Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronic 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

internal bias current compensation-op amp

Status
Not open for further replies.

lasteem1

Newbie level 5
Joined
Apr 28, 2010
Messages
9
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Location
US
Activity points
1,336
I see many experienced analog designers that impedance match the non-inverting and the inverting pins when an op-amp is internally compensated. Do most of you guys do this too? If not, what are Rules of Thumb for handling op-amps that compensate bias currents internally?
 

crutschow

Advanced Member level 5
Joined
Feb 22, 2012
Messages
3,789
Helped
903
Reputation
1,804
Reaction score
888
Trophy points
1,393
Location
L.A. USA Zulu -8
Activity points
21,353
The rule of thumb is, how much offset does the input bias current generate in the input impedance and can you tolerate that in your circuit. If that value is too high then you need to add the equivalent resistance on the other input so that the offset equals the offset current (difference in bias current between the two inputs) through the input impedance, not the bias current.
 

lasteem1

Newbie level 5
Joined
Apr 28, 2010
Messages
9
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Location
US
Activity points
1,336
The rule of thumb is, how much offset does the input bias current generate in the input impedance and can you tolerate that in your circuit. If that value is too high then you need to add the equivalent resistance on the other input so that the offset equals the offset current (difference in bias current between the two inputs) through the input impedance, not the bias current.
Usually when op-amps are internally compensated for bias currents their residual bias currents aren't equal and, in fact, they can be opposite in polarity because of slight mis-matches in the transistors or current sources. When this is the case I don't think you EVER want to have the equivalent impedance on the other input. However, if you have to add some divider network to add a DC bias(say to find a difference between an input and a known DC value) then what is the ROT for those resistors(and associated decoupling cap)? Do you make the impedance as small as possible?
 

Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top