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What you need to know about analog keys and multiplexers

For about twenty-five years, integrated semiconductor analog switches and multiplexers created on their basis, faithfully serve the developers of electronic products. The manufacturing process was perfected, the design of the microcircuits was changed - all this made it possible to reduce the supply voltage, power consumption, resistance of the public key, injected charge, switching time. What interesting things does Maxim offer in this group of 325 devices?

The architecture of keys and multiplexers has not changed much over the years, but the constant demand for improved performance makes manufacturers develop more and more new devices to meet the demands of developers. For quite a long time, MOS (metal oxide semiconductor) transistors were used as analog switches. Possessing low resistance in a conducting state and extremely high resistance in a cut-off state, with small leak points and small capacitance, they were almost ideal analog voltage-controlled switches. The need to switch signals that are equal or close in magnitude to the supply voltage, forced to solve this problem using switches on complementary MOSFET (CMOS). The well-known circuit 4066 is the classic analog key circuit for signals ranging from “ground” to a positive supply voltage (Maxim manufactures this chip called MAX4066). It is controlled by a unipolar signal from logic chips. A single p-channel or p-channel field-effect transistor operating in the enrichment mode can serve as an analog switch, but its resistance in the open state will significantly depend on the size of the switched signal.

The connection of the n-channel and p-channel MOS transistor in parallel dramatically reduces this dependence. Only one condition is needed - these transistors must be turned on and off simultaneously. Long-term improvements of the analog key based on CMOS transistors lowered the turn-on voltage threshold to 2.5-5.0 V. Adding a level converter made it possible to obtain gate control signals for complementary MOS transistors from logic level input signals. At the same time, the analog key can now switch the analog signal at a level of ± 15 V. A diagram of the modern key is shown in Fig. one.

Fig. 1. Scheme of the modern CMOS key

The control signal has a TTL logic level. In this case, the CMOS switch on transistors Q9 and Q10 can pass analog signals of ± U level. Transistors Q11 and Q12 shown in the diagram improve key performance by reducing key leakage and reducing modulation of open channel resistance. These two transistors should never be turned on at the same time. Otherwise, the negative power bus will be connected to the load and the on / off time will increase. The safety mode of operation of transistors Q11 and Q12 should be provided constructively. Relatively good parameters for the value of the resistance of the public key, for leakage currents and dynamic distortions of the transmission of a large signal at a frequency of up to 500 KHz are implemented in the MAX3XX keys. The easiest way to improve the above parameters is to connect in parallel with the keys on the chip. So MAX351, having 4 keys, with a parallel connection has a typical open-resistance of 5.5 ohms and the maximum - 11.25 ohms. At the same time, the maximum change in the key resistance from a change in the value of the switched signal does not exceed ΔR open ≤ 1.25 ohms.

Through the open transistors of the key current flows the switched signal. From the power sources, the current in the key practically does not flow. But to offset the levels and to control the key current is needed.

The increase in current occurs at a voltage of about 0.8 V and 2.4 V, which is associated with the transition of the transistors from the open to the closed state (and vice versa) and their transition to this time in the linear mode. If the logic and analog voltages of the power supplies are equal, then the currents through the microcircuit flow at the level of leakage current - less than 1 μA. For normal operation of a switch with different voltages (for example, +5 V and ± 15 V), it is necessary to install shunt 10 μF capacitors in parallel with 100 nF to each source terminal.

Dynamic key errors are determined by the fact that the control signal passes through several stages, and each has a delay. This is especially important in multichannel multiplexers, for example, 8 in 1. It is impossible to implement channel switching on here, if the previous one is not turned off. That's why, in the MAX338 chip, the guaranteed switching delay time is structurally introduced - at least 10 ns. When the key is turned on and off, the control signal through the capacity of the transistors of the preliminary stages injects some charge into the conducting channel of the key. This leads to an error when transmitting the signal through the key. The value of the injected charge is smaller, the smaller the resistance of the open channel. For the same reasons, the rise and fall times of the logic signal at the input for most MAXIM key schemes should not exceed 20 ns.

Knowing the intricacies of building keys, their strengths and weaknesses, one can find the widest use of semiconductor switches and multiplexers in electronic equipment. They can operate with radio frequencies up to 1 MHz and higher. Most analog switches provide little power dissipation and require a simple logical interface. The operation of the keys depends on the signal current in the switching element and, to reduce transmission losses, is usually limited to milliamperes.

To reduce crosstalk at frequencies of the order of 10 MHz and above, you can use keys (standard MAX312, MAX383, video T-keys MA4545) connected in a T-shaped pattern (Fig. 2). One or two keys are connected to earth with low impedance (typical -40 ohm) and excellent decoupling factor (-80 dB at 10 MHz). However, we must remember that with an increase in the operating frequency of the signal, crosstalk and decoupling become unsatisfactory.

Fig. 2. T-shaped key switching circuit for a 10 MHz signal

A simple oscillator circuit for 2 frequencies, stabilized by quartz resonators, is obtained using a chip with four keys (MAX 383) with a supply of ± 8 V or, using MAX 411, ± 18 V.

Integrated circuits of switches and multiplexers can be very useful for automatically setting the gain, frequency, phase, or voltage. For example, if you send a signal to a non-inverting input of an operational amplifier, and set up a serial resistive matrix at the output, then using a 16-channel multiplexer such as MAX 306, you can choose one of 16 gain levels. At the same time, each key is connected from one side to its “own” resistor, and the second side of all the keys is combined and connected to the inverting input of the operational amplifier.

