12345678901234567890123456789012123456789012345678901234567890121234567890123456789012345678901212345678901234567890123456789012123456789012

12345678901234567890123456789012123456789012345678901234567890121234567890123456789012345678901212345678901234567890123456789012123456789012

5

PS8188C

10/24/02

PS323/PS324/PS325

Precision, Single Supply SPST Analog Switches

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

remainlow,buttheswitchresistancemayincreaseatlowsupplyvoltages.

V+

Positive Supply

COM

NO

Vg

V-

Figure 1. Overvoltage Protection

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.