Composite amplifier (low noise + chopper) thread (diff between ltc1052 & ltc1151 - Page 1
Forum section: electronics / projects

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 17, 2013, 12:03:03 am
content: So I saw that you can built a pretty nice op amp by hooking up a chopper amplifier into the offset pin of a low noise amplifier, so you get the low noise properties along with low offset and low drift.I tried building one of these circuits, specifically a merge of the LT1028 and the LT1151.http://www.edn.com/design/analog/4408239/5/Composite-Amplifiers-Here is the circuit that I used, pretty much.Unfortunately Jim (I think he wrote this) does not give many details about why he chose the values that he did. I had a LTC1151 chopper stabilized amplifier handy, which is pretty much a dual +-15V version of the LTC1052 used in the app note and a LT1028. I built the circuit in the same way (using a gain of 100 rather then 1000), discarding the diodes and the 0.1 capacitors (the 1151 does not need them).*I used a 10ohm and a 1000 ohm rather then a 10 ohm and a 10000 ohm* THe LT1028 uses low value feedback resistors.However, this circuit does not work. The offset voltage is actually increased when I connect the chopper stabilized amplifier, by about 5 times, and I don't think I see a drift benefit (it is certainly not 50nV/degree C since I can detect the change when I heat the LT1028 with my finger tip, even considering the gain it should only be ~5uV/degree c).I soldered this circuit up on a piece of prototyping board, a high quality one made of just holes (no connections) that are through hole plated. I soldered everything together using bread board wire and I am confident that the connections are made, Ichecked everything with a ohm meter.How come this circuit does not work? Is the LTC1151 an inappropriate replacement for the LTC1052? A scan of the datasheet made it clear to me that its pretty much the same part with higher voltage rails, I don't see any game changing differences.Also I welcome any discussion on this type of amplifier design, pros cons questions comments. 'THis would be such a powerful tool if I could only figure out how to use it...*when I saw offset increased, I mean like an order of magnitude sometimes. And drift, it would slowly oscillate around some hunting point.

author: Rerouter
author_rank: Super Contributor
timestamp: December 17, 2013, 01:16:40 am
content: i could be completely wrong, but those 2 diodes on the chopper could have been to limit its output so that it did not blow up the offset pin on the low noise amplifier?

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 17, 2013, 01:19:39 am
content: Quote from: Rerouter on December 17, 2013, 01:16:40 amThe 1N758 Zeners allow the LTC1052 to function from ±15V rails. -articleThe LT1052 device is not a +-15V device, even with those diodes he is still technically exceeding the specification.The absolute maximum for the LTC1052 is actually 18V (TOTAL). He is giving it 20V in this app note. I don't understand.

author: wiss
author_rank: Frequent Contributor
timestamp: December 17, 2013, 09:59:16 am
content: Quote from: SArepairman on December 17, 2013, 01:19:39 amThe zeners remove 10 V from the 15 V supply, => +-5 V, it will work.Is the the 1152 oscillating?

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 17, 2013, 11:22:43 am
content: Quote from: wiss on December 17, 2013, 09:59:16 am]I notice probing pin 8 on the LT1028 with a keithley 199 DMM really changes the offset voltage, by milivolts, and this thing is supposed to have a super high input impedance.Same thing with a 34401a in high z mode

author: wiss
author_rank: Frequent Contributor
timestamp: December 17, 2013, 11:30:22 am
content: What does you source-circuit look like, what is on the input of the amp?

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 17, 2013, 11:34:25 am
content: It is grounded for nowhow come probing pin 8 causes an offset to change?

author: wiss
author_rank: Frequent Contributor
timestamp: December 17, 2013, 12:02:21 pm
content: Quote from: SArepairman on December 17, 2013, 11:34:25 amI guess you are in the 30 V range? you will load one side of the input pair collector with 11 MOhm + any bias-current from the DMM?Is the 1152 connected when you do this?

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 17, 2013, 12:08:23 pm
content: Quote from: wiss on December 17, 2013, 12:02:21 pmYes it is connected but I am probing it with the >10 gigaohm mode. It is connected. And yes the device is being powered by +-15V.

