COM

NO or

NC

SUPPLY VOLTAGE

ANALOG

INPUT

SIGNAL

COM

D1

NO

GND

0.047 PF

LMS4684

www.ti.com

SNOSAL0C – DECEMBER 2004 – REVISED APRIL 2013

It is also good practice to provide adequate supply bypassing to all analog circuits. We recommend a that

minimum bypass capacitor value of 0.047µF be provided for the LMS4684. An inadequate bypass capacitor can

lead to excessive supply current.

Figure 17. Input Over Voltage Protection Circuitry

OFF-ISOLATION

Analog switches are composed of FETs (field Effect Transistors). The channel resistance is low when the pass

transistors are "on" and that resistance is high when the pass transistors are "off". However, when the pass

transistors are "off", the source to drain capacitance of the pass transistors will pass some energy. This

capacitance is inversely proportional to the switch "on" resistance, so a switch with a low "on" resistance may not

be suitable for some high frequency applications.

Figure 18 shows the equivalent circuit of an analog switch. Unless the load impedance after the switch is

relatively low, the switch capacitance will couple excessive energy across the "open" switch at higher

frequencies, degrading off isolation performance. Off Isolation of the LMS4684 is specified with a 50

Ω load.

Higher load impedances will degrade off isolation performance compared with what is specified.

Figure 18. Equivalent Circuit of an Analog Switch

Off isolation may be improved by decreasing the LMS4684 load impedance below 50

Ω. When doing this, be sure

that the LMS4684 maximum current rating is not exceeded. Also, decreasing the load impedance too much can

result in excessive signal distortion because the channel resistance variation with input signal voltage would then

be a greater percentage of the load impedance.

If it is desired to extend the usable bandwidth of the LMS4684 while maintaining reasonable off-isolation is

through the use of the circuit of Figure 19.

Copyright © 2004–2013, Texas Instruments Incorporated

Submit Documentation Feedback

9

Product Folder Links: LMS4684