Instrumentation amplifier not amplifying?

[amspire]

The LM7171 data sheet says it is stable at gains of -1, or +2 (or more). I would try putting another pair of 1K resistors across R2 and R5 to increase the gain of the differential stage to 2. The LM7171 does not have great phase margin down near 0dB gain, so for this amplifier, the more gain, the more stable. If you need a gain of exactly 11, you could increase the 100 ohm resistor to compensate for the extra gain in the differential stage.

[danadak]

You have what appears to be an oscillation at 50 Mhz according to your timebase
on scope shot. Very fast amps have to have very careful low L layout and excellent
bypassing. Look carefully at bypass C specs, you should have a .01, 01 uF ceramic
as well as bulk bypass, eg, 1 uF or greater. If you look at ceramic datasheets not
all ceramics have good esr curves at RF.


https://www.intersil.com/content/dam/Intersil/documents/an13/an1325.pdf

http://cds.linear.com/docs/en/application-note/an47fa.pdf


Regards, Dana.

[torch]

After bumping the Windows priority up to "High" for LTSpice and leaving it run overnight, it was all the way up to about 75% done by morning. If I'm doing this right, the simulator shows negative gain (green line is V1, blue line is Vout):

select "modified trap" for the integration method and "alternate" for the solver.

"Modified Trap" was already selected. "Alternate" made little difference. However, changing the "matrix compiler" to "Pseudo code" and "Default DC solve strategy" to "Noopiter" and "Skip Gmin Stepping" really sped things up -- a 5ms sec test took just a couple of minutes. I have no idea what those things do, but the output graph looks about the same as the overnight result.

When I think of an InAmp, I think of low noise, high CMR amps.  The LM7171 is neither of these. It is a (somewhat) high speed, high current output amp.   It might be that is is not suited for use as an InAmp.  Something like an AD8671 (my workhorse) is what I would prefer.   That isn't to say you can't clean that output up a bit.

I was experimenting last night with different supply voltages, filter caps and termination and exploring the results with the scope probe. At -15/+15 supply, I accidentally shorted V1 and the adjacent negative rail pin. Apparently, that will kill the LM7171  . So maybe I'll try the AD8671 next...

[torch]

You have what appears to be an oscillation at 50 Mhz according to your timebase
on scope shot.

Yes.

Very fast amps have to have very careful low L layout and excellent
bypassing. Look carefully at bypass C specs, you should have a .01, 01 uF ceramic
as well as bulk bypass, eg, 1 uF or greater.

From my limited understanding of reducing inductance, I tried to keep signal paths as short as possible and used metal film resistors. I understand that SMD resistors may be preferable to through-hole, but I have found tiny smd components do not play well with old eyes and shaky hands.

The initial capacitor values were chosen from the TI spec sheet -- .01uF bypassing the chip pins, and 2.2uF bypassing the rails. TI did spec ceramic for the .01 and I used polyester film because a) that's what I had in that value and b)I understand they are more linear in response. Similarly, TI specified 2.2 tantalum caps and I used aluminium electrolytic at the rails because that's what I had. All that said, the power supply seems rock steady with no significant ripple.

TI did not specify the type of feedback capacitor, merely advising "For the LM7171, a feedback capacitor of 2pF is recommended". I used a ceramic disk -- again, because that's what I have.

If you look at ceramic datasheets not all ceramics have good esr curves at RF.

Given that I bought a grab-bag of various size capacitors from Radio Shack a few years back and later augmented that with a variety pack purchased on-line (don't remember exactly where from), I cannot characterize them better than the general construction type.

[danadak]

Look at section 9.1.3 of datasheet (TI) , comments about absolute value of
Feedback R.

http://www.ti.com/lit/ds/symlink/lm7171.pdf

To look at C effectiveness at RF a bridge is required.

http://techdoc.kvindesland.no/radio/passivecomp/20061223155312558.pdf

 You will be amazed just how crappy many ceramics are when you get into
the 10's of Mhz range expecting them to function as a bypass. I can attest
to not having looked at datasheets on passives and having poor results when
working at RF. Radio Shack grab bag components OK for generic low speed,
low DC constraint type of work. Otherwise choose your passives with care.

Lastly if your baseband signals are not RF, save yourself a lot of aggravation
and use low speed OpAmps for the IA. Note this OpAmp does not have a CMR
that includes ground, at low Vsupply its quite poor, another reason to consider
lower speed general class of OpAmps.

Regards, Dana.

[torch]

I was aiming for a high impedance amplifier with bandwidth up to at least 10MHz @ 10x amplitude. I picked the LM7171 because a) the unity gain is 200MHz, so the response should be pretty flat to 10MHz and b) I had some left over from another project (a little single-chip amplifier with 50ohm input impedance that works quite nicely).

[danadak]

Then you picked right part, but if you want CMR out to 10 Mhz you
will most likely have to include AC trim to get it. What is your CMR
goal at 10 Mhz ?

Rehards, Dana.

[torch]

As high as possible 

I thought the CMRR was primarily a function of the op amp itself, provided external conditions and components were equal. What do you mean by "AC trim"? At a guess, I suspect it must be related to correcting any AC phase mis-match between the buffer stage outputs and the amplification stage inputs?

[danadak]

The CMR is set by external accuracy of matching feedback R's
and Aol and G. Attached is an analysis done in Matlab that
can be used to evaluate component matching effects and Aol
OpAmp to OpAmp effects.

I was referring to use of a C trimmer to take care of layout differences
and stray C affecting matching.



Regards, Dana.

[torch]

That active compensation circuit looks intriguing. Almost too good to be true. I wonder what the downsides are?

[danadak]

It was done with a 400 Khz GBW OpAmp, trying to do this at 10 Mhz
I would guess is not a tea party. But then external trimming would possibly
work.

Another approach is using a differential high speed A/D, but then you
have to convert back to analog, unless you are taking measurements
and digitizing the output already. This approach has effectively trimmed
CMR due to the die matching and synchronicity of the samplers.

http://www.analog.com/media/en/technical-documentation/data-sheets/AD7402.pdf

Regards, Dana.

[danadak]

I happened to come across this today, might be of interest -

https://xellers.wordpress.com/electronics/1ghz-active-differential-probe/

Regards, Dana.