N implant
resistor to GND on the digital input pin which acts as a diode. This would
appear to explain the clamping of the low logic level that you are seeing.
The absolute maximum ratings for Digital inputs may be a bit misleading as they
state: VSS − 2 V to VDD + 2 V or 30 mA, Whichever Occurs First. On the other
hand these are absolute maximum ratings to avoid damaging the device, they are
not operational specs.