# How does common mode range include the negative rail?

How do the devices like AD8220 handle the common mode voltage equal to the negative rail or even lower voltage (Take a look at figure 30 of AD8220 datasheet) ?

Does the operation of the amplifier degrade if the Vcm be close to the end of its range? Or it will work good as long as Vcm be within the range no matter where in the range?

• e2e.ti.com/blogs_/archives/b/thesignal/archive/2012/05/08/…
– G36
Commented Oct 16, 2018 at 21:28
• electronics.stackexchange.com/questions/234575/…
– G36
Commented Oct 16, 2018 at 21:29
• @alireza: It's a good idea to capitalise your sentences and part numbers correctly for legibility and credibility. If English is not your first language you can add this into your user profile so we can make allowances - but what language does not use capital letters at the start of sentences? Commented Oct 16, 2018 at 21:40
• @Transistor Chinese, Hindustani and Arabic comes to my mind. The first the third and the fifth most used languages in the world? Commented Oct 17, 2018 at 8:24

We know what the conditions are for dual rails (±5V) and single supply (0 to 5V), so if you had one rail at say -2.5V and one at 5V then the common mode hexagon would be streched from the

Something like this:

The hexagon or diamond plot is scaled with the Vcc range: Source: AN-1401

Although in-amps appear agnostic to the input common-mode voltage, internally, they till must address this voltage. Common-mode voltages, especially as they approach the supplies, can cause the internal nodes to saturate when the external input and output voltages may otherwise be within range. The diamond plot represents this imitation by plotting the combination of every headroom limit, including the input range, the output range, and the internal nodes. The diamond plot is a boundary plot that shows he achievable output voltage (VOUT) range for any given input common-mode voltage (VCM), or equivalently, the input common-mode voltage range for which a given output voltage can be produced.

Source: AN-1401

How do the devices like AD8220 handle the common mode voltage equal to the negative rail or even lower voltage (Take a look at figure 30 of AD8220 datasheet) ?

It scales with the diamond plot, the negative rail would be the lowest voltage

Does the operation of the amplifier degrade if the Vcm be close to the end of its range? Or it will work good as long as Vcm be within the range no matter where in the range?

The signal will not be a subtraction when the input signals are outside of the common mode range, so operation is affected. If you are in the common mode range of both the inputs then the instrumentation amplifier will operate as expected.

How does the circuit implement common-mode down to or past the lower rail? They use PFETs, perhaps with the bulk tied somewhat above the lower rail, to raise the threshold voltage, like this

simulate this circuit – Schematic created using CircuitLab