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HdrChopper

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Voltage instrumentation amplifier commmon mode stability
Jun 11^th, 2009, 7:49pm

Dear all,

I'm currently designing a resistive feedback voltage instrumentation
amplifier with a closed loop gain of 100x. A high open loop gain allows me to achieve such closed loop gain. At the same time I�m trying to achieve the highest possible bandwidth without affecting too much the common mode stability.
As you know the common mode gain is 1X. Therefore the GBW for the CM loop gain will be 100x higher than for the differential loop and therefore stability is way tighter.

Could you point me to a good reference or briefly comment on any technique you may know of for improving common mode stability and at the same time keep the differential GBW unaffected?

Your comments or suggestions will be highly appreciated.
Best regards

Tosei

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sos

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Re: Voltage instrumentation amplifier commmon mode stability
Reply #1 - Jun 24^th, 2009, 8:48am

What comes to mind is to use current feedback amplifiers for the input amps. They will be stable to unity gain, and retain a lot of their bandwidth at high gains. x100 is pretty high, your required bandwidth at this gain is probably a determining factor in which op amp to use (I don't have any special recommendations).

The downside of using current feedback amplifiers is that you will increase the inverting input current errors, both DC and noise. The DC terms will be rejected by the cmrr of the instrumentation amp, the noise will not.

Steve

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aaron_do

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Re: Voltage instrumentation amplifier commmon mode stability
Reply #2 - Jun 24^th, 2009, 5:45pm

Hi,

just wanted to refer you to a paper which talks about CMRR for the instrumentation amplifier.

M. A. Smither et. al, "C.M.R.R. ANALYSIS OF THE 3-OP-AMP INSTRUMENTATION AMPLIFIER", ELECTRONICS LETTERS 29th September 1977 Vol. 13 No. 20.

Here he shows that the CMRR of an instrumentation amplifier is not improved by increasing the buffer gain.

However, I think if your objective is to have a high ratio of differential-mode to common-mode gain then high buffer gain is necessary.

cheers,
Aaron

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HdrChopper Community Fellow Offline Posts: 493	Re: Voltage instrumentation amplifier commmon mode stability Reply #3 - Jun 24^th, 2009, 6:52pm Thanks folks for your valuable suggestions. Steve: unfortunately I cannot use current feedback in my design and I have to stick to the simpler resistive feedback. Aaron: I'll check that reference. Many thanks Tosei
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sos

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Re: Voltage instrumentation amplifier commmon mode stability
Reply #4 - Jun 30^th, 2009, 9:02am

It doesn't sound like you follow my suggestion, unless I don't follow your comment.

Current feedback in my post refers to the internal construction of the two op amps making up the input stage of a traditional three op amp instrumentation amp. They still have just resistors around them like normal.

Steve

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HdrChopper

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Re: Voltage instrumentation amplifier commmon mode stability
Reply #5 - Jul 5^th, 2009, 4:34pm

Hi Steve,

I understood what you were suggesting Steve. I actually was not totally clear when I asked the question: Instead of using the classical 3-opamp structure, I use only the 2buffer opamps and keep the differential output signal instead of converting it into single-ended (see attach). Instead of gaining 1x in the buffer stages I gain 100x, but I doubt I will be able to use current feedback since area is really tight in my circuit; unless I'm able to really save the day concerning the CM stability with the current feedback.
Since I'm not familiar with the current feedback in IAs, could you point me to a good reference about them?

Many thanks & Regards
Tosei

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sos

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Re: Voltage instrumentation amplifier commmon mode stability
Reply #6 - Jul 10^th, 2009, 1:48pm

I don't know of a reference talking about using current feedback opamps in an instrumentation amp, but maybe just an application note on current feedback would be useful. I, of course, recommend my company's app note, The Ins and Outs of Current Feedback:
http://cadeka.com/uploaded_files/AN-3_The%20Ins%20and%20Outs%20of%20Current%20Fe...
You may have to splice the URL together, or you can hunt for AN-3 on the company home page.
Many other companies have similar app notes. Ours descends directly from Comlinear->Kota->Cadeka, which are all pretty much the same people doing the writing and designing. So I think ours is maybe more polished than our competitors, but I'm biased.

Sorry I took so long to reply, I guess if I'm going to reply, I should read the board more often.

Steve Smith

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pauloau

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Re: Voltage instrumentation amplifier commmon mode stability
Reply #7 - Jul 15^th, 2009, 1:27pm

Hi Tosei,

As I understand, you are designing an InAmp with discrete op-amps. Am I right?
In that case, if you want to use current-mode op-amps following Steve's suggestion, you could take a look on this big appnote from Analog Devices: http://www.analog.com/static/imported-files/design_handbooks/5812756674312778737...)
There is a lot of info there and part numbers that maybe you could use.

Now if you are designing an integrated InAmp, you might want to take a look at the following references:
B.J. van den Dool, J.H.Huijsing, "Indirect Current Feedback Instrumentation Amplifier with a Common-Mode Input Range that Includes the Negative Rail," IEEE J. Solid-State Circuits, Vol.28, No.7, pp. 743-749, Jully 1993.
C. Toumazou, F. J. Lidgey, "Novel Current-Mode Instrumentation Amplifier", Electronics Letters, Vol. 25, No. 3, pp. 228-230, Feb., 1989.

In Toumazou's paper, a discrete realization is described, but an integrated one would be better in terms of matching.

Regards,
Paulo

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HdrChopper Community Fellow Offline Posts: 493	Re: Voltage instrumentation amplifier commmon mode stability Reply #8 - Jul 15^th, 2009, 7:12pm Hi Paulo, Thanks for your useful suggestions. I�m actually designing an integrated IA so I will look for the corresponding references you pointed out. Best Regards Tosei
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