# In-Amp Variable Reference Voltage as Active Buffer

Discussion in 'The Projects Forum' started by witssq, Mar 5, 2011.

1. ### witssq Thread Starter Active Member

Mar 29, 2009
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I would like to make a circuit of low noise adjustable reference voltage to instrumentation amp.

Using the attached doc's figure 11 "An Op Amp Buffer Connected as an Active Filter Drives the Reference Pin of an In-Amp", Not destroying purpose, I would like to
adjust reference voltage from -Vs to +Vs(dual supply rail).

The simple expected revision is

1) C2(1uF) to GND -> C2(1UF) to -VS (for dual supply)
2) R2(200kOhm) to GND -> R2(200kOhm) to -VS (for dual supply)
3) Supply rail GND -> -VS
4) R1(200KOhm) and R2(200KOhm) -> (20KOhm) + Trimpot(400KOhm) + (20KOhm) (Fixed to Adjustable, 90% of supply rail)

Could anyone help me about other component calculation and the above approach.

Best regards,

SunSung

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2. ### wayneh Expert

Sep 9, 2010
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Those changes make sense to me. What are you having trouble with?

3. ### Ron H AAC Fanatic!

Apr 14, 2005
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The whole point of the active filter is to remove power supply noise from the reference created by the voltage divider. When you return C2 to -VS, you defeat this purpose. Noise on -VS will appear at the op amp output unattenuated. Use a nonpolarized 1uF cap, connected to GND as shown.

4. ### witssq Thread Starter Active Member

Mar 29, 2009
48
0
After testing this circuit, I will respond with result.

I have one more question about REF Voltage.
I would like to know about exact transfer function of AD8221 with REF voltage term included.
I expect that it is : VOUT = ((VIN+ - VIN-)/(1 + (49.4K/RGAIN))) + VREF(REF Pin Voltage).
But on test, It seems not to be correct especially on REF term.

Best regards.
SunSung

5. ### Ron H AAC Fanatic!

Apr 14, 2005
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I don't know any more than what the datasheet says, but it is probably correct. Did you measure Vref at pin 6 (REF)?

6. ### witssq Thread Starter Active Member

Mar 29, 2009
48
0
Thanks,

Yes, VREF(TP2) on the attached schematic is correct with +/- 8V swing.
It seems that Transfer function is not simple addition of VREF.

Although doing more test for quantifying, if you have some idea about

Best regards,
SunSung

7. ### witssq Thread Starter Active Member

Mar 29, 2009
48
0
It seems that the above transfer function is correct.

Problem is different point
VREF swing should be above +/- 15V to amplyfy input (12.4-71.1mV) to output(-10V - +10V) with -42.7mV as Zero Point, as shown on the attached excel file image.

But REF pin cannot accept above Vs(+/- 10V).

Without enlarging supply voltage swing, How could I solve this problem?

Best Regards,
SunSung

• ###### AIR.jpg
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Last edited: Mar 7, 2011
8. ### Ron H AAC Fanatic!

Apr 14, 2005
7,049
674
I don't understand your problem description or your spreadsheet. Can you post the equation for the transfer function that you need, and the schematic of what you have?

9. ### witssq Thread Starter Active Member

Mar 29, 2009
48
0
Sorry, I missed then schematic upload.

My transfer function is the above reply. It is correct.

I need to make VDIFF(-42.7mV) to VOUT(Zero) on excel sheet.
To establish this level shift, +15V level shift is needed.
But level shift on reference pin more than +10V is not allowed referring to
data sheet.

Best regards,

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10. ### Ron H AAC Fanatic!

Apr 14, 2005
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674
But that is not an equation. What I meant was something like this:
Vout=kVin+Voffset
Where you fill in k and Voffset.

Vout=341.690*Vin+14.590

Is this correct? If so, you need to run the IA at lower gain and offset, and then post-amplify the output with a conventional noninverting op amp stage.
For example:
Vout=10*(34.169*Vin+1.459)
In this example, the post-amp would have a gain of 10. You can make the post-amp gain any value that will allow you to keep the IA offset below 10V.

11. ### witssq Thread Starter Active Member

Mar 29, 2009
48
0

Transfer function you requested is
VOUT = ((VIN+ - VIN-)/(1 + (49.4K/RGAIN))) + VREF(REF Pin Voltage).

You seems to understand correctly spreadsheet.

About +/- 5V output swing can be achieved current schematics. post-op amp for +/-10V seems to make more noise between AD8221 and ADC thant direct conversion with +/-5V. Is this correct?

Is thera a possibility to solve this problem using some network between AD8221 output pin and REF pin?

Best regards,
SunSung

12. ### Ron H AAC Fanatic!

Apr 14, 2005
7,049
674
Do you have an A/D that can accept ±10V input?
How many bits is it?

Mar 29, 2009
48
0

regards,

14. ### Ron H AAC Fanatic!

Apr 14, 2005
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Sorry for all the questions, but I'm trying to get a handle on the noise question you asked.

15. ### witssq Thread Starter Active Member

Mar 29, 2009
48
0
Thanks,

Please let me know about what system bandwidth means, because I don't have much experience.

Best regards,
SunSung Hwang

16. ### Ron H AAC Fanatic!

Apr 14, 2005
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674
What is generating the voltage you are amplifying and digitizing?

17. ### witssq Thread Starter Active Member

Mar 29, 2009
48
0
It's pressure sensor with bridge.
Please refer to the attached docs.

Best regards,
SunSung

18. ### Ron H AAC Fanatic!

Apr 14, 2005
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674
There are no documents attached.
What is your A/D sample rate?

19. ### witssq Thread Starter Active Member

Mar 29, 2009
48
0
Sorry,
I'm attaching documnets
I'll return with sample rate.

Best regards,
SunSung

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20. ### witssq Thread Starter Active Member

Mar 29, 2009
48
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The sample rate to need is maximum of AD7734