Question
The data sheet spec for the powerdown pin quotes voltages e.g. 3.2V as if
relative to a ground pin. However there is no ground pin. Some of the graphs
quote supplies of +/-2.5V so 3.2v would not seem to be a good idea. Is the
device actually controlled by the voltage relative to the +Vs or to the -Vs
pins?
Answer
The levels in the spec tables can be referred to the positive supply. So, for
the “Enabled” case of >3.2V, it would mean “greater than 1.8V down from the
positive supply – the same approach can be used for the “Powered Down” case of
similar situation often occurs with output swing levels and input common-mode
range, which are sometimes specified in absolute volts, and sometimes relative
to the supplies. The numbers are nonetheless correct, since “ground” on +/-5V
supplies, where the specifications are defined, is zero volts, and there is
generally a ground plane defining zero volts between the two supplies.