I am using the THS4504/5 fully differntial amplifier and have a question about the data sheet specification for common mode input range. The common mode input range is pecified for +/-5V power supplies, and is -5.1/+2. This is an overall range of 7.1V. The common mode range is also given for single supply +5V to 0V, and is -0.1 to +2V. If the negative power supply is changed to -8V, does the upper value of the common mode input range also change? Does the 7.1V range shift to -8.1V/-1V or does the upper limit remain constant at 2.1V no matter the negative supply value? It is unclear from the data sheet specifications and TI technical suipport does not seem to know the answer and has directed me here to pose the question.
For nearly all op amps the input common mode range as well as the output swing ranges are determined by a fixed offset from the supply rails. To determine the ranges for a new range you would take the datasheet specifications and determine the offset from the range to either supply and then reapply that offset to the new supplies.
In your example, for +5 V and -5V supplies the upper limit is 3V less than the + supply and 100mV below the lower supply.
For the new supplies of -8V and +5V the new range would be (-8.1V) to 5V-3V = 2V
Likewise the new output voltage range would shift from -4.6V to +3.8 V to the new range of -7.6V to +3.8V.
If you use +5V and -8V the total supply voltage is now 13V which is comfortably under the 16.5V abs max listed in the datasheet.
As you can see giving the device more voltage range on the supply pins gives a one for one increase in available input and output range. There may be second order impacts that slightly decrease the range under different supply conditions, but they are generally related to increased power loads and higher temperatures due to the increased quiescent power dissipation.
There is a THS4505 model in TINA-TI spice which is available free on the TI website. You may want to consider simulating your application in that tool. SPICE models are usually very good at modeling DC operating points and voltage ranges with respect to new supply conditions.
Thanks for the excellent posting! I knew about the output voltage swing from the rails, but did not consider that the input side of the device would/must act similalry.
One more question, if using the SPICE to verify, would one just use a DC input and change the value of the DC input unitl the ouput voltage does not change with it in a one for one manner?
I usually set up the SPICE model with the same supply voltage and input common mode voltage as my system. I then put in a fairly large input siganl at a frequency similar to my signal. Then I run a transient response.
This way I can usually see if I'm hitting a supply rail, but if I hit a slew rate limit first that will show up as well. You may also see saturation recovery or snap-back issues that you wouldn't see with a static DC sim.
By the way, I'm new to TI and have not worked with this particular amplifier. I came to TI through the National Semiconductor merger.
I'm not a power user yet, but I really like the TINA-TI simulator tool. If you haven't already, you can download a reference schematic at the link below. Look for the file I highlighted below the link. The latest version of TINA also has the National Semiconductor SPICE models in the library as well at the TI parts.
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