Power Supply for operational amplifier

[JiL0]

Got a question regarding the power supply. When should we use +3.3V to -3.3V power supply when designing an opamp and when should be use 3.3V to 0V instead?

I'm designing an opamp for medical application. Does this mean the amplifier has to be working at -3.3V to 3.3V since we cant add 1.65V to the biosignal to make it operate around the analog common ground?

 

[flatulent]

You are usually better off by using two opposite polarity supplies. That way you eliminate the need for AC coupling and synthetic grounds. Since biological signals are low frequency, this eliminates large value coupling capacitors. It also allows you to swing the inputs and outputs to 0V without using special rail to rail amplifiers. You should size the two supply voltages to allow the full output swing you need.

 

[JiL0]

I see. Thanks.

Is rail to rail amplifier an amplifier that can have an maximum output one Vth below and above Vdd and Vss? Whats so special about rail-to-rail amplifier?

 

[flatulent]

They are special in that they have a large output swing and frequently large input common mode range. You pay a price for this in other parameters being lower in value or higher economic cost.

 

[alphi]

use mc34063 to generate +-3.3V power source.

 

[RegUser_2]

Got a question regarding the power supply. When should we use +3.3V to -3.3V power supply when designing an opamp and when should be use 3.3V to 0V instead?

I'm designing an opamp for medical application. Does this mean the amplifier has to be working at -3.3V to 3.3V since we cant add 1.65V to the biosignal to make it operate around the analog common ground?

There are special class of OP's which are designed to operate from a single polarity power supply, like for instance the Burr Brown's OPA350 ->

DESCRIPTION

OPA350 series rail-to-rail CMOS operational amplifiers are
optimized for low voltage, single-supply operation. Rail-torail
input/output, low noise (5nV/√Hz), and high speed operation
(38MHz, 22V/µs) make them ideal for driving sampling
Analog-to-Digital (A/D) converters. They are also well suited
for cell phone PA control loops and video processing (75Ω
drive capability) as well as audio and general purpose applications.
Single, dual, and quad versions have identical specifications
for maximum design flexibility.

The OPA350 series operates on a single supply as low as 2.5V
with an input common-mode voltage range that extends
300mV below ground and 300mV above the positive supply.
Output voltage swing is to within 10mV of the supply rails with
a 10kΩ load. Dual and quad designs feature completely independent
circuitry for lowest crosstalk and freedom from interaction.

 

[JiL0]

In this case, if we want to use such amplifier in our circuit, does this mean we have to add 1.65V to our input signal? Is this possible for medical applications which translate to connecting the negative electrode to 1.65V instead of ground?

 

[JiL0]

I have another question. What is the difference between a circuit with a single 3V power supply and a circuit with dual power supply of -1.5V and 1.5V? Since potential is a relative term, the circuit should behave the exactly the same rite?

This led me to think about what will be bulk potential for the n-mos transistor. the bulk potential of the n-mos is suppose to be the lowest potential in a circuit. Does this mean the substrate of the n-mos transistor is connected to -1.65V in a dual-supply circuits?

 

[batdin]

A "normal" opamp cannot tolerate input and output voltages near Vee or Vcc.That's why its inputs should be biased so that its input differential amplifier and output stage can work in a linear mode.
If you use +3V only,you should at least make a "virtual" ground for the inputs,and use capacitors for DC decoupling.This means additional components and transients due to charging/discharging of input and output capacitors.
As a summary: using PS with dual polarity can simplify design but is just recommended,not obligatory.