Switches Figure 3. Off-isolation plots.


Figure 4. Insertion loss plots.


these architectural details, it becomes easier to analyse performance metrics like capacitance, RON, and switching behaviour in evaluation studies, ensuring the correct switch type is chosen for specific applications.


 


A better way to qualify and quantify the switches is to look at the added value contribution in a system design application. As mentioned, for the application referenced in Figure 1, the ADG1412 is well suited for such applications and can be easily deployed in lieu of a PhotoMOS switch. This CMOS switch is a quad single-pole, single- throw (SPST) device with excellent features like power handling, response time, or on-


Benchmark Criteria


Leakage COFF RON (CxR) product


Drain Capacitance [CD(OFF)]


Turn-on speed


Voltage, current capability


Cost/channel


PhotoMOS 1-Form-A (1)


1nA 0.45pF 12 5.4pF. 1pF 200μs (32V,c 120mA) High Package area size 3.55mm2 Figure 5. Switch turn-on time.


resistance and low leakage. By comparing the CMOS switch performance across relevant metrics as illustrated in Table 1, a scorecard evaluation allows designers to quantify its merit against other alternatives. This could reveal more about the device’s efficiency in signal switching, especially in complex or sensitive electronic systems.


ADG1412 (Quad-SPST)


30pA 1.6pF 1.5 2.4pF. 23pF 100ns (32V,c 250mA) Low 4.00mm2


* CD (OFF) affects the CxR product performance Table 1. Switch specifications


www.cieonline.co.uk Value Add


Ideal for leakage testing; less voltage error contribution at output


Less waveform distortion, higher isolation


Low signal voltage drops at output, insertion loss reduced


Less waveform distortion, higher isolation, and low signal loss


Higher value degrades the CxR performance-distorting input signals, reducing off-isolation


Fastest switching ability


Ability to pass more output drive current to load


Promotes channel density increase and enabling up to 50% cost reduction


per switch Switch area size closely match each other after layout


 


The off-isolation plots (Figure 3) for both switches indicate a high suppression (–80dB at 100kHz) of input signals not reaching the output. PhotoMOS performs marginally better with -10dB as frequency increases. For the switching application shown in Figure 1,


Scorecard CMOS switch better


PhotoMOS switch better


CMOS switch better


PhotoMOS switch marginally better due to low drain capacitance


PhotoMOS switch better


CMOS switch better CMOS switch better


CMOS switch better Close match


which indicates direct current (DC) switching, the switch capacitances do not matter - rather the switch benchmark parameters of interest are low leakage current, fast turn-on speeds, and insertion loss.


Insertion loss: Switch on-resistance A switch with low RON is critical. The I*R (voltage) drop limits the system performance. Low RON variation from part to part and over temperature reduces measurement error. The insertion loss plot in Figure 4 shows the PhotoMOS switch tailoring down –0.8dB at 100kHz compared to the CMOS switch (–0.3dB) at the same frequency. This reinforces the low RON (1.5Ω) for the CMOS switch.


Switch turn-on time


From the onset of a drive enable/logic voltage applied to either switch that causes the switch to close and pass input signals to output, expect a significant delay time for the case of using a PhotoMOS switch (Figure 5). This slow turn-on speed is due to the input capacitance on the LED input stage and the delay due to internal circuitry used for energy conversion from current to voltage required to drive the gates of the MOSFETs. These slow turn-on speeds have been a notable point of dissatisfaction among customers and one that slows and impacts the overall system application. The CMOS switch has a fast turn-on speed (100ns) desired in system applications over a PhotoMOS switch (200,000ns). This represents a reduced magnitude of two thousand times (×2000).


Continues on page 24 Components in Electronics October 2025 23


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