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 17, 2013, 12:16:00 pm
content: Hmm, I wonder if maybe there are flux leakage currents messing things up considering its so sensetive to a damn multimeter on those pins. I am going to give it a nice good cleaning before further bench work.there is alot of flux near the pins on the bottom of the board, I forgot to clean it off (the chips are mounted into two dip8 machine pin sockets, not the cheap ass ones). I am confident (*hopeful*) that the sockets are not the problem but the shorting flux under them might be.

author: wiss
author_rank: Frequent Contributor
timestamp: December 17, 2013, 12:28:50 pm
content: Quote from: SArepairman on December 17, 2013, 12:08:23 pmWhat is the bias current of the DMM? what does the it read with a 1 Meg "short" in the input?

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 17, 2013, 12:36:34 pm
content: The specifications for my 34401a meter claim the bias current is <30pA. I put a 1 meg resistor short in it andI see it jump from 24uV to 1uV in <10gohm mode with highest ac filter and 6.5 digits and 10 gigaohm mode.on the 10 megaohm input impedance mode i see it also jump from 0-25uV roughly.

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 17, 2013, 12:41:44 pm
content: I made a nicer short for it (used a banana to BNC then a BNC to screw terminal/banana connector) and its still pretty noisy. better then the test lead clips by far though.But now its only jumping from 0-3uV. The connector is maybe 3 inches long.

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 17, 2013, 12:43:38 pm
content: when I replace the 1 meg resistor with a short it is about 2uV in the test jig that I have setup but much less noisy.It should be 30 microvolts though right? 30pA * 1meg = 30uV...I can probably measure it with my electrometer. But I think its safe to just use the manufactures specifications that its 30pA for both modes, considering that the offset change was the same for both modes, assuming of course that it is the input bias current

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 17, 2013, 01:24:10 pm
content: :phew:no dice, i gave it a nice clean and its still doing the same old shiteven if i touch the ground with the multimeter (one lead connected) the offset shoots up.how the fuck am i supposed to test this god damn thing

author: wiss
author_rank: Frequent Contributor
timestamp: December 17, 2013, 01:25:21 pm
content: Quote from: SArepairman on December 17, 2013, 01:24:10 pmUhu? Ground-loop? Power it from two 9V batteries...

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 17, 2013, 01:33:22 pm
content: nmv it still changes osdfisdihjidsfhdsffsdhsuduhfusuh i need a brreak too fustatedsdfsadf sdzffvzvzvdzfwhy is there a 100k resistor linking the inverting pins?

author: wiss
author_rank: Frequent Contributor
timestamp: December 17, 2013, 02:10:31 pm
content: Quote from: SArepairman on December 17, 2013, 01:33:22 pmTo prevent the 1052 feedback-cap from swamping the 1028 feedback. And maybe to block the switch-charge from the 1052 from reaching the 1028.

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 18, 2013, 09:44:12 am
content: well I don't fucking know. I tried a LT1151, LT1051, LT1150. I don't have a LT1052.what makes the lt1052 so special?I guess the bias resistors must be different for some reason, thats the only thing I can think of.I am going to attach afew trim pots to this circuit to see if i can get soemthing that resembles a composite amplifier..i wonder what makes the 1051 different then all these other devices.I guess if the 1052 worked then they would have made appnote notes that it does work...

author: wiss
author_rank: Frequent Contributor
timestamp: December 18, 2013, 01:25:15 pm
content: I don't think it is special, I know that LTC1050 behaves like this, no way to get simultanious high bandwidth and high input impedance.

