The precision of INA105 for voltage addition and voltage subtraction

[[email protected]]

I'm doing a simple project which will do voltage
calculations(A+D)-(B+C)/(A+B+C+D) which some of you have known through
my previous post.

I'm using INA105 for the addition part. With Vcc +5 (+10) and Vdd -5
(-10), i obtained very poor results. For example, A=-5, B=-5, A+B=-10
which is very inaccurate. Based on your experience, are there any
problems with the precisions of the INA105 itself which is not 100%
unity gain or some noise may be incurred?

I'll add a VR in the feedback loop to fine tune. I may be wrong. Do you
have any suggestions/opinions which may help to quicken the design
process?

Fyi, the link to technical info of INA105 is as below:
http://www.us.oup.com/us/pdf/microcircuits/students/amps/ina105-burr.pdf
Thanks in advance.

Regards,
Albert

 

[Uwe Bonnes]

I'm doing a simple project which will do voltage
calculations(A+D)-(B+C)/(A+B+C+D) which some of you have known through
my previous post.

I'm using INA105 for the addition part. With Vcc +5 (+10) and Vdd -5
(-10), i obtained very poor results. For example, A=-5, B=-5, A+B=-10
which is very inaccurate. Based on your experience, are there any
problems with the precisions of the INA105 itself which is not 100%
unity gain or some noise may be incurred?

I'll add a VR in the feedback loop to fine tune. I may be wrong. Do you
have any suggestions/opinions which may help to quicken the design
process?

Fyi, the link to technical info of INA105 is as below:
http://www.us.oup.com/us/pdf/microcircuits/students/amps/ina105-burr.pdf
Thanks in advance.

Be sure to keep all intermediate and final voltages well inside the output
range.

Bye

 

[[email protected]]

Dear Uwe Bonnes,

thanks. Quoting your post, i assume that you meant the intermediate and
final voltages must be within the Vcc &Vee. Am i right? Correct me if
i'm wrong.

Dear all,

Well... After testing the circuit and connecting the circuit to the
correct ground, i was able to obtain the more desirable results., which
is +/-3%.

Now, i'm facing a problem regarding Vref and GND. My circuit is
basically designed to obtain 4 signals (namely A, B, C and D) from 4
testing points of a CD drive and do calculation of output
((A+D)-(B+C))/(A+B+C+D). All the 4 signals and output must be connected
to Vref as a reference point instead of GND. Apart from that, the Vcc
and Vee of the op-amps used are also connected to the Vref.

Vref and GND mentioned above are both points on the CD drive also .
When we take GND as the reference point, everything works perfectly.
However, if Vref is taken as the reference point, some component(s)
which are in between Vref and the GND is/are burnt.

My question is that how can we take Vref as the reference point without
causing damage to the CD drive while getting a desirable results? Hope
to get your response soon. The Vref for different drives are different.

Fyi, the requirement is to take Vref as the reference point. I'm not
sure my explanation here is clear. If not, please feel free to
highlight to me. I'll try my best to make the picture clearer.

Sorry that my circuit diagram is not yet ready in soft copy.
Thanks.

Regards,
Albert