Accuracy of Differential opamp over entire input voltage range?

• Jul 14, 2015

• #1

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: 118
• Differential opamp.txt

  2 KB · Views: 71

• Jul 14, 2015

• #2

Audioguru

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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.

 

• Jul 14, 2015

• #3

FvM

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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.

 

• Jul 14, 2015

• #4

T

treez

Guest

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

Click to expand...

..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?

 

• Jul 14, 2015

• #5

FvM

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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.

 

• Jul 14, 2015

• #6

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

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• Instrumentation opamp.txt

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• Jul 15, 2015

• #7

FvM

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No advantage at all, but adding offset voltage.

 

• Jul 15, 2015

• #8

T

treez

Guest

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

 

• Jul 15, 2015

• #9

KlausST

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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

 

• Jul 15, 2015

• #10

T

treez

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https://datasheet.octopart.com/MAX4372TEBT+T-Dallas-Semiconductor-datasheet-13699810.pdf

...sorry but as you can see none work down to less than around 2.7v, and with three nimh batteries and cable loss, that's not low enough....it has to be either diff amp or IN amp.

 

• Jul 15, 2015

• #11

KlausST

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Hi,

sorry but as you can see none work down to less than around 2.7v,

Click to expand...

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

Click to expand...

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

 

• Jul 15, 2015

• #12

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

 

• Jul 15, 2015

• #13

KlausST

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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

Click to expand...

Why not?

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

Klaus

 

• Jul 15, 2015

• #14

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?

 

• Jul 15, 2015

• #15

KlausST

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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

Click to expand...

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.

Click to expand...

Klaus

 

• Jul 15, 2015

• #16

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.

 

• Jul 15, 2015

• #17

KlausST

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Hi,

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

Klaus