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 19, 2013, 01:39:06 am
content: Do you think I could combine this with a sallen key low pass filter to remove any offset errors? Are there any op-amp topologies that this composite amplifier configuration would not be usable with *assuming I buy the LT1052 and follow the app note to the letter?*I don't see why not but that was the point of my testing, to make low noise low offset filters with excellent temperature stability.

author: Rerouter
author_rank: Super Contributor
timestamp: December 19, 2013, 07:15:53 am
content: as a test i would up that 10n feedback capacitor to 100n, and really slow things down, as the offset is hardly going to change that fast,and to assure it remains stable, put a 1K resistance straight after the output and before tying in with the capacitor, as they never like directly driving a capacitive load,

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 19, 2013, 07:59:18 am
content: Quote from: Rerouter on December 19, 2013, 07:15:53 amwill do and it will work, hopefully, but if it does not what is your suspicion of why the non lt1052 amplifiers don't work?I figured there must be a reason why they don't include the same app note in the other chopper amplifiers, only the lt1052 has a composite amplifier app note, even though the specs on all these guys are almost the same. Does the internal sample and hold capacitance on the non lt1052 amplifiers have some kind of problem preventing it from doing what its supposed to do? The only difference between them is that the other amplifiers don't need sample and hold capacitors.

author: Rerouter
author_rank: Super Contributor
timestamp: December 19, 2013, 09:10:06 am
content: i would like to point out that the lt1052 datasheet also uses a 100n capacitor,also the 100K resistor isolating the negative feedback of the chip should have a low temp coefficient, even the heat from your hand near this could easily throw it out by a few 10's of uVmy suspicions are something is either missing from your circuit or there schematic,equally that your meter touching ground causes it to go into fits slightly implies noise is getting injected, and slowing down the feedback mechanism can help with this,and just as i haven't seen it mentioned in this thread, you are locally decoupling the op amps right?

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 19, 2013, 02:10:59 pm
content: Quote from: Rerouter on December 19, 2013, 09:10:06 amYes. I have four decoupling capacitors connected to the V+ and V- terminals of the op amps, 100n each, dead bugged to the star ground, which is connected to the ground of my power supply (I am using a dual low noise (3uV rms) power supply to make a +- 15V source). The ground point is connected to the power supply by a thick 16 gauge wire. The resistors that I am using are roughly 50ppm metal film resistors. I have let the system stabilize. The non inverting amplifier feedback resistor is also connected to this star ground.The ground injection noise issue was removed once the remnants of a high pass filter circuit were cut away from the power supply rails, I think a 10nF capacitor was causing problems.The resistors were measured, ~68.5 and~ 129.5 ohms. The 10nF feedback capacitor is a film capacitor. The decoupling capacitors are 100nF ceramics (through hole).I am going to try your proposal now and give you the numbers that I get, I had to get away from this thing for a little while.I will also try to add decoupling electrolytic capacitors., and possibly tantalums.

author: Lunasix
author_rank: Regular Contributor
timestamp: December 19, 2013, 02:36:29 pm
content: I can't really help but I used, many years ago, a similar circuit with LT1007 stabilized by a a LTC1052. There are small diferencies but principal is the same. Design was from somebody else and the board was not working. I made corrections to have it working fine. There was errors in resistor's values and chopper amplifier was unable to correct offset.Try to open the offset loop to verify voltages values, and verify that inputs of LTC1052 are in a valid range.

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 19, 2013, 02:45:40 pm
content: Quote from: Lunasix on December 19, 2013, 02:36:29 pmI saw that circuit in another app note. I believe it might be in the 1052 datasheet.

author: Lunasix
author_rank: Regular Contributor
timestamp: December 19, 2013, 02:52:04 pm
content: The board was designed in 1990 and 20 to 30 boards were produced, all working fine with the right values.I will try later to scan the part of schematic.

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 19, 2013, 03:28:18 pm
content: well, hmm, i just noticed something. I was trying to make heads or tails out of the LT1028 schematic and I noticed that the internal resistors in the unit are both 130 ohms. He chose 130 ohms for one resistor (same as internal resistor) and then 68 for the other resistor.The voltage divider in the lt1028 isIs a 68 ohm resistor not throwing it out of balance? Then again I don't really know whats going on in there.

author: Lunasix
author_rank: Regular Contributor
timestamp: December 19, 2013, 03:39:42 pm
content: Have you tried to replace output of chopper amplifier with voltage generator and verified that low noise amplifier's offset react as wanted, for all equivalent chopper output swing, without any latch (I think I have seen this problem on my board when values were incorrect) ?

