Remember that ground is ground. The only issues are color coding and noise. The OP wants to use one wire instead of two. He can, and it will work fine, as long as the 24AC is grounded, and the 24DC supplying the PLC Analog card is also grounded at the same point or potential.
From another site:
In section 66.5.3, the UL508A standard lists the following
color coding for internal power wiring.
Black - all ungrounded control power circuit conductors
regardless of voltage
White or natural grey - grounded AC current-carrying power
circuit conductor, regardless of voltage
In section 66.9, the UL508A standard lists the following
color coding for internal control wiring.
Black - all ungrounded control circuit conductors operating at the supply voltage
Red - ungrounded AC control circuits operating at a voltage less than the supply voltage
Blue - ungrounded DC control circuits
Yellow - ungrounded control circuits or other wiring, such
as for cabinet lighting, that remain energized when the main disconnect is in the "off" position
White or natural grey - grounded AC current-carrying control circuit conductor, regardless of voltage
White with blue stripe - grounded DC current-carrying
control circuit conductor
White with yellow stripe - grounded AC control circuit
current-carrying conductor that remains energized when the
main disconnect is in the "off" position
The UL508A standard does not apply to wiring outside the
industrial control panel.
From another site:
In section 66.5.3, the UL508A standard lists the following
color coding for internal power wiring.
Black - all ungrounded control power circuit conductors
regardless of voltage
White or natural grey - grounded AC current-carrying power
circuit conductor, regardless of voltage
In section 66.9, the UL508A standard lists the following
color coding for internal control wiring.
Black - all ungrounded control circuit conductors operating at the supply voltage
Red - ungrounded AC control circuits operating at a voltage less than the supply voltage
Blue - ungrounded DC control circuits
Yellow - ungrounded control circuits or other wiring, such
as for cabinet lighting, that remain energized when the main disconnect is in the "off" position
White or natural grey - grounded AC current-carrying control circuit conductor, regardless of voltage
White with blue stripe - grounded DC current-carrying
control circuit conductor
White with yellow stripe - grounded AC control circuit
current-carrying conductor that remains energized when the
main disconnect is in the "off" position
The UL508A standard does not apply to wiring outside the
industrial control panel.