# Where exactly appears the channel capacitance and Ron when switch on DG442 switches

I have this switch in a circuit, actually there is 30 switches.

https://www.vishay.com/docs/70053/dg441.pdf

When system is not executing any operation, switches are off. Sx and Dx are not connected. But some system operation will switching them on, connecting Sx and Dx.

In the worst case I have to take into consideration the effects of some associated parameters such as capacitances or intern Rs. I am analyzing the circuit in the worst case and I need to know how to take into account the channel capacity when switch ON. The datasheet tells us that there are 16pF for each channel of the switch.

My question is:

1.- This capacity refers to a capacity between the switch output dx pin and GND? Does it mean that when say "channel"? It also indicates a Ron = 50Ohms per channel. Is it supposed to be parallel to this channel Capacitor? or is it serial with capacitor?

2.- Where can it be in the simplest circuit where a signal S1 is switched on with CHA = GND and CHB = signal M is switched on with D1, where M has connected RL = 12K serial with V= 12V? VBA is my Vout as seen here:

I would like to model this circuit adding Ron and Cp. But I'm not sure how to add them to B and A nodes, specially when switching S1 with GND. That would mean S1 != GND?

The on capacitance is across the switch analog terminals.

From the schematic in the datasheet, I have added where it exists:

The capacitor exists across the analog switch channel (which was not identified in the schematic) so the A and B analog terminals are where I have put the capacitor.

This is also the primary cause for off isolation degrading with frequency coupled with the isolation resistance and load; as you can see it is a classic single pole with 20dB/decade characteristics.

When I did a quick calculation, I got a value of about 15pF for the source to drain capacitance, close enough for confirmation.

For a really good overview of analog switches, I recommend this application note.

• I have updated the answer; the analog terminals are where the capacitor has been added. Commented Feb 20, 2019 at 16:37
• Thank you, so I guess Ron is also R across the channel, between drain and source terminals. Is it? Commented Feb 21, 2019 at 9:45

The FET likely looks like this

simulate this circuit – Schematic created using CircuitLab

• Yes, but I don' t know what CDon certainly applys; C between D and substrat (normally connected to GND) (this is not the ON channel in a TRT, or if it refers to CDS: C between Drain and source (this is the channel). Commented Feb 21, 2019 at 12:06

I got this image from app notes, shared by Peter Smith, I think that CD refers to C between drain and GND. And Ron is R between drain and source. Althought they talk about "channel" I think they mean CD. If not, they would talk about CDS. That's my opinion. If I'm wrong, please correct me. This is what I have just undertsood from app notes.