RON Modulation in CMOS Switches and Multiplexers; What It Is and How to Predict Its Effect on Signal Distortion* By John Wynne on-channel resistance with input signal is termed RON modulation, and the actual spread of maximum channel resistance to minimum channel resistance over the signal swing of interest is represented by DRON. A single CMOS switch or a single channel of a CMOS multiplexer essentially consists of an N-channel and a P-channel MOSFET transistor in parallel. Figure 1a shows this basic arrangement. The respective drains and sources of the two transistors are tied together to become the switch terminals while the gates of the two transistors are usually driven with the power supply voltages, VDD and VSS, to control the on/off action of the switch. RON RON (N-CHANNEL) Essentially, the N-channel is on for positive gate-tosource voltages and off for negative gate-to-source voltages while the P-channel is vice versa. RON (P-CHANNEL) CMOS RESULTANT RON VDD(+) 0 Q1 VDD VSS N-CHANNEL Q4 Figure 1b. Individual MOSFET R ON Profiles vs. V S(V D) Figure 2 shows typical RON profiles for Analog Devices ADG508A/ADG509A multiplexers. Three RON curves are shown for three power supply ranges. As might be expected, both the absolute value of the channel RON and the DRON increase as the power supplies are reduced. It is instructive to compare the RON profiles from this generation of multiplexers with devices from recent generations. Figure 3 shows the RON profile for the 4-/8channel ADG608/ADG609 multiplexers. Due to the use of a 3 mM, 12 V process, these devices are limited to ±5 V operation rather than the ±15 V operation of the ADG508A/ADG509A series, which is built on a 6 mM, 44 V process. More modern examples of the emphasis within ADI to reduce RON modulation are single SPDT switches, ADG619/ADG620, running off of ±5 V supplies or a single +5 V supply. The typical DRON for these switches is below 1 W with a typical on resistance of 4 W. P-CHANNEL D Q2 N-CHANNEL VSS(–) Figure 1a. Basic CMOS Switch With a fixed voltage on the gate the effective drive voltage for either transistor varies in proportion to the polarity and magnitude of the analog signal passing through the switch. In Figure 1b, where RON, the resistance of the on switch, is plotted against applied analog switch voltage, VS or VD, the resistance of the N-channel increases with positive voltage and the resistance of the P-channel increases with negative VS or VD. The resultant parallel combination of these two characteristics (heavy line in Figure 1b) results in the well-known crown or twin-peak characteristic. This variation in a *Based on Application Note AN-251, which appeared in the ADI Applications Reference Manual of 1993. REV. 0 VS (VD) S P-CHANNEL Q3 + –1– © Analog Devices, Inc., 2002 700 VIN1 600 RS1 S1 VDD = +5V VSS = –5V D RON – 500 VIN8 400 300 RS8 VOUT RL S8 VDD = +10.8V VSS = –10.8V 1 OF 8 DECODER 200 A0 100 0 –20 VDD = +15V VSS = –15V –15 –10 –5 0 5 VD (VS) – V 10 15 Also included in Figure 4 are any source resistances, RSX, which may exist for each channel. Over the signal range of interest, the amount of distortion generated through any one channel is proportional to the ratio of DRON to total minimum channel resistance: 20 50 ( ) Distortion a DRON / RL + RSX + RON min TA = +25C 40 ON RESISTANCE – A2 Figure 4. Multiplexing High Level Signals Figure 2. R ON as a Function of V D(V S): Dual Supply Voltage, T A = 25 C, ADG508A/ADG509A 45 A1 For any given multiplexer or switch with a given value of RON min and a maximum value of DRON over the signal range, the distortion generated can be kept low by choosing a relatively large value for RL, which does not compromise the performance of the circuit due to the additional thermal noise. 