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.

Accuracy of Differential opamp over entire input voltage range?

Status
Not open for further replies.
T

treez

Guest
Hello,

We wish to monitor the current from a battery as in the schematic shown of a differential opamp with LT6220 opamp.

However, the LT6220 datasheet only gives the CMRR for Vin from 0 to 3.5V, and only when the supply to the opamp is 5V.

However, we have an 8V supply to the opamp, and the common mode voltage can go up to 4.8V.
Therefore, do you think our setup will be very accurate? We don’t need fantastic accuracy, but if the opamp output voltage is more than 10% away from what it should be, then that may be a problem.


LT6220 Opamp datasheet
http://cds.linear.com/docs/en/datasheet/622012fc.pdf

Please also find attached the LTspice simulation of it. (this shows no problem but I think the opamp models are idealised in the simulator, and have infinite CMRR over the entire input range, so the simulation cannot be relied upon.
 

Attachments

  • Differential opamp schematic.pdf
    7.8 KB · Views: 55
  • Differential opamp.txt
    2 KB · Views: 17

Audioguru

Advanced Member level 5
Joined
Jan 19, 2008
Messages
9,397
Helped
2,149
Reputation
4,292
Reaction score
1,993
Trophy points
1,393
Location
Toronto area of Canada
Activity points
59,185
The opamp has rail-to-rail inputs that work fine when between 0V or its positive supply voltage. CMRR is simply the rejection of hum on the positive supply.
 
  • Like
Reactions: treez

    T

    Points: 2
    Helpful Answer Positive Rating

FvM

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
50,020
Helped
14,496
Reputation
29,262
Reaction score
13,426
Trophy points
1,393
Location
Bochum, Germany
Activity points
287,099
LT6220 is probably not first choice for a high side current monitor.

You are right that CMRR and PSRR are unspecified for common mode voltage near the positive rail. Also the input offset is increased. But in most cases, resistor mismatch will be the dominant source of power supply power supply dependency.

Respectively, the best way to achieve high PSRR is a dedicated current monitor amplifier utilizing current sources.
 
  • Like
Reactions: treez

    T

    Points: 2
    Helpful Answer Positive Rating
T

treez

Guest
LT6220 is probably not first choice for a high side current monitor.

..the problem is , that no current monitor IC's that work in your recommended manner actually work with voltages below 3V.

For example, the LTC6102 will not work when the battery is down at 2.9V

LTC6102 datasheet
http://cds.linear.com/docs/en/datasheet/6102fe.pdf

..so anyway, do you think that the circuit I show in post #1 will be more than 10% inaccurate?

..should I really be using an Instrumentation opamp?
 

FvM

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
50,020
Helped
14,496
Reputation
29,262
Reaction score
13,426
Trophy points
1,393
Location
Bochum, Germany
Activity points
287,099
I was assuming that you'll use the OP with Vs = Vin. With Vs = 8V, the accuracy will be surely better. I would expect similar CMRR values as specified for Vs = 5V.

The achieved CMRR will mainly depend on the resistor matching, you can calculate the maximum error for e.g. 1 % or 0.1% resistors.
 
  • Like
Reactions: treez

    T

    Points: 2
    Helpful Answer Positive Rating
T

treez

Guest
Thanks, do you think the accuracy, CMRR & input offset voltage will be better for the following instrumentation opamp version of the circuit in post #1?

LTspice simulation and pdf schematic attached
 

Attachments

  • instrumentation opamp SCHEMATIC.pdf
    8 KB · Views: 52
  • Instrumentation opamp.txt
    2.5 KB · Views: 19

FvM

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
50,020
Helped
14,496
Reputation
29,262
Reaction score
13,426
Trophy points
1,393
Location
Bochum, Germany
Activity points
287,099
No advantage at all, but adding offset voltage.
 
  • Like
Reactions: treez

    T

    Points: 2
    Helpful Answer Positive Rating
T

treez

Guest
so the IN amp is worse?....the web says IN amps are better than diff amps
 

KlausST

Super Moderator
Staff member
Joined
Apr 17, 2014
Messages
21,980
Helped
4,591
Reputation
9,196
Reaction score
4,808
Trophy points
1,393
Activity points
145,109
Hi,

look for TI`s LMP8640, INA282, or LT´s 1999, or MAXIM MAX4372FESA.

As far as i can see all work down below 2.9V Vcc and have at least common mode voltage range of 0...28V.

Klaus
 
  • Like
Reactions: treez

    T

    Points: 2
    Helpful Answer Positive Rating

KlausST

Super Moderator
Staff member
Joined
Apr 17, 2014
Messages
21,980
Helped
4,591
Reputation
9,196
Reaction score
4,808
Trophy points
1,393
Activity points
145,109
Hi,

sorry but as you can see none work down to less than around 2.7v,
All have a CMVR less than 2.7V.

It seems you mix it up with VCC. Yes, the most of them work down to 2.7V.
However, we have an 8V supply to the opamp
With VCC of 8V the CMV can be down to zero and less with every given device...
So why now 2.7V? (CMV or VCC?)

In post#4 you talk about 2.9V Vcc.
So what is your specification for VCC?
And what is your specification for CMVR?

Klaus
 
  • Like
Reactions: treez

    T

    Points: 2
    Helpful Answer Positive Rating
T

treez

Guest
those 'hang off the rail' current monitor ic's cannot be used to monitor current from 3 nimh cells in series, especially when cable drop is also accounted for
 

KlausST

Super Moderator
Staff member
Joined
Apr 17, 2014
Messages
21,980
Helped
4,591
Reputation
9,196
Reaction score
4,808
Trophy points
1,393
Activity points
145,109
Hi,

those 'hang off the rail' current monitor ic's cannot be used to monitor current from 3 nimh cells in series, especially when cable drop is also accounted for

Why not?

-> use seperate wires for connecting the ICs with the shunt (sense signals).
-> Connect IC GND to analog GND (ADC)

Klaus
 
  • Like
Reactions: treez

    T

    Points: 2
    Helpful Answer Positive Rating
T

treez

Guest
thanks, but for three nimh cells, and monitoring the current from them , surely a diff amp is all that is needed?.....is there anything special about those "hang off the rail" current monitor ics?
 

KlausST

Super Moderator
Staff member
Joined
Apr 17, 2014
Messages
21,980
Helped
4,591
Reputation
9,196
Reaction score
4,808
Trophy points
1,393
Activity points
145,109
Hi,

Give more information: What especially is the problem with "three nimh cells"?

is there anything special about those "hang off the rail" current monitor ics
Yes, the are specially designed for this (your) application = optimized. Regarding supply current, CMRR, CMVR...;-)

from the MAX4372 datasheet:
The input common-
mode range of 0 to 28V is independent of the supply
voltage and ensures that the current-sense
feedback remains viable even when connected to a
2-cell battery pack in deep discharge.

Klaus
 
  • Like
Reactions: treez

    T

    Points: 2
    Helpful Answer Positive Rating
T

treez

Guest
Thanks very much Klaus, now i see your point....that max4372 HAS a separate vcc pin, so that is good, it doent rely on the battery voltage for power.....thankyou very much indeed.......that is what I have been looking for......I will just check if it is more accurate than a diff amp though.
 

KlausST

Super Moderator
Staff member
Joined
Apr 17, 2014
Messages
21,980
Helped
4,591
Reputation
9,196
Reaction score
4,808
Trophy points
1,393
Activity points
145,109
Hi,

Any of those ICs i´ve listed has seperate VCC.

Klaus
 
  • Like
Reactions: treez

    T

    Points: 2
    Helpful Answer Positive Rating
Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top