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ADG723BRM Datasheet(PDF) 8 Page - Analog Devices |
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ADG723BRM Datasheet(HTML) 8 Page - Analog Devices |
8 / 8 page ADG721/ADG722/ADG723 –8– REV. 0 APPLICATIONS INFORMATION The ADG721/ADG722/ADG723 belongs to Analog Devices’ new family of CMOS switches. This series of general purpose switches have improved switching times, lower on resistance, higher bandwidths, low power consumption and low leakage currents. ADG721/ADG722/ADG723 Supply Voltages Functionality of the ADG721/ADG722/ADG723 extends from +1.8 V to +5.5 V single supply, which makes it ideal for battery powered instruments, where important design parameters are power efficiency and performance. It is important to note that the supply voltage effects the input signal range, the on resistance and the switching times of the part. By taking a look at the typical performance characteristics and the specifications, the effects of the power supplies can be clearly seen. For VDD = +1.8 V, on resistance is typically 40 Ω over the tem- perature range. On Response vs. Frequency Figure 8 illustrates the parasitic components that affect the ac performance of CMOS switches (the switch is shown surrounded by a box). Additional external capacitances will further degrade some performance. These capacitances affect feedthrough, crosstalk and system bandwidth. CDS S VIN CD CLOAD RLOAD D VOUT RON Figure 8. Switch Represented by Equivalent Parasitic Components The transfer function that describes the equivalent diagram of the switch (Figure 8) is of the form (A)s shown below. A(s) = R T s( RON CDS ) +1 s( RON CT RT ) +1 where: CT = CLOAD + CD + CDS RT = RLOAD/(RLOAD + RON) 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. Therefore, in order to maximize bandwidth a switch must have a low input and output capacitance and low on resistance. The On Response vs. Frequency plot for the ADG721/ADG722/ADG723 can be seen in Figure 7. 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 as shown in Figure 9. CDS S VIN CD CLOAD RLOAD D VOUT Figure 9. Off Isolation Is Affected by External Load Resis- tance and Capacitance The larger the value of CDS, larger values of feedthrough will be produced. The typical performance characteristic graph of Fig- ure 5 illustrates the drop in off isolation as a function of fre- quency. From dc to roughly 1 MHz, the switch shows better than –80 dB isolation. Up to frequencies of 10 MHz, the off isolation remains better than –60 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 as 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. A(s) = s( RLOAD CDS ) s( RLOAD )(CLOAD + C D + C DS ) +1 OUTLINE DIMENSIONS Dimensions shown in inches and (mm). 8-Lead SOIC (RM-8) 8 5 4 1 0.122 (3.10) 0.114 (2.90) 0.199 (5.05) 0.187 (4.75) PIN 1 0.0256 (0.65) BSC 0.122 (3.10) 0.114 (2.90) SEATING PLANE 0.006 (0.15) 0.002 (0.05) 0.018 (0.46) 0.008 (0.20) 0.043 (1.09) 0.037 (0.94) 0.120 (3.05) 0.112 (2.84) 0.011 (0.28) 0.003 (0.08) 0.028 (0.71) 0.016 (0.41) 33° 27° 0.120 (3.05) 0.112 (2.84) |
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