PS323 ETC, PS323 Datasheet - Page 5

• Manufacturer Part Number: PS323
• Description: Precision / Single-Supply SPST Analog Switches
• Manufacturer: ETC

1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 1 2 3 4 5 6 7 8 9 0 1 2
1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 1 2 3 4 5 6 7 8 9 0 1 2
Application Information
The PS323, PS324, & PS325 dual analog switches precisely switch
inputs with a single 2.7V to 12V supply, low on-resistance (30Ohm)
and high speed operation (tON = 85ns, tOFF = 25ns). The devices
are suited to portable battery powered equipment due to a low
supply voltage (2.7V), low power consumption (5mW), and low
leakage currents (0.1nA). High frequency applications benefit from
the high bandwidth, high off isolation, and low crosstalk.
Proper Power Supply Sequencing & Over-voltage
Protection
With any CMOS device, proper power supply sequencing is needed
to protect the device from excessive input currents, which may
permanently damage the IC. All I/O pins contain ESD protection
diodes from the pin to V+ and to GND (see Figure 9 below).
To prevent forward biasing of these diodes, V+ must be applied
before any input signals, and input signals must swing between
V+ and GND. If these conditions cannot be guaranteed, then one
of two suggested protection methods must be employed. Protect
the logic inputs by adding a 1kOhm resistor in series with the input
(see Figure 9). The resistor limits the currents below the threshold
that can cause permanent damage to sub-micro Amp levels. This
reduced input current produces an insignificant voltage drop during
normal operation. A series resistor is not desirable, but small-signal
diodes can be added in series with the supply pins to provide
over-voltage protection for the IC. The diodes limit the analog
signal from 1V below V+ to 1V above GND. The leakage current will
remain low, but the switch resistance may increase at low supply voltages.
V g
Figure 1. Overvoltage Protection
NO
Positive Supply
V+
V-
COM
5
Power-Supply Considerations
The PS32X construction is typical of most CMOS analog switches,
except that they have voltage supply pins: V+ and GND. These
pins power the internal CMOS switches and set the analog voltage
limits. Unlike switches with a 12V maximum supply voltage, the
PS32X series is made from a 17V-supply voltage technology that
provides room for 10-20% tolerance on 12V supplies. This
technology gives room for overshoot and noise spikes. While the
minimum recommended supply voltage is 2.7V, it is important to
note that the input signal-range, switching times, and on-resistance
degrade at lower supply voltages. Refer to the electrical
specifications for details. V+ and GND also power the internal
logic drivers (turn the switch on & off). These switches can be
operated with bipolar supplies if the voltage range from V- (at the
GND pin) to V+ does not exceed a total of 12V.
Logic-Level Thresholds
The switch logic is TTL compatible (0.8V & 2.4V) over a supply
range of 3V to 11V. At 12V the VIH level is about 2.5V. This is below
the TTL guaranteed high output minimum level of 2.8V, but noise
margin is reduced. For best results with a 12V supply, use logic
drive that 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 digital input signals from GND
to V+ with a fast transition time minimizes power dissipation.
High-Frequency Signal Passing/Isolation
In 50Ohm systems, signal response is flat even past 300MHz.
An OFF switch is like a capacitor and passes high frequencies
with low attenuation, resulting in signal passing from the switch
input to output. OFF Isolation is the resistance to passing signals
while the switch is OFF, while Crosstalk indicates the amount of
signal noise that crosses over from one switch to another. The
OFF Isolation is about 50dB in 50Ohm systems. Larger load
impedances reduce Off Isolation and Crosstalk due to the voltage
divider action of the switch OFF impedance and the load.
Leakage Current
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.
Most of the analog leakage current comes from the ESD diodes to
V+ or GND. Although the ESD diodes on a given signal pin are
identical and therefore fairly well balanced, they are reverse biased
differently. Either V+ or GND and the analog signal bias each
diode. Hence, leakage currents will vary as the signal varies. The
difference in the diode leakage currents to the V+ and GND pins
creates the analog signal-path leakage current. Also, analog leakage
currents flow between each pin and one of the supply terminals,
not to the other switch terminal. This is why both sides of a given
switch can show leakage currents of the same or opposite polarity. There
is no connection between the analog-signal paths and V+ or GND.
Precision, Single Supply SPST Analog Switches
PS323/PS324/PS325
PS8188C
10/24/02