We often get similar questions posted about TI signal switch products. This list will grow as we receive more questions that have common answers.
Can the voltage on the I/O pins of a switch be greater than VCC or less than 0 V?
-It depends on thje Absolute Maximum Ratings table.
Sometimes the input will have a limit of "-0.5 V (min) to VCC + 0.5 V (max)," which means that there is a forward-conducting ESD diode from the input back to VCC and from the input to ground. In this case, an input higher than VCC or lower than 0V is not allowed unless there is a series resistor limiting the current into the input to less than the IIK or IOK maximum clamp current rating.
-If the input has a limit of, for example, "-0.5 to 7 V," then the maximum voltage is not dependent on VCC, there is no diode to VCC, and the input is overvoltage-tolerant, allowing you to have inputs greater than VCC. However, inputs greater than VCC will not pass through unattenuated on most switches.
What is the maximum signal rate or data rate of a signal switch?
-The best way to figure out the maximum signal rate or data rate is to estimate based on bandwidth. TI specifies switch bandwidth as the point where the output is attenuated -3 dB from the standard DC attenuation. On most switches, there is a bandwidth curve in the datasheet or in an application report.
To calculate the maximum data rate the bandwidth can support you can use the theoretical formula DataRate = 2xBandwidth.
In practice we recommend selecting switches with more bandwidth than what is theoretically required for a particular data rate to account for system losses that occur on PCB traces etc. The rough formula we use to select switches is Bandwidth = (3/2)xDataRate which will provide sufficient margin.
Are TI analog/digital switches/multiplexers bidirectional?
-Yes, all analog and digital bus switches are bidirectional.
This means that the switches do not have their body terminal tied to any one terminal, which means the drain and source terminal are interchangeable deeding on where you are applying your signal. Please see the diagram below on what a FET switch looks like on the silicon level:
May I use a differential switch for a single ended application?
TI's differential switches are high performance devices designed to have excellent channel matching which is a key requirement for most differential signaling applications. You may use these differential switches in single ended applications.
What should I do with unused inputs/outputs of a switch?
-Switch channel inputs can either be left floating or terminated to GND (which can help reduce signal reflections). However, switch logic inputs, such as Select pins, OE pins, and other logic inputs, should never be left floating. They should be, by default, tied to either VCC or GND.
What is the difference between an analog switch and a digital bus switch?
TI's analog and digital bus switches are electrically equivalent and both share the common switch architectures found in the semiconductor industry.
Single FET switch
Bilateral FET switch
for more information about these switch architectures see this app note, "Selecting the right Texas Instruments Signal Switch."
Even thought the different ICs have the same electrical structures, the terms "analog switch" and "digital bus switch" were introduced to help TI categorize its large switch portfolio into 2 different groups, high channel count switches and low channel count switches.
Typically you will find the high channel count switches in the portfolio labeled "digital bus switches" because digital buses usually have many more signal paths than analog signal paths for their common use of transmitting 8, 16, 32 bits of data. As TI's switch portfolio continues to expand this loose distinction between analog and digital devices are becoming irrelevant.
Where does the exposed center pad for RGY and RGT packages connected to?
It is suggested to soldered the exposed center pad to the PCB for thermal and mechanical performance. The pad may be electrically connected to ground. Please refer to the datasheet mechanical information for detailed information and could refer to the app note here .
Signal switch naming convention and vocabulary
Does TI have any Normally Closed switches when Vcc = 0V ?
TI's signal switches are solid state switches but they steal the naming conventions from mechanical switches. The terms Normally Closed (NC) and Normally Open (NO) describe the normal (unenergized) physical position of the mechanical switch. The figure below shows that the 1-3 path as normally closed and the 3-2 path is the path that connects when the conducting wire physically moves to connect the 2-3 path.
Unlike mechanical switches, solid state switches do not have any moving parts internal to the IC. Technically the Normally Closed (NC) path and (NO) are always physically connected inside TI's signal switches. To turn "ON" the switch or connect the signal path from one side to the other requires a bias voltage to the gate to allow current to flow between the drain and the source.
Solid state switches that conduct with out a bias voltage applied to the power supply Vcc utilize depletion FETs which will conduct without a bias voltage and isolate when biased. Currently TI does not offer a signal switch with depletion FETs to create a "normally closed" solid state switch. If you need this type of device we would be happy to talk about creating this type of device and how using depletion FET switch fits into your system.
What is On/Off Leakage?
Off leakage is when you turn off the switch and there is still a parasitic current which bleeds through the switch onto the output. This physical phenomenon that occurs is because a switch is not a perfect signal block when turned off. It behaves very closely to one, but if you view the image of a NMOS, you can see that although there is no voltage applied to the gate, you can still have charge slip through to the other side. Problems that come from this is your device is offset voltage sensitive, you will begin to see a change in voltage on the other side of your MUXed application depending on the load resistance. An image of how we test for off leakage can be seen below:
On leakage, however, is the when there is additional current that appears on the output or is sources from the input. This phenomenon happens due to the parasitics of the device mentioned above. These parasitics will draw current into other portions of the device, and likewise other portions will leak charge into the output of the device. The same problem can occur as that of the Off state leakage, which makes using sensitive sensors tricky. Please see below for the the On-state switch leakage current:
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