Widespread use found analog keys in sound systems. When a signal passes through a key, the signal should not deteriorate, introduce any new information into it, distort the wave form and phase. Completely avoid this can not be. Obviously, all distortions should be minimized. The total value of the nonlinear distortion coefficient (ТHD) is defined as the ratio of the square root of the sum of squares of the second, third and higher harmonics to the value of the main (first) harmonic. The choice of an analog key with a minimum of THD requires one - low resistance in the open state (R on ) and, therefore, a slight unevenness of resistance R on or flatness.

Flatness is defined as the difference between the maximum and minimum resistance values ​​in the open state, measured in a given range of an analog signal. Often, (unless otherwise indicated in the documentation), flatness is assumed to be 10% of the open channel resistance. Distortions are the result of parallel connection of p-and n-channel transistors, which have non-linear resistance characteristics in the open state.

Practically, the maximum nonlinear distortion is determined by the following relationship:

where r is heat - the load connected in series with the key.

Fig. 3. The dependence of the total nonlinear distortion coefficient (THD) for the frequency

In fig. 3 shows the dependence of THD on the frequency for the three keys MAX 4501, MAX4544, and MAX4621 with a test load R load . = 10 kΩ

These graphs show that in sound systems to minimize total nonlinear distortion, it is necessary to select keys with very low resistance in the open state.

CMOS analog keys undoubtedly have many useful qualities, so most developers consider them as the norm and use them in a wide variety of applications.

Let's pay attention to some technical parameters of keys. Today there are many analog switches working with a single low-voltage power supply. Low-voltage switches with unipolar power supply and logic signals according to CMOS standards and TTL levels are also used. But there are also keys that operate from a supply of ± 15 V or ± 12 V. To control them, another power source is required, labeled V L , which is usually 5 V or 3.3 V.

If the logic signal is at the level of V + (or V L , if present), then the analog keys do not substantially flow current from the power supply. Applying TTL levels at a five-volt voltage V L , it is possible to increase the current from the power source by more than 1000 times. To avoid unnecessary current consumption from a power source, you should avoid using TTL levels - inherited from the 1980s.

Switching time (t-on and t-off) for most analog switches is in the range of 60 nsec. up to 1 msec.

For MAXIM's “clickless” sound switches, the switching time is increased to the millisecond range, which will eliminate the audible clicks.

So, we see that to transmit a signal with minimal distortion, you need either the minimum key resistance in the open state, or the maximum possible load at the key output. Consider another aspect when switching - the effect of charge injection. To obtain a low R ON value, a channel area extension is required. The result is a large input capacitance and the corresponding charge: an increase in the dissipated power from the charge-discharge current in each switching cycle. The constant charging time t = R × C depends on the resistance (R ON ) and the capacitance (C) of the load. This usually lasts for several tens of nanoseconds, but low-resistance keys have a longer on / off period. Keys with high R ON are faster. MAXIM offers both types of keys - with the same base and in the same SOT-23 package. MAX4501 and MAX4502 have a higher resistance R ON , but a short on / off time MAX4514 have a lower resistance R ON , but a longer switching time.

Another negative consequence of low impedance switches is a higher level of charge injection caused by an increased level of current through the gate capacitance. This is especially important when using keys in a sample / hold device for accurate conversion to ADC.

Protection of keys against electrostatic charge (ESD) is based on the achievements of MAXIM in this area. They allowed to increase the protection of new analog switches up to ± 15 kW according to the recommendations of IEC 1000-4-2 level 4 (the highest level). All analog inputs for ESD tests use a model of the human body, as well as contact and discharge through the air gap specified in the IEC 1000-4-2 methodology.

Thus, the released keys MAX4551 - MAX4553 are pin-compatible with most of the standard four key chips such as DS201 / 211, MAX391, etc. Now you do not need to protect analog inputs using expensive restrictive diodes, since protection against electrostatic discharges (up to 15 kV) incorporated in the scheme of keys and multiplexers.

The following important characteristic should be noted in modern keys. Usually the allowable range of the input signal voltage is limited by the voltage on the power supply buses. If the analog signal exceeds the voltage of the power source, current flows through the reverse-biased parasitic diodes. In the case when this current has no limitation, the microcircuit fails due to overheating. Therefore, most of the old keys and multiplexers could work with currents not exceeding 10 ÷ 20 mA.

New MAXIM keys have built-in protection against breakdown, when they remain up to ± 25 V (some up to 36 V) of the input signal with 15 V power supply and ± 40 V with power off. In this case (in case of overvoltage), the key assumes a high impedance at the input of the analog signal, regardless of the state of the switch or the load resistance. Only the leakage current making up the nanoamperes can flow from the signal source. One circumstance is very important here: these keys do not require a certain order of supplying the supply voltage and voltage of the analog signal. Even with the power removed, the key does not break from the analog signal. The breakdown-protected MAX4511 ÷ MAX4513 keys are pin compatible with DS411 ÷ DS413.

In a brief journal article it is impossible to make a detailed description of all the properties of keys and multiplexers. Anyone who is interested in such information, I suggest visiting the site or the site of the official MAXIM distributor - Rainbow Technologies. At these addresses you will find a lot of useful information for the correct selection and use of this type of devices.