MT-088: Analog Switches and Multiplexers Basics

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MT-088 Figure 11: Off Isolation Versus Frequency Figure 12 shows typical CMOS analog switch OFF-isolation as a function of frequency for the ADG708 8-channel multiplexer. From dc to several kilohertz, the multiplexer has nearly 90-dB isolation. As the frequency increases, an increasing amount of signal reaches the output. However, even at 10 MHz, the switch shown still has nearly 60 dB of isolation. Figure 12: OFF-Isolation Versus Frequency for ADG708 8-Channel Multiplexer Page 8 of 23
MT-088 Another ac parameter that affects system performance is the charge injection that takes place during switching. Figure 13 shows the equivalent circuit of the charge injection mechanism. V DD V SS Step waveforms of ± (V DD –V SS ) are applied to C Q , the gate capacitance of the output switches. Figure 13: Dynamic Performance Considerations: Charge Injection Model When the switch control input is asserted, it causes the control circuit to apply a large voltage change (from V DD to V SS , or vice versa) at the gate of the CMOS switch. This fast change in voltage injects a charge into the switch output through the gate-drain capacitance C Q . The amount of charge coupled depends on the magnitude of the gate-drain capacitance. The charge injection introduces a step change in output voltage when switching as shown in Figure 14. The change in output voltage, ΔV OU T, is a function of the amount of charge injected, Q INJ (which is in turn a function of the gate-drain capacitance, C Q ) and the load capacitance, C L . 0V Figure 14: Effects of Charge Injection on Output Page 9 of 23
Page 1 and 2: MT-088 TUTORIAL Analog Switches and
Page 3 and 4: MT-088 SWITCH DRIVER SWITCH Figure
Page 5 and 6: MT-088 DC errors associated with a
Page 7: MT-088 Figure 9: Bode Plot of CMOS
Page 11 and 12: MT-088 Crosstalk is related to the
Page 13 and 14: MT-088 APPLYING THE ANALOG SWITCH S
Page 15 and 16: MT-088 It is important to know how
Page 17 and 18: MT-088 R F = 10kΩ - 625Ω 1.43kΩ
Page 19 and 20: MT-088 The ADG918 is an absorptive
Page 21: MT-088 If diode protection is used,

MT-088: Analog Switches and Multiplexers Basics

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