Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

Op-amp circuit with different cutoff frequencies

Status
Not open for further replies.
J

jimmykk

Full Member level 3
Joined
Oct 2, 2014
Messages
158
Helped
0
Reputation
0
Reaction score
1
Trophy points
1,298
Activity points
2,863
Star Ground Single Supply op amp

Hi
I was going through a design recommendation and came across this Star Ground concept. Why is the +Vs has bypass capacitors connected to star ground. Is it this ground same as actual ground.
opamp.PNG
Also, the Vs/2 voltage shown at negative input of op amp has to be connected physically to the resistor divider network or will it automatically appear
that point.
 

Re: Star Ground Single Supply op amp

As for the "star ground concept", I'm with the Analog Devices application note Staying Well Grounded. https://www.analog.com/en/analog-dialogue/articles/staying-well-grounded.html

The star grounding philosophy, while reasonable theoretically, is difficult to implement in practice.
Click to expand...

It serves a purpose for some power supply circuits, for general analog circuit design, you better consider ground planes. OP power supply bypassing need to be located as near to each device as possible.

The single supply amplifier circuit needs a dedicated bias voltage divider, nothing appears automatically for general purpose OPs. Some application specific amps, e.g. audio power amplifier may provide a built-in bias network.
 

Re: Star Ground Single Supply op amp

Hi,

Vs/2 is generated with the voltage divider RA and RB.
No DC current can flow through capacitors, thus VS/2 is at noniverting input.
The Opamp (circuit) regulates the output in a way that the same voltage appears at the inverting input.

No need for extra circuitry.

Klaus
 

Re: Star Ground Single Supply op amp

Ground planes are good for signal but throwing high
switching currents or noisy supply current returns
onto it, makes these "signal" as well (which you do
not want, mixed in). For such things a star scheme
(or, a "clean" ground plane and a "dirty" ground-return
plane, themselves "starred" at the best point you can
pick) is likely still better.

You might consider current loops and what they contain
as a way to lead you to ground partitioning.
 

Re: Star Ground Single Supply op amp

Thanks for the reply.
One thing more about this circuit that i would like to know is :
How would you know that what frequencies are allowed to pass and which are stopped by looking at these bandwidth equations in the circuit. I get that the last filter at output is a high pass filter. But
what about the others. Suppose, i have Ra= Rb= Rin =12k and R2=17.4K, R1= 2.2K and my load resistance is a transducer which varies from 500 to 2200 ohm.
I am interested in frequencies from around 50k to 1.2Mhz to pass through this circuit. Assuming i have used .1uF capacitors in this circuit.
 

Op amp circuit with different cut off frequencies

Hi
About this circuit that i would like to know is :
opamp.PNG
How would you know that what frequencies are allowed to pass and which are stopped by looking at these bandwidth equations in the circuit. I get that the last filter at output is a high pass filter. But what about the others. Suppose, i have Ra= Rb= Rin =12k and R2=17.4K, R1= 2.2K and my load resistance is a transducer which varies from 50 to 2200 ohm.
I am interested in frequencies from around 50k to 1.2Mhz to pass through this circuit with same gain. Assuming i have used .1uF capacitors in this circuit. What should be the values of the passive components in this circuit then
 

Re: Op amp circuit with different cut off frequencies

You'll usually start the design at a different point, after defining intended gain and bandwidth, you'll chose a suitable OP. OP1177 has GBW of 1.3 MHz and can't achieve your specification.


Secondly, you'll select resistors first according to OP and transducer parameters, then chose capacitor values.


Yes, BW2 and BW2 are high-pass corners and should be below the operation band. Also supply filter low-pass corner BW1.

BW1 calculation is only valid for Rin >> RA.
 

Re: Op amp circuit with different cut off frequencies

Hi
I forgot to mention that Op amp i used is LT1210.
LT1010 is used to provide the 12V Bias voltage of 12V into the + input terminal as Vs+ is 24V.
"Yes, BW2 and BW2 are high-pass corners and should be below the operation band"
So, BW2 and BW4 are high pass filters and is BW3 also low pass filter.
But in example Ra and Rin are same. Do i have to increase the value of Rin.
 

Re: Op amp circuit with different cut off frequencies

But in example Ra and Rin are same. Do i have to increase the value of Rin.
Click to expand...
Not necessarily. I just mentioned that the calculation for BW1 is no longer correct. Depending on the ratio of Cin to C2, BW2 may vary as well. Seting C2 >>Cin is another way to cancel BW2-BW1 dependencies.

The simplified calculation can't account for all possible cases, you better check the behavior of your circuit implementation with a circuit simulator.

LT1210 is a current feedback amplifier, the feedback network design must follow quite different rules than for regular voltage feedback type.
 

Re: Op amp circuit with different cut off frequencies

FvM said:
Not necessarily. I just mentioned that the calculation for BW1 is no longer correct. Depending on the ratio of Cin to C2, BW2 may vary as well. Seting C2 >>Cin is another way to cancel BW2-BW1 dependencies.
Click to expand...

So if i set C2=1uF with same resistor values i.e. 12k for Ra, Rb and Rin. Would it help to pass the frequencies from 50khz to 1.2Mhz.

As i see BW1 as a low pass corner which has cut off freq less than 1khz. How can it allow my frequency band to pass
 
Last edited:

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top