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
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–4–
E02995-0–x/02(0)