[Shishuo Design] How to compare the performance of solid state relays and CMOS switches?

The turn-off capacitance CDS(OFF) between the source and drain can be used to measure the ability of the source signal to couple to the drain after the switch is turned off. It is a common specification parameter in solid-state relays such as PhotoMOS®, OptoMOS®, photorelays, or MOSFET relays, and is often referred to as the output capacitance COUT in solid-state relay data sheets. CMOS switches typically do not include this specification, but off isolation is another way to characterize the same phenomenon. Off isolation is defined as the amount of signal coupled to the source of the drain when the switch is off. This article will discuss how COUT is derived from off-isolation and how it can be used to more effectively compare the performance of solid-state relays and CMOS switches. This is important because CMOS switches are suitable for many applications where solid-state relays are used, such as switching DC signals and high-speed AC signals.

Figure 1 shows a typical performance plot of off-isolation versus frequency for the ADG5412. The graph shows that as the frequency of the signal at the source increases, the off-isolation decreases.

Figure 1. ADG5412 turn-off isolation vs. frequency

(±15 V dual power supply).

This means that as the signal frequency increases, more of the signal on the source appears on the drain of the off switch. This behavior is not surprising if you look at how the equivalent circuit of the switch behaves in the off state, as shown in the test circuit in Figure 2. When the switch is in the off state, there is a parasitic capacitance between the source and drain, which is C _DS(OFF) in the figure . This parasitic capacitance allows high frequency signals to pass through, and the shutdown isolation diagram is designed to identify these characteristics.

Figure 2. Turn-off isolation measurement test circuit.

_{We take V} and V _from the test circuit shown in Figure 2 and plug them into the following equation to calculate the off-isolation:

Applying the results obtained from the off isolation diagram to the equivalent circuit of the open switch, the C _DS(OFF) of the CMOS switch can be calculated . First, if we consider turning off the switching channel and load, we can think of the circuit as a high-pass filter, as shown in Figure 3.

Figure 3. C _DS(OFF) and RL high-pass filter.

The transfer function for the circuit shown can be calculated by:

Next, consider the source voltage V _S and its impedance, as shown in Figure 2. The source impedance RS _is 50 Ω and matches the 50 Ω load impedance _RL . Assuming that under ideal conditions, C _{DS(OFF) is shorted, then VS} is 2 times V _IN when the impedances are equal . This means that when the transfer function is calculated based on V _S , the entire transfer function is doubled.

Therefore, the conversion function of the entire system is:

This transfer function can then be substituted into the off-isolation formula to obtain:

Then, re-transform the formula to find the value of C _DS(OFF) :

This means that if you know R _L , the frequency f of the input signal, and the off isolation specification value (dB), you can calculate C _DS(OFF) . These values ​​can be found in the data sheet of the switch or multiplexer product in the Analog Devices product line. The following example shows how to do it.

This example uses the ADGS1612, a 4-way SPST switch controlled by SPI. The off-isolation specification for the ADGS1612 is −65 dB and can be found in Table 1 in the data sheet. According to the test conditions section of the shutdown isolation specification, R _L is 50 Ω and the signal frequency f is 100 kHz. Substituting these values ​​into the C _DS(OFF) formula, the capacitance value can be calculated.

Note that in the switch-to-multiplexer off-isolation measurement circuit, an additional 50 Ω termination resistor may be included before the source pin of the switch channel, as shown in Figure 4. With the off-isolation specification measured in this way, it can still be calculated using the C _DS(OFF) formula. However, if the source pin uses a 50 Ω termination resistor (which is subsequently used in the C _DS(OFF) equation), 6 dB needs to be added to the off isolation specification given in the data sheet. This is to compensate because the 50 Ω termination resistance at the source will halve the voltage, equivalent to −6 dB.

Figure 4. With 50 Ω termination resistor on source

Off isolation test circuit.

_{Table 1 shows the C DS(OFF)} values ​​for selected switch products from the Analog Devices product family . The ADG54xx and ADG52xx families can handle signal voltage swings up to 44 V, and the ADG14xx and ADG12xx families can transmit signal voltage swings up to 33 V. This comparable signal ranges from 30 V to 40 V solid state relays. The last column in the table also shows how to use C _DS(OFF) and the switch on-resistance to calculate the R _ON , C _DS(OFF) product, which is used as an order of merit in solid-state relays. The product of R _ON and C _DS(OFF) shows that when the switch is on, it has very little effect on the attenuation of the signal, and how strong the switch is at blocking high-speed signals when it is off. The table shows that the product of R _ON and C _OFF of ADG1412 is less than 5, which is quite advantageous among the solid state relays on the market.

Table 1. Selection within the Analog Devices product family

_{C DS(OFF)} of SPST × 4 switches

CMOS switches offer several advantages over solid-state relays. Specifically include:

The typical digital input current for most Analog Devices CMOS switches is 1 nA, while the recommended forward current for diodes in solid-state relays is 5 mA. This means that CMOS switches are easily controlled directly by GPIOs on the microcontroller.

The typical turn-on time of the ADG1412 is 100 ns, and the turn-on time of solid-state relays is several hundred milliseconds.

For example, the ADGS1414D comes in a 5 mm × 4 mm package and has 8 switching channels, 1.5 Ω on-resistance, and 5 pF C _DS(OFF) . That is, one switch per 2.5 mm2 of package area.

The switch's ability to block signals in the off state is critical. In solid state relays, the C _OFF specification is a measure of the capacitance across the switch that allows the input signal to couple to the output of the off switch. In CMOS switches, this capacitance is not measured directly; however, it can be inferred from the off-isolation specification. By deriving the transfer function for an open switch, C _DS(OFF) can be determined using the off-isolation value (dB), the frequency of the input signal, and the load resistance . C _DS(OFF) is an important value when comparing the C _OUT specifications of CMOS switches and solid-state relays . In addition, C _DS(OFF) can be used to calculate the R _ON , C _DS(OFF) product, which is a rank value that shows the overall off-isolation and signal loss performance of the switch. In this way, when selecting a switch for an application, you can compare and select CMOS switches and solid-state relays more intuitively. CMOS switches also have many advantages over solid-state relays, such as easier driving of switching logic, faster switching speeds, and the ability to integrate more switches in the package.