QI am using the multiplexer ADG1204 (LFCSP_VQ package)for low frequency
applications and long switching time, where VDD is 8V and VSS is -5V. Please
let me know:
1. What is the optimal impedance level for source and load to apply?
2. Is there any importance in connecting the exposed paddle to VSS?
AThe ADG1204 is optimised for low capacitance and charge injection and therefore
has a relatively high ON-resistance - with your 13 V total supply this is
typically 250-300 Ω. But the architecture of all CMOS analog switches and
multiplexers, not simply this one, causes their ON-resistance to vary as the
channel voltage moves from close to one supply to close to the other. This is
illustrated in Figs 4-8 on the Data Sheet.
It is therefore advisable to use a load resistance which is much larger than
the switch ON-resistance. Just how much larger depends on how much gain loss
and distortion your system can tolerate. It is quite common for people to use
an op-amp configured as a unity gain buffer to present a high impedance to the
CMOS switch and a low impedance to their ultimate load.
You may drive the input of the switch from whatever impedance you wish, but
this source impedance will add to the switch impedance.
The paddle is internally connected to Vss so it is not necessary to make
another connection. If large currents are flowing in the switch you may wish to
make thermal contact between the paddle and a heat sink. This heat sink must be
at Vss potential.