Offsetting Input Of a Instrumentation Amplifer

[Stanfi]

I have an application where I have a 0-36mv dc signal going into an instrumentation amplifer, and I set the gain to give me around 0 to 10VDC out.

In my application the lowest the my mv singal may go is 20mv, so my scalable signal is only 0 to 36mv an span of 16mv. I would like to be able to ingnore this 20mv, and scale my input so that I can amplify 0 to 16mv and get 0 to 10V out. I am limited on my output, and I cannot get enought output singal to allow me to offset the output, so would like to find a way to do it on the input, but I do not want to introduce uncessary distortion.

Any suggestions, would be appreciated.

 

[felixc]

Is the instrumentation amp a black box with no offset control, or is it an IC that will appear on your circuit? Some IC amps do have pins for offset control, or app note from manufacturers will offer typical ways to offset the inputs with their product.

 

[Stanfi]

I am using an IC.. Right now a burn brown INA125. However, if I could find one that allowed for offset control, I would be willing to switch my design.

Anyone have a suggestion of a IC that would allow this?

 

[felixc]

Perhaps you can find some ideas in this manual:

http://www.analog.com/analog_root/s...InstrumentationAmplifiers/designersGuide.html

 

[Warpspeed]

You should be able to add the offset to the output easily enough. But as you say this is not straightforward.

I think the first thing to do would be to set the gain to something that is not going to saturate the instrumentation amplifier. Say x100 or something. That will give you 3.6 volts output. Then offset that with 2.0 volts at the output, giving an output signal of 0 to 1.6 volts for your 0 to 16mV input.

That would then need to go into another amplifier to increase the gain sufficiently to get the desired 0 to 10v span. A gain of x6.25 should just about do it.

Quite a few extra parts unfortunately, but I think that may be about the only practical way around the problem.

 

[Stanfi]

Thanks guys,

I'll look at that manual felixc

Warpseed,

That is a good idea, something I may have to do. My first try is going to be to change out my power supply to 24V this will raise my limit on the output on the amplifer from 14 to 23 or so, allowing me to increase my gain. I am hoping this will work as this unit has been inproduction for a while, and would allow me to used the PCB boards we have already had manufactured.

 

[Warpspeed]

If you can change the supply voltages, that might help a great deal.

Having the positive supply much higher than the negative supply may also help, but be very cautious about losing negative common mode headroom on the inputs.

 

[wrong100]

Sranfi.
The easy way is it use a standard voltage offset cir.
It is easy just inject enough current from a ref voltage
(not p s )into the correct opamp input to make the total difference between the two, zero volts, at the required offset. This cir is used in analog cir all the time. It can be injected in two ways:
1) as a constant current source sample (resistor = 100 times
the normal source resistance and error will be 1/100.
2) as a constant voltage source (not good in all applications) but same resolution, (100 source/1 ref) just not as much ref voltage needed.
You can get a description of the workings from the application notes or help pages of many (I know it is hard
to find items in TI and its bought conpanies) IC mfg.
Linear T., and Nat. are a couple that have addressed
this problen (1970 to 1980) everyone did.

If you use an op amp with an offset adjust, and use it to
offset the output, you could get into temp, as well as
p. s. voltage offset problems, this offset feature
has MANY implications. It was first used to trim temp (or
and) offset voltages in an opamp such as Vbe verses current
"gain" and temp tracking (for the first stage). This was
an attempt to get offsets into the low mil range, and
you had to do temp testing to ensure this.

I do not know what your needs are all, aspects must be considerted (including how long will this produce be
here).

I must stop trying to give full courses.