Hi.
I am using the Texas instruments INA125 chip for an analogue amplifier as I have shown on the attached image. INA125 data sheets says Nonelinearity -+0.002% of FS at Gain = 500
I am trying to use both the single and dual supply modes by connecting Vs- to ground or to a negative supply
The Dual supply operation seems to work fine (I have connected it almost identical to figure 1 of the INA125 data sheet) Its giving me an acceptable linear output 0.07% of FS for a gain of up to 4000.
The Problem is when I connect it in a Single Supply Mode (Figure 5 of the data sheet attached) with V-,VrefCom and IAref all connected to ground.
The linearity is way out of spec (I am getting 1-3% of FS) and I have a minimum Output of around 65mV (even placing offset adjustment resistors I cannot go below this value in single supply mode).
I also measured IAref before connecting it to ground at it was = 2.5V. A bit like in figure 6 of the data sheet, but how is this possible if I did not connect IAref to Vref2.5. I have a feeling watever the problem is, it probably has something to do with this IAref, but I cannot seem to know why..please help!

 

The Problem is when I connect it in a Single Supply Mode (Figure 5 of the data sheet attached) with V-,VrefCom and IAref all connected to ground.
The linearity is way out of spec (I am getting 1-3% of FS) and I have a minimum Output of around 65mV (even placing offset adjustment resistors I cannot go below this value in single supply mode).

Over what output voltage range did you measure the linearity?

Did you have a load on the output when you measured the minimum output?
If not, a load of a few kΩ to ground, may reduce that minimum.

 

Over what output voltage range did you measure the linearity?

Did you have a load on the output when you measured the minimum output?
If not, a load of a few kΩ to ground, may reduce that minimum.

My application is for a 0-10V output or 0-5V output. I am using minimum 7V input for the single supply 0-5V and 12V for 0-10V. The Maximum I am using is about 32V.
No I did not have a load on the output, I only connected a DMM +terminal to Vout and negative terminal to gnd. Is the load necessary especially because I need a 0-5V output to be displayed or for something else, how is this going to work with the load there?...If you could please help me to understand why there is this Minimum 65mV instead of Zero it would really be useful, am I missing something here? I do not think the ground is at 65mV, so how come If I connect IAref to ground I then get 65mV?

 

I do not think the ground is at 65mV, so how come If I connect IAref to ground I then get 65mV?

Because the output circuit is not designed to pull the output totally down to ground.
(See data sheet discussion below, emphasis mine):


Note that if you measure the linearity to below 150mV output, it will give significant error.

 

Because the output circuit is not designed to pull the output totally down to ground.
(See data sheet discussion below, emphasis mine):
View attachment 144240

Note that if you measure the linearity to below 150mV output, it will give significant error.

Okay thanks. Is the only way to get around this to make it a 0.2 - 5.2V output? or there are other ways to get around this because I really need a 0-5 or 0-10V output?

 

You already know a way to do it. Use a negative supply. Single supply can never get you accuracy near zero.

Bob

 

You already know a way to do it. Use a negative supply. Single supply can never get you accuracy near zero.

Bob

Yes I understand but my main requirement is to have the circuit to work with only a single power supply.
Do you have any suggestion of maybe additional circuit that I could add like a power supply converter? Something Like a virtual ground using voltage dividers or maybe TLE2426? help here would be appreciated because I do not know how I can connect this to my current circuit? but it would need to work for supply voltages of (7V to 12V) Minimum and (32V or 36V) Maximum.
Another thing is I could use the IAref (which is 2.5V) as my output reference. So that I can have my outputs as 0-5 or 0-10V (in reality they will actually be 0-2.5V and 0-7.5V if referenced to ground). Will this be a bad/wrong thing to do? I will just have to make sure that instead of 7V minimum for 0-5V output I can say maybe 10V minimum to power the circuit?

 

Use a TL7660 to convert the +10 volts to negative.
SG

 

Okay thanks. Is the only way to get around this to make it a 0.2 - 5.2V output? or there are other ways to get around this because I really need a 0-5 or 0-10V output?

Yes that's one way. You can set the range from .2 to 5.2 volt output followed by an op amp to zero offset the .2 volt.
SG

 

Use a TL7660 to convert the +10 volts to negative.
SG
View attachment 144300

Hi.
Thanks for the help, I am not sure this will work because it is a 10V converter and the INA125 needs something like almost 12V supply to produce a 0-10V output or 7V supply for 0-5V output. I also would need the circuit to work for 7V-36V in a single supply mode or -+(7V to 15V) in a dual supply.
Is it also a good idea as an alternative approach resulting in 0V to 5V range, to use LM7705 (a negative charge pump with -230mV output) to bias the negative rail (i.e connect the LM7705 output to pin 3 of the ina125)?

 

Hi.

Is it also a good idea as an alternative approach resulting in 0V to 5V range, to use LM7705 (a negative charge pump with -230mV output) to bias the negative rail (i.e connect the LM7705 output to pin 3 of the ina125)?

You can but would need to use a voltage regulator in front of the LM7705 as max Vcc is 5.75volts. Similar situation with the TL7660 as the output voltage follows the input, a 5 volt regulator preceding the chip would produce a -5 volt supply for negative Vs under all Vcc voltages.
SG

 

You can but would need to use a voltage regulator in front of the LM7705 as max Vcc is 5.75volts. Similar situation with the TL7660 as the output voltage follows the input, a 5 volt regulator preceding the chip would produce a -5 volt supply for negative Vs under all Vcc voltages.
SG

How about if I just power the LM7705 using the Vref_5V precision voltage reference from the INA125? Since I am already using this 5V precision reference for bridge excitation, I might as-well use it to power the LM7705. (I assume it should be able to handle this?) i.e connect pin 4 of INA125 to the input of the LM7705 and output of LM7705 to pin 3 of INA125?
This might save me having to place a voltage regulator for the TL7660?

 

How about if I just power the LM7705 using the Vref_5V precision voltage reference from the INA125? Since I am already using this 5V precision reference for bridge excitation, I might as-well use it to power the LM7705. (I assume it should be able to handle this?) i.e connect pin 4 of INA125 to the input of the LM7705 and output of LM7705 to pin 3 of INA125?
This might save me having to place a voltage regulator for the TL7660?

Good idea! Yes it will handle the current easily. It's possible the INA125 may require more than -.23 volts since it is not rail to rail.
SG