7
FN6095.4
October 28, 2010
Detailed Description
The ISL84781 analog multiplexer offers precise switching
capability from a single 1.6V to 3.6V supply with ultra low
ON-resistance (0.41Ω) and high speed operation
(t
ON
=16ns, t
OFF
= 13ns) with +3V supply. The device is
especially well-suited for portable battery powered
equipment thanks to the low operating supply voltage (1.6V),
low power consumption (0.2µW), and low leakage currents
(70nA max). High frequency applications also benefit from the
wide bandwidth, and the very high off isolation and crosstalk
rejection.
Supply Sequencing and Overvoltage Protection
With any CMOS device, proper power supply sequencing is
required to protect the device from excessive input currents
which might permanently damage the IC. All I/O pins contain
ESD protection diodes from the pin to V+ and to GND
(See Figure 7). To prevent forward biasing these diodes, V+
must be applied before any input signals, and the input
signal voltages must remain between V+ and GND. If these
conditions cannot be guaranteed, then one of the following
two protection methods should be employed.
Logic inputs can easily be protected by adding a 1kΩ
resistor in series with the input (see Figure 7). The resistor
limits the input current below the threshold that produces
permanent damage, and the sub-microamp input current
produces an insignificant voltage drop during normal
operation.
This method is not applicable for the signal path inputs.
Adding a series resistor to the switch input defeats the
purpose of using a low r
ON
switch, so two small signal
diodes can be added in series with the supply pins to provide
overvoltage protection for all pins (see Figure 7). These
additional diodes limit the analog signal from 1V below V+ to
1V above GND. The low leakage current performance is
unaffected by this approach, but the switch signal range is
reduced and the resistance may increase, especially at low
supply voltages.
Power-Supply Considerations
The ISL84781 construction is typical of most single supply
CMOS analog multiplexers, in that it has two supply pins: V+
and GND. V+ and GND drive the internal CMOS switches
and set its analog voltage limits. Unlike switches with a 4V
maximum supply voltage, the ISL84781 4.7V maximum
supply voltage provides plenty of room for the 10% tolerance
of 3.6V supplies, as well as room for overshoot and noise
spikes.
The minimum recommended supply voltage is 1.6V but the
part will operate with a supply below 1.5V. It is important to
note that the input signal range, switching times, and
ON-resistance degrade at lower supply voltages. Refer to
the electrical specification tables and “Typical Performance
Curves” beginning on page 8 for details.
V+ and GND power the internal logic and level shifters. The
level shifters convert the logic levels to switched V+ and
GND signals to drive the analog switch gate terminals.
These multiplexers cannot be operated with bipolar supplies,
because the input switching point becomes negative in this
configuration.
Logic-Level Thresholds
This device is 1.8V CMOS compatible (0.5V and 1.4V) over
a supply range of 2.0V to 3.6V (See Figure 12). At 3.6V the
V
IH
level is about 1.27V. This is still below the 1.8V CMOS
guaranteed high output minimum level of 1.4V, but noise
margin is reduced.
The digital input stages draw supply current whenever the
digital input voltage is not at one of the supply rails. Driving
the digital input signals from GND to V+ with a fast transition
time minimizes power dissipation.
High-Frequency Performance
In 50Ω systems, signal response is reasonably flat even past
10MHz with a -3dB bandwidth of 52MHz (See Figure 16).
The frequency response is very consistent over a wide V+
range, and for varying analog signal levels.
An OFF switch acts like a capacitor and passes higher
frequencies with less attenuation, resulting in signal feed
through from a switch’s input to its output. Off-Isolation is the
resistance to this feed-through. Figure 17 details the high Off
Isolation provided by these devices. At 100kHz, Off Isolation
is about 65dB in 50Ω systems, decreasing approximately
20dB per decade as frequency increases. Higher load
impedances decrease Off Isolation due to the voltage divider
action of the switch OFF impedance and the load
impedance.
Leakage Considerations
Reverse ESD protection diodes are internally connected
between each analog-signal pin and both V+ and GND.
One of these diodes conducts if any analog signal exceeds
V+ or GND.
FIGURE 7. OVERVOLTAGE PROTECTION
GND
V
COM
V
NOx
OPTIONAL PROTECTION
V+
LOGIC
DIODE
OPTIONAL PROTECTION
DIODE
OPTIONAL
PROTECTION
RESISTOR
FOR LOGIC
INPUTS
1kΩ
ISL84781
