Newer Model Instrumentation Amp

[Paul Henderson]

I have previously used AMP 01, INA114 and AD620 instrumentation amps
for EEG apps. All maximum gain 10,000.

Since these packages are all about ten years old now, I would like to
update my design, if appropriate.

Can someone advise regarding more recent and better spec'd IC's of
this general type?

Paul Hendersen

 

[Winfield Hill]

Paul Henderson wrote...

I have previously used AMP 01, INA114 and AD620 instrumentation amps
for EEG apps. All maximum gain 10,000.

Since these packages are all about ten years old now, I would like to
update my design, if appropriate.

Can someone advise regarding more recent and better spec'd IC's of
this general type?

I recently finished updating our Selected Instrumentation Amps
table for H&H AoE III. Except for low-voltage-process designs,
I did not notice a lot of new activity in the last 15 years.
Most of the classic parts are doing fine, especially if they're
available in small SMT packages.

Commenting on your choices, but for gains up to 1000 (most of
the data sheets don't address G=10k, that's a lot in one stage).

The AMP01 comes in a large awkward package, but it seems to
do well at very high gains. However, it didn't make our list.
I wasn't able to find distributor inventory. Lack of interest?

The INA114 is rather slow at high gains, the INA128 has similar
input characteristics, but is about 10x faster at high gains.
Both are fine at distributors, maybe the INA128 has the edge.

The AD620 is a respectable part, popular, inexpensive. But
its 80dB CMRR at 10kHz pales compared to the INA128's 105dB.
The AD8221 is similar to the AD620, cheap, with 87dB CMRR.
The AD620 and AD8221 both let you directly bypass the input
transistors for RFI suppression. The INA128 may as well, but
TI hides the circuit details from the engineer. Too bad.

You might want to consider some JFET parts. JFETs usually do
better than bipolar in the ignoring-RFI department. The AD8220
is interesting, inexpensive, but for some reason distributors
don't have any stock. Sold out? It does well for bandwidth
at high gains, but not so well for CMRR at high frequency.
The venerable INA110, for example, beats the pant off of it.

 

[Winfield Hill]

John Larkin wrote...

On a current design, I had to make my own. I wanted lots of
overvoltage protection, logic-switchable gains from 0.05 to 256, high
precision, and at least +-12 volts of common-mode range, 120 dB CMRR
at high gain. I wound up with a classic 3-opamp diffamp, using an
LT1124 dual opamp, four Supertex depletion mode fets for protection, a
discrete string of thinfilm resistors, one DPDT gain switch relay, two
analog muxes, and an INA154 as the second stage. Two tiny trimpots
tweak cmrr. Times 16 on one board. I'd love to get all that in a SO-8!

Indeed.

What was its CMRR at 1 and 10kHz, that's a tough region.

 

[Joerg]

On a current design, I had to make my own. I wanted lots of
overvoltage protection, logic-switchable gains from 0.05 to 256, high
precision, and at least +-12 volts of common-mode range, 120 dB CMRR
at high gain. I wound up with a classic 3-opamp diffamp, using an
LT1124 dual opamp, four Supertex depletion mode fets for protection, a
discrete string of thinfilm resistors, one DPDT gain switch relay, two
analog muxes, and an INA154 as the second stage. Two tiny trimpots
tweak cmrr. Times 16 on one board. I'd love to get all that in a SO-8!

Maybe different in your case because you guys make low-volume specialty
equipment, and I have never designed an EEG. But I did design ECGs and
not in my wildest dreams would have considered instrumentation amps.
They are IMHO way overpriced, at $3-4 and up. I like to do that for less
than a buck

Sometimes it may pay off, while thinking about a change anyhow, to
ponder whether a transition to jelly-bean parts might make sense.

