Question about the common mode level

[sixth]

What's the best way to generat a common mode level? Using a bandgap voltage reference or just divide the supply voltage by 2?

sixth

 

[vbhupendra]

Bandgap is better option

 

[laglead]

What's the best way to generat a common mode level? Using a bandgap voltage reference or just divide the supply voltage by 2?

sixth

It depends on the required accuracy of the common-mode level.

 

[sixth]

Bandgap is better option

Why bandgap is better? Bandgap will provide a fixed level, when the supply voltage decrease, the output swing will be decreased.

sixth

 

[laglead]

Bandgap is better option

Why bandgap is better? Bandgap will provide a fixed level, when the supply voltage decrease, the output swing will be decreased.

sixth

Actually, we need common-mode level in differential sigals. Power supply may have and only have influence on differential swing. Common-mode level is uneffected.

 

[surianova]

What's the best way to generat a common mode level? Using a bandgap voltage reference or just divide the supply voltage by 2?

sixth

If you divide the power suppy by 2, the fullchip current will change a lot when
power suppy change. ( example 3V to 3.6V), the reference will change from 1.5V to 1.8V. Imagine this reference voltage going to bias CMOS gate. The small variation of gate voltage will generate large variation of tail current. This large variation of current also change the gain or bandwith of opamp very significantly.
Or we can said the PSRR will be very bad.

 

[jerryzhao]

Some time we need the common mode level that divide the supply voltage by 2. and it must follow the power voltage.
I don't think this voltage bias the other circuit.
The most easy way: divide the power by 2 resister. don't use bandgap.
Such as in pipeline ADC you need common mode volgat VCM=vdd/2. But the reference voltage maybe is VCM±0.5.
If want get this reference volgate that is not easy.