10
Detailed Descript ion
The IS L43140–ISL431 42 quad analog s witches offer p recise
switching capability from a bipolar ±2V to ±6V or a single 2V
to 12V supply with low on-resistance (50Ω) and high speed
switch ing (tON = 40ns, tOFF = 15ns). The devices are
especi ally we ll suited to portable battery po wered equip ment
thanks to the low operating supply voltage (2V), low power
consumption (1µW), low leakage currents (1nA max), and the
tiny QFN packa ging. High frequency applica t ions also benefit
from the wide bandwidth, and the very hi gh off isolatio n 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 perm anently d amage the I C. All I/O p ins conta in
ESD protection diod es from the p in to V+ and to V- (see
Figure 8). To prevent forward biasing these diodes, V+ and
V- must be applie d befo re any input signa ls, a nd inp ut sig nal
voltage s must remain be tween V+ and V-. I f these condit ions
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 8). 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.
Adding a series resistor to the switch input defeats the
purpose of using a low R ON switch , so two small si gnal
diodes can be added i n ser ies with the supp ly pins to prov ide
overvoltage protection for all pins (see Figure 8). These
additio nal dio des l imit th e anal og signal from 1 V bel ow V+ to
1V above V-. The low leakage current performance is
unaffected by this approach, but the switch resistance may
increase, especially at low supply voltages.
Power-Supply Considerations
The ISL4314X construction is typical of most CMOS analog
switches, in that they have three supply pins: V+, V-, and
GND. V+ and V- drive the internal CMOS switches and set
their analog voltage limits, so there are no connections
between the analog signal path and GND. Unlike switches
with a 13V m aximum suppl y voltage, the ISL4314X 15V
maximum supply voltage provides plenty of room for the
10% tolerance of 12V supplies (±6V or 12V sing le sup ply),
as well as room for overshoot and noise spikes.
This fami ly of swit ches per forms equ all y well whe n operated
with bipolar or single voltage supplies. The addition of the
GND pin allows for asymmetrical bipolar supplies (e.g. +5V
and -3V). The minimum recommended supply voltage is 2V
or ±2V. 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 for deta ils .
V+ and GND power the internal logic (thus setting the digital
switching point) and level shifters. The level shifters convert
the logic levels to switched V+ and V- signals to drive the
analog switch gate terminals, so switch parameters -
especi ally RON - are strongly influenced by V-.
Logic-Level Thresholds
V+ and GND power the internal logic stages, so V- has no
affect on logic thresholds. This switch family is TTL
compatible (0.8V and 2.4V) over a V+ supply range of 2.5V
to 10V (see Figure 17). At 12V the VIH level is about 2.7V,
so for best results use a logic family the provides a VOH
greater than 3V.
The digital input stages draw supply current whenever the
digital input voltage is not at one of the supply rails. Driving
the digita l input si gnals from GND to V+ with a fast trans ition
time minimizes power dissipation.
High-Frequency Performance
In 50Ω system s, signa l respons e is reasonab ly flat even past
100MHz (see Figure 18). Figure 18 also illustrates that the
frequenc y response is very consiste nt over a wide V+ rang e,
and for varying analog signal levels.
An off switch acts like a capacitor and passes higher
frequencies with less attenuation, resulting in signal
feedthrough from a switch’s input to its output. Off Isolation
is the resistance to this feedthrough, while Crosstalk
indicates the amount of feedthrough from one switch to
another. Figure 19 details the high Off Isolation and
Crosstalk rejection provided by this family. At 10MHz, off
isolation is about 50dB in 50Ω systems, decr easing
approximately 20dB per decade as frequency increases.
Higher load impedances decrease Off Isolation and
Crosstalk rejection due to the voltage divider action of the
switch OFF impedance and the load impedance.
FIGURE 8. OVERVOLTAGE PROTECTION
V-
VCOM
VNO or NC
OPTIONAL PROTECTION
V+
INX
DIODE
OPTIONAL PROTECTION
DIODE
OPTIONAL
PROTECTION
RESISTOR
ISL43140, ISL43141, ISL43142