 

[Paul Henderson]

On a current design, I had to make my own. I wanted lots of
overvoltage protection, logic-switchable gains from 0.05 to 256, high
precision, and at least +-12 volts of common-mode range, 120 dB CMRR
at high gain. I wound up with a classic 3-opamp diffamp, using an
LT1124 dual opamp, four Supertex depletion mode fets for protection, a
discrete string of thinfilm resistors, one DPDT gain switch relay, two
analog muxes, and an INA154 as the second stage. Two tiny trimpots
tweak cmrr. Times 16 on one board. I'd love to get all that in a SO-8!

If that's not a proprietary design John, any chance of posting a link
to the schematic?

Paul Hendersen

 

[[email protected]]

Yes, it is proprietary but, hell, I *am* the boss, so here it is:

ftp://jjlarkin.lmi.net/22S490B_ch12.pdf

in hopes that it will invoke an entertaining flurry of pecking and
clucking.

Clucking_mode=on.

Doesn't your common mode rejection depend on the tracking of the two
DACs/resistor strings?

I've done something similar, but with R407, if I understand the design
correctly, being controlled (ADI 1024-tap digi-pot, in my case), rather than
the two feedback resistors. In my case, the common mode rejection depends on
the two fixed feedback resistors, which is simpler. Of course the gain is
backwards, but that's the DSPs job. ;-)

I don't totally like the style of the schematic; I drew it on D-size
vellum "my way" and The Brat entered it into PADS. It would be too
much work to push 16 channels of stuff around at this point.

Agreed. ;-)

 

[Spehro Pefhany]

Yes, it is proprietary but, hell, I *am* the boss, so here it is:

ftp://jjlarkin.lmi.net/22S490B_ch12.pdf

in hopes that it will invoke an entertaining flurry of pecking and
clucking.

I don't totally like the style of the schematic; I drew it on D-size
vellum "my way" and The Brat entered it into PADS. It would be too
much work to push 16 channels of stuff around at this point.

John

That bipolar relay driver is a thing of beauty.



Best regards,
Spehro Pefhany

 

[[email protected]]

DACs?

Resistors + switches = multiplying DAC

I looked into using digi-pots as the CMRR trims, but they had poor
resolution, poor TCs, and/or tons of capacitance. Plus, we were almost
out of digital interface resources. The trimpots we're using are about
the size of a 1206 resistor.

The point is to not need trimming, particularly gain sensitive trim.

 

[Joerg]

That bipolar relay driver is a thing of beauty.

Sure is.

But John calls them "K", as in kontactor or kool kampground )

 

[Spehro Pefhany]

Sure is.

But John calls them "K", as in kontactor or kool kampground )

K is the standard designator for relays. Don't know why.


Best regards,
Spehro Pefhany

 

[[email protected]]

K is the standard designator for relays. Don't know why.

Because they kost more than an 'R'.

 

[John Devereux]

Yes, it is proprietary but, hell, I *am* the boss, so here it is:

ftp://jjlarkin.lmi.net/22S490B_ch12.pdf

in hopes that it will invoke an entertaining flurry of pecking and
clucking.

I don't totally like the style of the schematic; I drew it on D-size
vellum "my way" and The Brat entered it into PADS. It would be too
much work to push 16 channels of stuff around at this point.

Thanks for posting a real-world design, it's nice to see that here.

Items I found "Interesting":

- I seem to recall you mentioning the use of the Supertex parts, not
seen them used like this before. Would have guessed the fault current
was too high, but in fact it looks like it is only a couple of mA.

- Use of latching relays and their driver. Never used latching ones
myself (or relays at all for that matter in low power circuits)!

- There seem to be twice as many gain setting switches/resistors as you
need. Is that for CMMR/layout symmetry reasons?

 

[Winfield Hill]

John Larkin wrote...

Yes, it is proprietary but, hell, I *am* the boss, so here it is:
ftp://jjlarkin.lmi.net/22S490B_ch12.pdf
in hopes that it will invoke an entertaining flurry of pecking
and clucking.

Thanks.

