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Original question:

TS3A5223: Upper voltage tolerance of NC1, NC2, NO1, NO2, COM1 and COM2

TS3A5223: reg: ON capacitance for the part TS3A5223RSWR

vinay kattela
Prodigy 30 points
Part Number: TS3A5223
Other Parts Discussed in Thread: TMUX136, TMUX1072

Hi,

As per the datasheet the ON capacitance for TS3A5223 part is shown as 115pF.

I think 115pF ON capacitance of the switch is definitely higher side and may causing ringing/possible oscillation.

So does that mean during ON period the impedance going to be as follows based on the frequency of the input signals :

 

Constant

Pi

ON Cap

10Mhz

2PifC

Impedance

2

3.141

1.15E-10

1.00E+07

7.22E-03

1.38E+02

Constant

Pi

ON Cap

20Mhz

2PifC

Impedance

2

3.141

1.15E-10

2.00E+07

1.44E-02

6.92E+01

 

In our design we do have the differential signals coming from catheter connected to ADL5565ACPZ  and the output of this IC is connected to the switch as above.

We want to make sure if it is not going to load the IC due to ON capacitance !

  • 0
    TI__Mastermind 45340 points
    Vinay,

    In a FET switch there is an inverse relationship between on-state capacitance and on-state resistance due the physical size of the FETs. Generally the larger FETs have lower on-state but due to their large size they have high on-state capacitance. The TS5A5223 device is less than 0.5 ohm onstate resistance which means this is a relatively large FET which will have high on state capacitance.

    What do you think about using low capacitance FET switches like the TMUX136 or TMUX154?

    Thank you,
    Adam
  • 0 in reply to Adam Torma
    Prodigy 30 points
    Query1: Question that i have is what is the Differential capacitance of the TS5A5223 IC ? As per the data sheet spec i could find only the singled ended capacitance. Since the Differential amp is going to drive the switch with diff signals, I would like to know the Cdiff ??.
    Query2: I really appreciate for suggesting the part TMUX136, do you have a part with this spec supporting +5V power supply ??
  • 0 in reply to vinay kattela
    TI__Mastermind 45340 points

    Vinay,

    1) If you are saying the differential capacitance is between the COM1 and COM2 pins we don't specifically specify that spec in the datasheet.  The on-state capacitance measurement will include the pin capacitance to ground as well as the adjacent pin capacitance.  Based on this you can assume the differential capacitance will be some value less than Con.    

    2) You can use the Ti.com switch selection tool filter for all switches that will support 5V power supply.  I found the TMUX1072 device when filtering for 2:1, 2-channel, Vdd > 5V.

    Thank you,

    Adam

  • 0 in reply to Adam Torma
    Prodigy 30 points
    Since our signals are differential coming out of the pre amplifier - I am really interested is there any reason why the specific switch that we used does not have the spec. In that perspective are we really using the right switch for our pre-amp that we are using is the current concern ?
  • 0 in reply to vinay kattela
    TI__Mastermind 45340 points

    Vinay,

    The reason there is not a specific spec for differential capacitance for the signal switch is that this value is included in the Con specification.  If the on state capacitance is too great for your system performance there is no reason to look further at the differential capacitance since this value will be less. 

    In differential signaling applications the skew between the COM1 and COM2 pins are important.  There are devices that take care on their bond wire and silicon layout to optimize the skew performance.  As you can see in the newer TMUX136 and TMUX1072 devices have excellent skew performance which makes them good for differential signaling applications.

    Thank you,

    Adam