Datasheet ADG701L, ADG702L (Analog Devices) - 10

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ADG701L/ADG702L Data Sheet APPLICATIONS INFORMATION
The ADG701L/ADG702L belong to the Analog Devices new The signal transfer characteristic is dependent on the switch family of CMOS switches. This series of general-purpose switches channel capacitance, CDS. This capacitance creates a frequency have improved switching times, lower RON, higher bandwidth, zero in the numerator of the transfer function, A(s). Because low power consumption, and low leakage currents. the switch on resistance is small, this zero usually occurs at high frequencies. The bandwidth is a function of the switch output
SUPPLY VOLTAGES
capacitance combined with CDS and the load capacitance. The Functionality of the ADG701L/ADG702L extends from 1.8 V to frequency pole corresponding to these capacitances appears in 5.5 V single supply, making the parts ideal for battery-powered the denominator of A(s). instruments where power, efficiency, and performance are important design parameters. The dominant effect of the output capacitance, CD, causes the pole breakpoint frequency to occur first. In order to maximize It is important to note that the supply voltage affects the input bandwidth, a switch must have a low input and output capaci- signal range, the on resistance, and the switching times of the tance and low on resistance. The on response versus frequency part. The effects of the power supplies can be clearly seen in the for the ADG701L/ADG702L is shown in Figure 11. Typical Performance Characteristics section and the Specifications section.
OFF ISOLATION
Off isolation is a measure of the input signal coupled through For VDD = 1.8 V operation, RON is typically 40 Ω over the an off switch to the switch output. The capacitance, CDS, temperature range. couples the input signal to the output load when the switch is
BANDWIDTH
off (see Figure 20). Figure 19 illustrates the parasitic components that affect the ac
CDS
performance of CMOS switches (a box surrounds the switch).
S D
Additional external capacitances further degrade some perform-
VOUT
ance. These capacitances affect feedthrough, crosstalk, and
V CD CLOAD RLOAD IN
-019 system bandwidth. 05486
C
Figure 20. Off Isolation Is Affected by External Load Resistance and
DS
Capacitance
S D VOUT RON
The larger the value of CDS, the larger the values of feedthrough
V CD CLOAD RLOAD IN
produced. Figure 10 illustrates the drop in off isolation as a 05486-018 function of frequency. From dc to roughly 1 MHz, the switch Figure 19. Switch Represented by Equivalent Parasitic Components shows better than −75 dB isolation. Up to frequencies of The transfer function that describes the equivalent diagram of 10 MHz, the off isolation remains better than −55 dB. As the the switch (see Figure 19) is of the form A(s), as shown in the frequency increases, more and more of the input signal is following equation: coupled through to the output. Off isolation can be maximized by choosing a switch with the smallest CDS possible. The values  s(R C ) 1  ( A s) ON DS  R   of load resistance and capacitance also affect off isolation, as T s(R C R )   1 ON T T  they contribute to the coefficients of the poles and zeros in the transfer function of the switch when open. and calculate the total capacitance, CT, with the following equation:  s(R C )  1  ( A s) LOAD DS  R   T s(R )(C )   1 C LOAD T  T = CLOAD + CD + CDS where C DS is the drain/source capacitance. Rev. A | Page 10 of 12 Document Outline Features Applications General Description Functional Block Diagrams Product Highlights Revision History Specifications Absolute Maximum Ratings ESD Caution Pin Configurations and Function Descriptions Typical Performance Characteristics Test Circuits Terminology Applications Information Supply Voltages Bandwidth Off Isolation Outline Dimensions Ordering Guide