I had not realised that the mosFET SSR acts as a capacitor when it changes to open.
At least we have two mosFETs in series and capacitors in series are effectively half value as far as blocking impedance goes.
Does the bypass diode have any effect on the open condition impedance and series capacitance?
At least we have two mosFETs in series and capacitors in series are effectively half value as far as blocking impedance goes.
Does the bypass diode have any effect on the open condition impedance and series capacitance?
The diode is a parasitic part and not added intentionally and so its capacitive effects become part of the device as a whole. You can't really separate the diode from the FET. The diode has a profound impact in the open condition and is why two devices back to back are needed to make a functional switch.
Thinking about that leads me to the conclusion that the capacitive impedance of one mosFET is bypassed by the Vf of the diode when voltage is applied in one direction.
When the voltage reverses to the other direction, then the other diode bypasses the other mosFET.
It would appear one does not get the advantage of two mosFETs in series when the SSR switches to open.
One usually sees capacitance across a mosFET changing with Vds. As Vds rises Cjunction drops.
Does the capacitance that remains vary with the applied voltage?
When the voltage reverses to the other direction, then the other diode bypasses the other mosFET.
It would appear one does not get the advantage of two mosFETs in series when the SSR switches to open.
One usually sees capacitance across a mosFET changing with Vds. As Vds rises Cjunction drops.
Does the capacitance that remains vary with the applied voltage?
I've no idea on that tbh although it seems likely that it would vary by some amount, how much though I couldn't say.
Got Wine loaded and got LTspice loaded. Seems to be working fine. Now just need to go through your tutorial and learn how to use it. Thanks Mooly.
If you look at an internal picture of an Ayre K-5xeMP you can see it's full of 74HC4052 for both input selection and volume control. By all accounts this is a good sounding pre-amp. As a note the 74HC4052 is not a solid state switch but an analog switch, it still uses mosfets for the switching but there is no opto-isolation.
Not sure about the sound but spec wise there are even better chips out there today. There is a trade-off between parasitic capacitance and on resistance, so you have to pic the spot that sounds best for you. The resistance is nonlinear which would cause distortion with loading, so it's best to buffer with something like a jfet and keep the load upwards of a meg. but that's just what the design sheet sagest.
The DG444/DG445 also seems like it's worth a try.
Not sure about the sound but spec wise there are even better chips out there today. There is a trade-off between parasitic capacitance and on resistance, so you have to pic the spot that sounds best for you. The resistance is nonlinear which would cause distortion with loading, so it's best to buffer with something like a jfet and keep the load upwards of a meg. but that's just what the design sheet sagest.
The DG444/DG445 also seems like it's worth a try.
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Analog switches aren't a product space I've reviewed in detail but FSA2257 is one reasonably performant starting point in package not especially involved to solder. Among Maxim's offerings the MAX4621/2/3 may be of more interest than the DG445 for their lower on state resistance and wider voltage range. Both should be OK with a 10k passive volume pot. Or one could just buffer the switch output.
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From what I have been reading it looks like the DG445 has some good numbers as far as crosstalk, capacitance are concerned. The Ron is a little higher @ 50 ohms compared to the Vishey SSR @ .25 ohms. DG445 off isolation @ 60db (1mhz) looks much better. If I am reading all this correctly. I am very new to all of this. Looks like the DG445 wins. I have the Vishay SSR's already so I am going to play with them some, but may change my mind. Thanks for the information LBHajdu
I tested my first AC SSR with a speaker fed through a 1k resistor to limit the volume when testing amplifiers.
It is clear that the capacitance, when the SSR is open, allows a lot of HF to pass to the load.
Is there a direct impact in relation to the voltage of a signal to the amount of bypass of high frequency? I am looking at line level. What do you think the signal strength was using the SSR on amplifier output after the 1k resistor? I guess my question is, would the HF bypass be much less at line level signal as opposed the the amplifier output? It sounds like off capacitance is a major factor. At line level signal, how much Ron is too much? I am not yet sure if my level of understanding of these things is allowing me to ask the right questions. I feel like the child in a room full of scholars-haha
Yes, its slightly audible if the volume is turned up and the switch open. Its not really a problem for this application though and certainly not worth making a more sophisticated arrangement of series/shunt.
The more signal level there is, the more will appear as an unwanted signal at the other side of the switch. Using a simple single electronic switch (such as a single JFET or single switch in a 4066 type package) for critical line level switching isn't really good enough.
That really depends on the surrounding circuitry although aiming to use devices with as low an Ron as possible makes sense. Once you get into many 10's of ohms then care is needed to take that into account in the overall design.
These are all interesting devices, but ultimately you need to draw up a specification for what you are trying to achieve. Is it line level, speaker level or power switching you want to do. All the devices (like this one) that I have seen are either not high enough rated to be useful for speaker or power switching (that we would use for amplifiers) or not ideally suited to line level use.
I think Analogue Devices and Linear Technology have some useful devices that would be more suited to line level use. And used correctly you might be surprised just how good the ancient stuff like 4066's can be.
I think Analogue Devices and Linear Technology have some useful devices that would be more suited to line level use. And used correctly you might be surprised just how good the ancient stuff like 4066's can be.
Ok, now I am confused. This thread started about using SSRs for line level switching. 4066 was brought up earlier. Then in #32 it was stated that 4066 wasn't good enough for this. Now stated #36 that I would be suprised how good a 4066 could be for this. Am I missing something?
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