Newer Model Instrumentation Amp
in [Design]
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From: Joerg on 20 Jun 2010 11:38 John Larkin wrote: > On 20 Jun 2010 05:38:13 -0700, Winfield Hill > <Winfield_member(a)newsguy.com> wrote: > >> 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. >> > > 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. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: John Larkin on 20 Jun 2010 13:14 On 20 Jun 2010 08:29:57 -0700, Winfield Hill <Winfield_member(a)newsguy.com> wrote: >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. I'm after common-mode rejection for reasonable ground loops (DC, 60 Hz, mild harmonics) and volts of DC from load cell bridges. We tune the CMRR with a 10 volt p-p, 100 Hz square wave applied to both inputs, and the residual, as you turn the tweak pots, looks pretty square. We are currently applying the square wave and looking at the IA output with the gain set to max, using signal averaging on a scope at 2 mv/div, which is 8 uv/div relative to the input. Seems to work. I want to do it all in software, with a lockin-type thing, based on reading our ADC and synchronously detecting it against the cm square wave; if we sell enough of these, it might be worth writing that code. Here it is: http://www.highlandtechnology.com/DSS/V490DS.html The analog stuff pretty much just worked. The big problems were 1. The AD7699 ADC data sheet is, to be polite, terse. The differential inputs are unusual. It is just barely possible, or maybe according to the specs impossible, to actually run it at 500 KHz. A timing diagram might be helpful. 2. I rashly promised 8-pole digital filtering down to 1 Hz, Bessel and Butterworth. That turns out to be non-trivial when you get to 500,000:1 sample/cutoff ratio. Three architectures later, one consultant hired/paid/dumped, great amounts of experiment and simulation and debate, it seems to work. That adventure deserves a thread of its own. One hint: the classic DSP butterfly filter explodes at ratios like this. 3. The lead customer liked the filtering concept so much he asked for two independent filters per channel, one for the realtime data and another for the FIFOs. That's 32 filters and 16 4Kx16 FIFOs and lots of other junk in a Spartan6/45. 4. The Xilinx software is a train wreck, which greatly complicated 2) and 3). Rev 12, supposedly the one that supports Spartan6 properly, is worse than 11. You say "place that flipflop in an i/o block" and it says, after considering the request for five or six hours, "drop dead." We may start using Altera for new designs. I'm tweaking the rev C board layout and finishing up the firmware this weekend. I get interrupted too much at work to let me concentrate on stuff like this. I can't multitask and get serious work done. This is Fathers Day and it's been Meat Weekend. Rubbed Tri-tip last night, BBQ ribs tonight. Nothing green in sight. John
From: Paul Henderson on 20 Jun 2010 18:01 On Sun, 20 Jun 2010 07:38:00 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> 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! > If that's not a proprietary design John, any chance of posting a link to the schematic? Paul Hendersen
From: John Larkin on 20 Jun 2010 18:28 On Sun, 20 Jun 2010 22:01:24 GMT, paulhendersen(a)qualcomm.com (Paul Henderson) wrote: >On Sun, 20 Jun 2010 07:38:00 -0700, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> 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! >> > >If that's not a proprietary design John, any chance of posting a link >to the schematic? > >Paul Hendersen 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
From: Jim Thompson on 20 Jun 2010 18:38
On Sun, 20 Jun 2010 15:28:53 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Sun, 20 Jun 2010 22:01:24 GMT, paulhendersen(a)qualcomm.com (Paul >Henderson) wrote: > >>On Sun, 20 Jun 2010 07:38:00 -0700, John Larkin >><jjlarkin(a)highNOTlandTHIStechnologyPART.com> 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! >>> >> >>If that's not a proprietary design John, any chance of posting a link >>to the schematic? >> >>Paul Hendersen > > >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. > [snip] Well? Peck! Peck! Cluck! Cluck! If there's anything creative there I don't see it. But it does keep the young bucks uneducated... and that's the important part for my personal economy ;-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Where is Joe McCarthy when you need him ?? |