The Turn-off Capacitance in Solid-State Relays:
A critical parameter in solid-state relays, such as PhotoMOS®, OptoMOS®, photorelays, or MOSFET relays, is the turn-off capacitance CDS(OFF) between source and drain. This parameter mirrors the degree to which the source signal influences the drain post turn-off. Typically, solid-state relay data sheets refer to this as COUT. Interestingly, for CMOS switches, this parameter isn't always highlighted; however, the concept of off-isolation serves a similar purpose. It measures the signal coupling from source to drain when the switch is inactive. Our exploration will delve into deriving COUT from off-isolation, a critical step for comparing solid-state relays and CMOS switches effectively. This knowledge is pivotal in selecting the right switch for various applications.
Turn-off Isolation's Frequency Dependency:
Take, for instance, the ADG5412 switch from Analog Devices. It exemplifies the typical off-isolation versus frequency relationship (Figure 1). The intriguing part? As the source signal frequency climbs, off-isolation diminishes. High-frequency signals tend to "leak" more readily to the off switch's drain. Delving into the switch's equivalent circuit (Figure 2) reveals a fascinating detail: a parasitic capacitance CDS(OFF) bridges the source and drain when the switch is off, allowing those high-frequency signals to slip through. This phenomenon's measurement is the essence of turn-off isolation.
Measuring Turn-off Isolation:
To tackle the off-isolation calculation, we start by extracting VS and VOUT values from the test circuit (Figure 2). These values then integrate into specific equations. First, we view the circuit as a high-pass filter (Figure 3), laying the groundwork for calculating the transfer function. By weaving in the source voltage VS and its impedance, the system's full transfer function emerges. The finale? Plugging this function into the off-isolation formula unravels the CDS(OFF) mystery. This step-by-step approach is not just methodical—it's crucial for accurately deducing the CDS(OFF) based on RL, signal frequency f, and turn-off isolation specifications.
CMOS Switches vs. Solid-State Relays:
Comparing CMOS switches and solid-state relays becomes intriguing when we scrutinize the CDS(OFF) values across Analog Devices' range. For example, the ADG54xx and ADG52xx series boast handling capabilities up to 44V, while the ADG14xx and ADG12xx series max out at 33V. We juxtapose these figures against 30V to 40V solid-state relays to spotlight performance variations. Moreover, calculating RON and CDS(OFF) products unveils the switch's influence on the signal in both active and passive states, offering a window into the comprehensive off-isolation and signal loss performance.
The Edge of CMOS Switches:
In many respects, CMOS switches outshine solid-state relays. Consider this: the typical digital input current for ADI's CMOS switches is a mere 1nA, starkly lower than the 5mA forward current recommended for solid-state relay diodes. This makes CMOS switches far more amenable to microcontroller GPIO direct control. Additionally, the turn-on time for a CMOS switch like the ADG1412 is a swift 100ns, dwarfing the sluggish milliseconds of a solid-state relay. And there's more—the capacity to house multiple switching channels in one compact package. For instance, the ADGS1414D fits 8 channels in a tiny 5mm × 4mm package.