35 30 VDD = +3V VSS = –3V 25 VDD = +5V VSS = –5V 20 15 10 PREDICT DISTORTION GRAPHICALLY Figures 5 and 6 show nomographs that can be used to quickly predict, to a first order, the distortion generated through a single channel. The nomogragh scaling is based upon the circuit configuration of Figure 4. The left-most scale represents the total channel resistance from source through to load and including the switch RON value at 0 V analog input. The middle scale represents the total harmonic distortion (THD) and the right-most scale represents the DRON over the signal range of interest. 5 0 –5.0 –4.0 –3.0 –2.0 –1.0 0 1.0 VD (VS) – V 2.0 3.0 4.0 5.0 Figure 3. R ON as a Function of V D(V S): Dual Supply Voltage, T A = 25 C, ADG608/ADG609 MODELING THE DISTORTION EFFECT Configuring the signal conditioning circuitry so that the switch or multiplexer operates directly into a summing junction of an op amp will obviously ensure a very low voltage across the switch which in turn virtually eliminates RON modulation problems. However it is not always possible or desirable to do this; many applications require high level signals to be passed through the channel. To use a nomograph, locate the total channel resistance on the left-most scale and the DRON on the right-most scale. A straight line is drawn between these two points, and where the line intersects with the middle scale is the approximate THD to be expected from the system. Figure 5 is scaled for switches and multiplexers exhibiting a DRON from 10 W to 100 W; Figure 6 is scaled for more modern switches and multiplexers that exhibit a DRON from 1 W to 10 W. Figure 4 shows a typical situation where high level signals are multiplexed into a common load resistance, RL. In this situation, RON modulation can be kept to a minimum if the analog input signal range is restricted. For instance, for the ADG508A operating on ±15 V power supplies, DRON is typically less than 20 W, with a ±5 V input signal range, increasing to over 50 W, for a ±10 V signal range. In contrast, for the ADG608 operating on ±5 V supplies, DRON is typically less than 3 W within its specified ±3 V signal range, increasing to approximately 21 W for a ±5 V signal range. Consider a switch with RON = 15 W at VS(VD) = 0 V and DRON = 3 W over the signal range of interest. With RS = 0 W and RL = 1 kW and using Figure 6, the THD through the channel is approximately 0.08% or –62 dB. Increasing the load resistor to 10 kW improves the THD to 0.0122% or –78 dB. –2– 100 90 10% 9% 8% 7% 6% 20dB 80 5% 70 4% 3% 2% 1% 1k 60 40dB 0.8% 0.7% 0.6% 2k 50 0.5% 0.4% 3k 0.3% 40 4k 5k 6k 7k 8k 9k 10k 20k 0.2% 0.1% 60dB 0.08% 0.07% 0.06% 30 0.05% 0.04% 30k 0.03% 40k 50k 60k 70k 80k 90k 100k 200k 0.02% 0.01% 80dB 20 0.008% 0.007% 0.006% 0.005% 0.004% 300k 0.003% 400k 500k 600k 700k 800k 900k 1M 0.002% 0.001% TOTAL CHANNEL RESISTANCE 100dB TOTAL HARMONIC DISTORTION (THD) 10 RON Figure 5. Nomogragh to Determine THD Through a Single Switch or Multiplexer Channel, DR ON from 10 W to 100 W REV. 0 –3– 10 9 10% 9% 8% 7% 6% 5% 20dB 8 7 4% 3% 2% 1% 100 6 40dB 0.8% 0.7% 0.6% 200 5 0.5% 0.4% 300 0.3% 4 400 500 600 700 800 900 1k 2k 0.2% 0.1% 60dB 0.08% 0.07% 0.06% 3 0.05% 0.04% 3k 0.03% 4k 5k 6k 7k 8k 9k 10k 20k 0.02% 0.01% 80dB 2 0.008% 0.007% 0.006% 0.005% 0.004% 30k 0.003% 40k 50k 60k 70k 80k 90k 100k 0.002% 0.001% TOTAL CHANNEL RESISTANCE 100dB TOTAL HARMONIC DISTORTION (THD) 1 RON Figure 6. Nomograph to Determine THD Through a Single Switch or Multiplexer Channel, DR ON from 1 W to 10 W a One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • Tel: 781/329-4700 • Fax: 781/326-8703 • www.analog.com PRINTED IN U.S.A. –4– E02995-0–x/02(0)