Basic questions about Analog ground

[suraj]

Analog ground

Hello Everyone,

I have some basic questions about analog

1. What is Analog ground?

2. Why we use +/- supply instead of using Vcc to 0?

3. How slew rate effect the peformance of OP AMP speed?

Thanks alot

Suraj

Moved here from "Analog IC design" by davorin
Use the right topic next time!!!

 

[papyaki]

Re: Analog ground

Hi,

1) It is the 0 volt point, reference of all analog potential. In some realisation, all wire connected to analog ground must converge to one and unique physical point.

2) In AC application, by using Vcc/0 power supply, you need to introduce a DC polarisation (Vcc/2) at the input of every stage, allowing a simetrical AC swing arround this value. You need also to add capacitor at the input, at the output and between every stage of your schematic to isolate every stage of this DC polarisation. All this is additionnal hardware compared to symetrical supplies

3) Slew rate is critical in switching systems. If your input is a square wave, your want the output to be also a square wave. Slew rate increases the rising and falling time of the output, changing the expected square wave into a trapezoid.

 

[Colbhaidh]

Re: Analog ground

Note that analogue ground when referring to a complex mixed mode IC is separated from digital ground for noise isolation considerations.

 

[white_shadow]

Re: Analog ground

2) You wonder why we use +/-, as opposed to Vcc/0.

I think beginners like me have at one time wondered about this. That is the voltage present in a system (or a circuit for beginners). If you take for instance a PC. There is 0, 5, and 12 volts. But if you look closer you have other voltages -5, -12. These are just voltage rails. Different aspects of a circuit will have varying voltage requirements. Such as biasing transistors or diodes.

Another thing you should look at is Vcc. Do you know why it is denoted Vcc?
I'm not sure but it could be V collector to collector.

 

[yjkwon57]

Re: Analog ground

Hi~~

Using +Vcc/-Vcc or Vcc1(= 2*Vcc)/GND depinds on applicxations. If your application needs a circuit to work from GND to Vcc1(=2*Vcc), then, you will use GND and Vcc1. More specifically, you may use GND/-Vcc1 if it is required from applications. By the way, since the full swing in the circuit operation is benefitial, you will change the DC offset in the input signal and understand the output signal to include the DC offset corresponding to the input signal offset.

 

[savadi]

Re: Analog ground

1) in the design of Mixed mode IC (Analog+Digital) due to the switching, the current of Vdd and also substrate is changed that causes noise. so that the analog ground is separated from digital
2) In todays IC technologies Vdd and ground is used not +-vdd. because making the positive and negative power supply is difficult.
3) If your input was Asinwt. so the maximum variation of your input is AW which happens at t=0. if you have a unity feedback AW=Slew rate. for a costant amplitude w can be yield.

 

[pixel]

Re: Analog ground

Dual supply has can make better performance in analog circuits, than single supply.
When you measure AC signal on 0-5v range if you set AGND to 2.5 V.
For voltage >2.5 signal is positive, and for <2.5 negative, but the problem is that 2.5 V is not so stable as a GND 0V.
When you have dual supply you have not this problem, because Analog gnd is at same potential as a GND, which is stabilized.
Dual power supply is something that engeneers like, but managers dislike, because of additional complexity, and price.

 

[mopengfei]

Analog ground

in a word ,u must know that the Analog ground can't connect to the Number groud.

 

[wwfieee]

Re: Analog ground

slew rate is important if your signal is changing faster than what the op amp can change. the limit can be found when you take the steepest derivative of your signal, and usually this is at the 0 crossing level for sinusoid signals. for inputs like a step function, the exponential rise of the signal is fastest at the beginning. in any case, your signal will be distorted which may or may not be good depending on your design. the amplitude at which the signal changes is also critical, the smaller the amplitude, the less likely the op amp becomes slew rate limited

 

[zareh]

Re: Analog ground

1) in mixed mode operation we isolate digital and analog ground to prevent digital circuit noise which also exist in ground potential to affect analog circuit operation and degrades its performance.

2)in bipolar opmaps output loads are often resistive and have small values.when circuit power supplies are +/- and we apply feedback to opamp then DC operating point reach their good condition for linear operation and output DC voltage approximately equals ground potential.
so in dc condition circuit saves very large power dissipation and consumes low power dissipation.

3)slewrate is a phenomenon that appears in large signal condition for example when we aplly a large sinewave to opamp input.in this case
slewrate affect output signal in addition to other circuit nonidealties and prevent output from fast transition and change sinwave to triangular.