13
FN6053.3
March 13, 2006
Detailed Description
The ISL43681 and ISL43741 multiplexers offer precise
switching capability from a bipolar ±2V to ±6V or a single 2V
to 12V supply with low on-resistance (39
) and high speed
operation (tON =38ns, tOFF = 19ns) with dual 5V supplies.
They have an inhibit and inhibit bar pin to simultaneously
open all signal paths. They also have a latch bar pin to lock
in the last switch address.
The devices are especially well suited for applications using
±5V supplies. With ±5V supplies the performance (RON,
Leakage, Charge Injection, etc.) is best in class.
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 V- (see
Figure
9). To prevent forward biasing these diodes, V+ and
V- must be applied before any input signals, and input signal
voltages must remain between V+ and V-. 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 1k
resistor in series with the input (see Figure
9). 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 RON switch, so two small signal
diodes can be added in series with the supply pins to provide
overvoltage protection for all pins (see Figure
9). These
additional diodes limit the analog signal from 1V below 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 ISL43681 and ISL43741 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 maximum supply voltage, the
ISL43681 and ISL43741 15V maximum supply voltage
provides plenty of room for the 10% tolerance of 12V
supplies (±6V or 12V single supply), as well as room for
overshoot and noise spikes.
This family of switches performs equally well when operated
with bipolar or single voltage supplies.The minimum
FIGURE 7. CROSSTALK TEST CIRCUIT
FIGURE 8. CAPACITANCE TEST CIRCUIT
Test Circuits and Waveforms (Continued)
0V or V+
ANALYZER
NOA or NCA
SIGNAL
GENERATOR
RL
GND
ADDX
50
N.C.
COMB
NOB or NCB
EN
V-
C
V+
C
COMA
EN, LE
ISL43741
GND
NO or NC
COM
ADDX
IMPEDANCE
ANALYZER
0V or V+
EN
V-
C
V+
C
EN, LE
FIGURE 9. INPUT OVERVOLTAGE PROTECTION
VCOM
VNO or NC
OPTIONAL PROTECTION
V+
LOGIC
DIODE
OPTIONAL PROTECTION
DIODE
OPTIONAL
PROTECTION
RESISTOR
FOR LOGIC
INPUTS
1k
V-
ISL43681, ISL43741