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ADG702LBRJZ-REEL7 Datasheet(PDF) 10 Page - Analog Devices |
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ADG702LBRJZ-REEL7 Datasheet(HTML) 10 Page - Analog Devices |
10 / 12 page ADG701L/ADG702L Rev. 0 | Page 10 of 12 APPLICATIONS INFORMATION The ADG701L/ADG702L belong to the Analog Devices new family of CMOS switches. This series of general-purpose switches have improved switching times, lower on resistance, higher bandwidth, low power consumption, and low leakage currents. SUPPLY VOLTAGES Functionality of the ADG701L/ADG702L extends from 1.8 V to 5.5 V single supply, making the parts ideal for battery-powered instruments where power, efficiency, and performance are important design parameters. It is important to note that the supply voltage affects the input signal range, the on resistance, and the switching times of the part. The effects of the power supplies can be clearly seen in the Typical Performance Characteristics and the Specifications sections. For VDD = 1.8 V operation, RON is typically 40 Ω over the temperature range. BANDWIDTH Figure 19 illustrates the parasitic components that affect the ac performance of CMOS switches (a box surrounds the switch). Additional external capacitances further degrade some perform- ance. These capacitances affect feedthrough, crosstalk, and system bandwidth. VIN SD RON CDS CD CLOAD RLOAD VOUT Figure 19. Switch Represented by Equivalent Parasitic Components The transfer function that describes the equivalent diagram of the switch (see Figure 19) is of the form A(s), as shown in the following equation: ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ + + = 1 ) ( 1 ) ( ) ( T T ON DS ON T R C R s C R s R s A where CT = CLOAD + CD + CDS. The signal transfer characteristic is dependent on the switch channel capacitance, CDS. This capacitance creates a frequency zero in the numerator of the transfer function, A(s). Because the switch on resistance is small, this zero usually occurs at high frequencies. The bandwidth is a function of the switch output capacitance combined with CDS and the load capacitance. The frequency pole corresponding to these capacitances appears in the denominator of A(s). The dominant effect of the output capacitance, CD, causes the pole breakpoint frequency to occur first. In order to maximize bandwidth, a switch must have a low input and output capaci- tance and low on resistance. The on response versus frequency for the ADG701L/ADG702L is shown in Figure 11. OFF ISOLATION Off isolation is a measure of the input signal coupled through an off switch to the switch output. The capacitance, CDS, couples the input signal to the output load when the switch is off (see Figure 20). VIN SD CDS CD CLOAD RLOAD VOUT Figure 20. Off Isolation Is Affected by External Load Resistance and Capacitance The larger the value of CDS, the larger the values of feedthrough produced. Figure 10 illustrates the drop in off isolation as a function of frequency. From dc to roughly 1 MHz, the switch shows better than −75 dB isolation. Up to frequencies of 10 MHz, the off isolation remains better than −55 dB. As the frequency increases, more and more of the input signal is coupled through to the output. Off isolation can be maximized by choosing a switch with the smallest CDS possible. The values of load resistance and capacitance also affect off isolation, as they contribute to the coefficients of the poles and zeros in the transfer function of the switch when open. ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ + + = 1 ) )( ( 1 ) ( ) ( T LOAD DS LOAD T C R s C R s R s A |
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