switch design in the cyclic adc

[A.jafari]

Hi
I've got to design a cyclic ADC and I have a problem with switch.
could you tell me that what is the value of switch Mosfet resistance in ON and OFF state ?
Thanks
with best wishes
A.Ja'fari

 

[erikl]

Which switch? There are so many. Analyze it by simulation!


Three witches watch three switches. Which witch watches which switch? ;-)

 

[A.jafari]

I didn't understand your answer but when I implemented switch in the cyclic adc my performance reduce that I don't know why?
Can you help me?
I have problem with on resistance of switches
best wishes.

 

[jeffrey samuel]

resistance of mosfet what resistance the drain resistance

Rd= Vds/ Id

and if you are talking about the Gate resistance then it is about a few hundred mega ohms

also try to post the schematic they may be very helpful in solving your issue

 

[A.jafari]

I attached my project's circuit.
Please help me.

 

[jeffrey samuel]

all i can see in the ckt is bunch of OP AMPS and an AND gate you said a MOS swith where is it
'

are you trying to implement a similar switch using MOS

 

[BradtheRad]

when I implemented switch in the cyclic adc my performance reduce that I don't know why?
Can you help me?
I have problem with on resistance of switches

Mosfets can be made with On-resistance down to 1/10 of an ohm or less.

However mosfets are a headache to arrange in the exact correct way, when all you want is to explore a conceptual design in simulation.

Therefore it is easy to use simulated switches. Sort of like a relay.

Have you been able to find a default resistance for the switches in the simulation?

The simulator I use gives 20 ohms as the default value. I once installed several in a switched-capacitor layout. I got poor performance at first.

I had to go and edit every one, so it got a value of maybe 1 ohm. Then I was able to have proper performance.

 

[KonstanU]

To A.jafari.
If you are going to simulate circuit, using relay as switch, then you must account for Cgd and Cgs capacitances of real MOS switch.
That is necessary because three mechanisms in MOS transistor operation introduce error at the instant the capacitance-switch turns off.
They are: channel charge injection, clock feedthrough and KT/C Noise.

to . BradtheRad

Could you tell me what the MOS size must be in order to provide 20 or 1 Ohms resistance.
I think that real value of on-resistance is 500-1000Ohms.

To A.jafari. Main source of creating off-resistance is leakage current. If accept that leakage current is 100fA and Vdd=1V than off-resistance will be about 10Gohm.

 

[erikl]

... accept that leakage current is 100fA and Vdd=1V than off-resistance will be about 10Gohm.

1V/100e-15A = 10e12Ω = 10TΩ

 

[KonstanU]

Thanks. You've corrected me.

 

[BradtheRad]

Could you tell me what the MOS size must be in order to provide 20 or 1 Ohms resistance.
I think that real value of on-resistance is 500-1000Ohms.

Mosfet descriptions often state an On-resistance which is in fractions of an ohm.

Here's a link to the IRF540 datasheet (popular low cost Nmosfet):

http://www.irf.com/product-info/datasheets/data/irf540n.pdf

It states On-resistance as 44 milli-Ohms.

The voltage-vs-current graph shows it conducts over 20A when applying 1V across drain-source. That makes .05 ohm.

To achieve that low value, the gate voltage must be raised at least 5V higher as referenced to the source terminal.

This can be a challenge, because there are times when you want to turn on the device, but the source has no definite path to ground (as in the switched-capacitor topology).

So how can you turn on the device?

To get past such hurdles takes ingenuity and effort.

 

[KonstanU]

Mosfet descriptions often state an On-resistance which is in fractions of an ohm.

Here's a link to the IRF540 datasheet (popular low cost Nmosfet):

http://www.irf.com/product-info/datasheets/data/irf540n.pdf

It states On-resistance as 44 milli-Ohms.

The voltage-vs-current graph shows it conducts over 20A when applying 1V across drain-source. That makes .05 ohm.

To achieve that low value, the gate voltage must be raised at least 5V higher as referenced to the source terminal.

This can be a challenge, because there are times when you want to turn on the device, but the source has no definite path to ground (as in the switched-capacitor topology).

So how can you turn on the device?

To get past such hurdles takes ingenuity and effort.

Pardon, we are talking about an IC switches not an discreet elements. Right? The Threshold voltage of IC N-MOS is usually 0.7V (or less) and maximum Vgs voltage cann't exceed Vdd (3.3V, 5.0V) in the sample-hold circuit.
IRF540 is power switch and it takes all the place inside package.It has a large size and its bulk and source terminals are connected together. For this reason, it is not possible to use this sort of switch in the sample-hold circuit (If it is not obvious for you I can explain why it is not so)
Next. Let accept, that s-h capacitance is 2-4pF. And If on-resistance is 500Ohm then time constant t=C*R= 2-4pF * 500Ohm = 1-2 ns. As you see for medium speed application it can be enough.

With best regards,
Kostiantyn

 

[A.jafari]

Thank you I try simulate your idea with Hspice .