Cluck. Peck. It doesn't have much in the way of
RFI suppression on the input. Cluck. Cluck.

 

[Winfield Hill]

Winfield Hill wrote...

John Larkin wrote...

Thanks.

Cluck. Peck. It doesn't have much in the way of
RFI suppression on the input. Cluck. Cluck.

Oh, sorry, I missed L25 and L26. What are they?

 

[Spehro Pefhany]

Winfield Hill wrote...

Oh, sorry, I missed L25 and L26. What are they?

Look like 0603 ferrite beads 1K @ 100MHz, so it should roll off at
maybe 10MHz with the 47pF caps.



Best regards,
Spehro Pefhany

 

[Spehro Pefhany]

Young Buck Alarm: Driving Inductive loads directly from CMOS logic is
NOT recommended without some kind of protective elements. Allowing
CMOS body diodes to be exposed to flyback currents can be hazardous,
possibly fatal to the chip if latch-up occurs. Observe data sheet
current limits carefully.

...Jim Thompson

Good advice in general. I've used emitter followers from CMOS for the
unipolar latching type, which don't require diodes.

Not sure which ones JL used, but some of those relays only draw 8mA or
so at 4.5V, and the diodes are shunted by MOSFETs under normal
conditions so it ought to work okay. In pathological conditions (say a
sudden drop of Vdd to 0 during the brief pulse when the relay is
energized) the diodes would conduct, but something like 8mA maximum is
pretty conservative for that particular part- data sheet says 100mA or
50mA.

A 20-cent H-bridge...

 

[Winfield Hill]

[email protected] wrote...
I've made a point about this here a few dozen times over the
years. "My favorite" LND150, I usually called it.

Right, 5.6nV * sqrt 2 = 8nV for two 2k parts. BTW, I like to
add a 1k resistor to control the current, in a scheme that only
needs a single resistor, hence less noise impact. Others have
also thought of this; I grabbed the drawing below from a 2002
post by Adam Seychell. Only I use 1k, 200 ohms won't do much.

.. ,-------------,
.. | |
.. ------- |
.. connector | | | S D
.. from o---+ +-+-- 200R--+-+ +-----> to non-inv
.. outside D S | | |
.. world | -------
.. | |
.. '-------------'

FWIW, Supertex's datasheet says "not recommended for new design."
They suggest LND150, which looks identical.

They simply renamed the LND250, which was the elegant SOT-23
version of the TO-92 LND150, to the LND150K1, and then the
old TO-92 part becomes the LND150N3, and they also added a
TO-243 part, LND150N8, which can handle more power. Nice.

This is all in keeping with Supertex' longstanding numbering
scheme, and it makes my remark easier, because now I don't
have to say, my favorite LND150 and LND250. With all the
choices, now the LND150 really is my favorite!

 

[Joerg]

Because they kost more than an 'R'.

And more than a kapacitor. I've seen K used more in power gear. In
electronics it's often RL or REL. But I go with whatever the client prefers.

 

[Winfield Hill]

John Larkin wrote...

How does that help? Idss is only a few mA max, and the
resistor just adds to the Johnson noise.

It helps if you want to protect against someone connecting
to say 230Vac, and leaving the connection for a while before
discovering their error. We need to protect the sot-23s.

 

[[email protected]]

And more than a kapacitor.

Dunno about that! I was just combing though some BOMs looking for a couple
nickels to squeeze and just happened to notice that we're using a $7 cap
(Case-D 220uF 20% 16V) on one product. I have no clue why we're using that
cap because we have a similar cap (Case-D 220uF *10%* 16V) that we pay $.70
for. The relays on the $7 cap board are only a buck or two (I'm not looking
at that board, so don't remember exactly).

I've seen K used more in power gear. In
electronics it's often RL or REL. But I go with whatever the client prefers.

I've never seen anything other than 'K' used. I'm with John, RL, REL, and all
other bastardizations are just that.