Application Skills of Integrated Multi-channel Analog Switches

Integrated multi-way analog switches (hereinafter referred to as multi-way switches ) are commonly used devices in important technical fields such as automatic data acquisition and programmable gain amplification. The quality of their actual performance has an important impact on the rigor and reliability of the system. Regarding the application technology of multi-way switches, some literature has two shortcomings: one is that there is more introduction to the device itself, but less introduction to the reasonable matching and coordination between the device and related circuits ; the other is that there is more introduction to principled things, but less introduction to operational things. Research shows that only by correctly selecting the type of multi-way switch, paying attention to the reasonable matching and coordination between the multi-way switch and related circuits , and ensuring that each circuit unit has a suitable working state, can the performance of the multi-way switch be fully utilized, and even make up for the lack of certain performance indicators, and achieve the expected effect. This article studies the application skills of multi-way switches from the perspective of application. At present, the multi-way switches on the market are mainly CMOS circuits, so the following discussion is aimed at this type of product unless otherwise specified.

1 Choice between “break before make” and “make before break”

Currently, the on-off switching mode of multi-way switches on the market is mostly "Break-Before-Make".

In automatic data acquisition, a "break before make" multi-way switch should be used. Otherwise, the two channels will be short-circuited, which may damage the signal source or the multi-way switch itself. However, in a programmable gain amplifier, if a multi-way switch is used to change the feedback resistor of an integrated operational amplifier to change the gain of the amplifier,
a "break before make" multi-way switch should not be used. Otherwise, the amplifier will be in an open-loop state. The open-loop gain of the amplifier is extremely high, which can easily disrupt the normal operation of the circuit and even damage components, and should generally be avoided.

2 Select the appropriate transmission signal input method

There are generally two ways to transmit signals: single-ended input and differential input, which are suitable for different occasions.

The single-ended input method is shown in Figure 1, that is, one end of all signal sources is connected to the same signal ground, the signal ground is connected to the analog ground of ADC, etc., and the other end of each signal source is connected to a multi-way switch. In the figure, Vs is the transmission signal, and Vc is the common-mode interference signal in the system.

The advantage of the connection method in Figure 1 (a) is that the common-mode rejection capability of the system can be guaranteed without reducing the number of channels by half. The disadvantage is that it is only applicable when all transmitted signals are referenced to a common potential and all signal sources are placed in the same noise environment, otherwise additional differential-mode interference will be introduced.

The connection method in Figure 1 (b) is suitable for the measurement of all transmission signals relative to the system analog common ground, and the signal level is significantly greater than the common mode interference in the system. Its advantage is that the maximum number of channels can be obtained, and its disadvantage is that the system basically loses the common mode suppression capability.

The differential input method is shown in Figure 2, that is, the two ends of all signal sources are connected to the input end of the multi-way switch. Its advantage is that it has a strong ability to resist common-mode interference, but its disadvantage is that the actual number of channels is only half of that of the single-ended input method. When the signal-to-noise ratio of the transmitted signal is low, the differential input method must be used.

3 Reduce the impact of on-resistance

The on-resistance RON of a multi-way switch (generally about 10Ω to 1kΩ) is much larger than the contact resistance (generally mΩ) of a mechanical switch, which has a significant impact on the signal transmission accuracy of automatic data acquisition or the gain of program control gain amplification. In addition, the RON channel changes with the power supply voltage, the amplitude of the transmission signal, etc., so its impact is difficult to correct later. In practice, it is generally tried to reduce RON to reduce its impact.

Taking CD4051 as an example, the test found [1] that RON of CD4051 changes with the power supply voltage and input analog voltage. When VDD=5V and VEE=0V, RON=280Ω, and changes suddenly with the change of V1; when VDD>10V and VEE=0V, RON=100Ω, and changes slowly with the change of V1. It can be seen that appropriately increasing the VDD of CD4051 is beneficial to reducing the influence of RON. It must be noted that when increasing VDD, the input logic level of the gate control terminals A, B, and C should be increased accordingly. For example: when VDD=12V (VEE=0V), the power supply voltage pull-up clamping method can be adopted, and the resistance of the pull-up resistor is set to be above 1.5kΩ, so that the effective high level of the gate control terminal signal is not less than 6V. In this way, the CD4051 is ideally turned on (RON is small), and the conversion between CMOS level and TTL level is realized (μP is generally TTL level).

It can be seen that, according to the specific situation, appropriately increasing the power supply voltage of the multi-way switch is an effective measure to reduce its RON effect. In addition, appropriately increasing the power supply voltage can also reduce the on-resistance difference ΔRON and speed up the switching speed at the same time.

4 Eliminating Errors Caused by Jitter

Similar to mechanical switches, multi-way switches also experience jitter when switching channels, which can cause transients. If the output signal of the multi-way switch is collected at this time, large errors may be introduced. For example [2]: A computer automatic data acquisition and processing system collects three analog quantities: pump speed, flow, and pressure. The TTL levels corresponding to the three analog quantities are 1.5454V, 1.5698V, and 2.9394V, respectively. The acquisition system collects these three analog quantities from channels 1, 2, and 3 for 10 consecutive times, and the acquisition results are between 1.8554 and 1.8603, 1.5625 and 1.5673, and 1.62207 and 1.62695, with large errors in channels 1 and 3. Research has found that this error is caused by the system collecting data before the multi-way switch is stable.

There are two common methods to eliminate jitter: one is to use hardware circuits (hardware method), that is, to use RC filters to eliminate jitter; the other is to use software delay methods to solve it (software method). In systems with μP, the software method is more advantageous than the hardware method. As in the above example, as long as a loop statement is added to the original QuickBASIC data acquisition program to delay appropriately, the acquisition results will be between 1.5454~1.5478, 1.5698~1.5722, and 2.9394~2.9418, the acquisition accuracy is significantly improved, and the acquisition results are normal.