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My digital Input is becoming high between at 1.6 and 1.7 only.

Hello all,
 
I am using PXI-6071E MIO card and i am trying to measure the analog input.
 
In the specifications of the card, it is stated that at 2.0V it would become logic high.
 
but in my observation it is becoming high between 1.6V and 1.7V.
 
Also i want to know what is the difference between TTL signa and CMOS signal.
 
If both are of same Voltage levels why they are stated separately?
 
please reply immediately
 
thankyou in advance
 
regards
 
Preetam
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LVTTL and CMOS are two different technologies used to implement logic.  TTL is constructed using a bipolar process (BJT transistors) whereas CMOS is the complementary MOS process.  From a digital perspective, they have very similar specifications and characteristics so are often lumped together.  However, they do behave slightly different, mainly in how they transition between logic states and the specific voltages that trigger that transition.

In response to your measured observation, LVTTL and CMOS devices will specifiy the MINIMUM voltage at which the device can guarantee the logic state to be valid HIGH.  That is, you could have a device that transitions at 1.6V, one at 1.7V, one at 1.9V but you will never have one that requires a voltage greater than 2.0V to change logic state.  Likewise, devices are specified with a MAXIMUM voltage at which the devices recognizes a logic LOW. 

Hope that helps
-Ryan
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Usually TTL levels are defined as follows: between 0 and 0.8V the input will be driven to a low state, between 2.3 and 5.0V it will be driven to the high state. You should avoid any voltage level in between.

CMOS logic circuits often require a higher voltage (I think it was between 4.0 and 5.0V) to be driven to a high state. Different from TTL levels CMOS levels do not require much current handling capacity. A TTL device must be able to sink (i.e. to give a path to GND) 400 micro-amps at least. CMOS logic is rather voltage driven than current driven.

Your observation is a (often found) misunderstanding of device specifications. Usually manufacturers rather specify requirements of the environment than actual performance of their devices. 2.0V for the input high state is the minimum requirement for the drive circuit, i.e. if the drive circuit is able to supply 2.0v the input is guaranteed to switch to the high state. It MAY switch to high state at a somewhat lower voltage but you ALWAYS should try to connect an input device which is guaranteed to supply 2.0V minimum for high state which will be sufficient for all boards with same specs. The difference between the actual voltage level driving the input to high state and the specified level is called "safety margin".
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