CBT Logic family

Thread Starter

mos_6502

Joined Dec 11, 2017
48
Hello everyone.
I have a question about CBT logic family/technology (example: SN74CBT16211A).
I would like to understand what a CBT device is (in this example, the devices are shown to me as particular mosfets), in particular:
1) On a constructive level, how is a single CBT "mosfet" constituted?
2) Are they bidirectional devices?
3) Can they pass analog signals (both positive and negative)?
4) are there single components capable of performing that function (a single CBT "mosfet")?

Thanks!
 

Marley

Joined Apr 4, 2016
489
1/ The diagram shown on the datasheet "Logic diagram - positive logic" is very simplified. Each channel probably consists of at least 2 FETs in parallel, possibly more complex.
2/ They are bidirectional.
3/ They probably could pass analog signals as long as the signal voltage stays between Vcc and GND. Although they are not intended for this. The ON resistance may not be constant over the range of signal voltage. Switches intended for analog applications take more care to linearize the resistance and usually have graphs in the datasheet showing the ON resistance over a range of signal voltages.
4/ There are "Analog switches". Normally in pairs or quads but single ones might be available.
 

Thread Starter

mos_6502

Joined Dec 11, 2017
48
Thanks for your answers.

I would like a device that can switch between a fixed positive voltage and a fixed negative voltage of LVTTL level.

I am currently using the cd4053 device which works as I would like (analog switch SPDT 2:1) but has the disadvantage of being very slow for my purpose.

I was looking for a device capable of switching in around 5nS and came across these analog CBT switches, but as I understand it they cannot handle negative signals (voltages).
 

Irving

Joined Jan 30, 2016
3,128
So a voltage of say +2.5 and -2.5 but not in between? And low current eg TTL levels or a few mA.

Regarding timing, the 5nS refers to propagation delay input to output or from selection to output?

Could you explain a little more what the fixed + and - voltage levels represent and/or provide a schematic of what you're trying to do - maybe there's another way to address the problem.
 

Thread Starter

mos_6502

Joined Dec 11, 2017
48
There are three fixed voltages that can be switched, one positive (+3.3), 0v and one negative (-3.3v).

For the timings I would like the switching time between one of the two inputs and the output to be about 5nS. In fact, I would like an SPDT switch with two inputs (analog, each could be + v, 0v, -v and it can vary between this value with a max frequency of 220MHz), and that the output can switch on one of these two inputs in about 5nS.

I actually do this with a CD4053.
 

MisterBill2

Joined Jan 23, 2018
13,115
There are a vast number of analog switch ICs and a fair number of FET devices that can also serve as switches. A safe way to search would be to visit the site of a reputable distributor, such as Digikey, and search through their listing of analog switches. Others may have favorite distributors that also include them.
How many lines do you need to switch? AND, is the application purely digital? +2.5 and -2.5 do not seem to be standard digital votages.
 

Thread Starter

mos_6502

Joined Dec 11, 2017
48
[...]
How many lines do you need to switch? AND, is the application purely digital? +2.5 and -2.5 do not seem to be standard digital votages.
I need 24 lines for my application.
As I have already explained, my signal is analog (even if it only has discrete values) with negative voltage values.
 

Irving

Joined Jan 30, 2016
3,128
There several devices from Analog Devices, TI, etc that can do analog switching but not bipolar and not at that speed. There are a few devices that can get close speed-wise but are only 1.8v operation.

A matched N- and P-channel mosfet pair is one option, see simulation below, available in SOT23-6 or SC70-6 packaging so relatively low footprint

1639433834940.png
1639433899851.png
 

MisterBill2

Joined Jan 23, 2018
13,115
I have a suggestion that may possibly be acceptable, presuming that all 24 need to be switched at exactly the same time, and presuming further that the signals will never be any higher. That idea is to level shift the switching system 2.5 volts below the common voltage of the rest of the system, so that the negative 2.5 volts would be zero volts relative to the switcher ICc, and the positive 2.5 volts would be +5 volts relative to the supply common of the switcher ICs. It will take two of the 24 channel devices because of having to select one of the two sources. The big challenge will be finding an isolating device able to handle that frequency.

Level shifting is less common but it should allow it to work.

And I am rather puzzled as to what sort of application this is. Quite unusual, really.
 
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