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 19, 2013, 04:39:55 pm
content: Quote from: Lunasix on December 19, 2013, 03:39:42 pmI will also try this. This is a good idea, I only hooked up a high precision power supply to the input of the op amp.

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 24, 2013, 04:48:12 pm
content: ah i think i have this fucking bastard licked finally.I figured to replace the 68 ohm resistor with a 630 ohm resistor (*just as an experiment). As I see it, with 68 ohms, the chopper amplifier is pretty much fighting the power supply, since only 68 ohms separate it from the 15V PSU. I also put a trimpot on the 130 ohm resistor. I then turned the circuit on, the offset was approximately 6 milivolts, I adjusted this trimpot to the max (1k ohm) and it went down to about 1 milivolt. I then plugged in my precision psu and found a certain voltage to dial in, I was able to get the amplifier offset to be like, almost dead on to 1 uV (though it jittered some because its low level measurement with air currents n other bullshit going on)I believe the datasheet meant to say a 1300 ohm and a 680 ohm. Having tiny ohm values does not make much sense to me, again, because the chopper is fighting the power supply, how could a op amp stand up to 68 ohms on a 15 volt rail? With this kind of impedance difference I believe the OPamp was trying to effect the entire 15V rail. I was clued in to this fact when I connected my precison power supply, which brought down the 15V rail to almost 0 volts as soon as it was connected, then I realized oh, you need some kind of impedance separating the two supplies.Anyway, this thing is finally behaving, it hunts though, I believe that the 100nf resistor is actually too large, leading to some kind of over shoot, but I am not sure. There is a tupperware container over this device (with about 1cm gap, highly nonideal) but the unit is pretty steady with about 5uV offset voltage, it hunts from about -10 to +10 uV now, according to my keithley 199. I believe that this is because of temperature problems due to the disgusting status of this prototype (it looks like modern art) and leakages from flux (its covered in flux again).I had enough of this shit on chirstmas but I think replacing the 100nF with a 10nF will allow it to compensate for errors faster and I suspect that a 1300 ohm is dead on, because 1kohm (the maximum of the cheap trimpot I installed on it) gets the offset to around 1mV, 300 ohms more and it might be like 200uV... I will also try to get rid of the 1kohm resistor seperating the 100nF (currently) capacitor from the chopper amplifier and I will try replacing the 15kohm that I installed with the datasheet 30kohm before I replace the 100nF with a 10nF.finally some progressI wish I realized earlier that 68 ohms does not make very much sense... unless I am missing something here........ I would love to be corrected. at least one good thing happened on this holiday.these experiment really needs a shield box, it does not even like when I move around it......

author: blackdog
author_rank: Frequent Contributor
timestamp: December 24, 2013, 06:24:49 pm
content: Hi SArepairman,If you look @ the datasheet of the LT1028 you can the schematic @ page 17.The are two resistors to make it posible to adjust the offset of the LT1028 and these are R5 and R6, both 130 Ohms.LT dit its best to make the offset as low as posible for you a user.The schematic of EDN witch is a from a aplication note of LT, is a automatic trim pot :-)The two resistors, 130 and 68 Ohm reduce the control range.The resistance of 68 Ohm creates an imbalance and is being compensated by the LTC1052 via the 30K resistor.It is pulls down current untill the difference between the two inputs of the LT1028 are the same.It is only possible to test this circuit @ uV level if you ground the +input.The 30K resistor should be enough to correct the offset of the LT1028.Measure the output level of the LTC1052, that its whitin the ouput commonmode range.If not, make the 30K resistor lower, do not go lower than 10K.Be aware of the bias current problems with Low Noise ApAmps, the LT1028 has max 180nA bias current!!!The LTC1052 is not there for fixing the bias current problems :-)I hope this helps you...Kind regarts,Blackdog

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 24, 2013, 06:37:26 pm
content: Quote from: blackdog on December 24, 2013, 06:24:49 pmI think that LT is just bloody wrong with the 68 and 130 ohm resistors, the thing won't bloody work with values this low. And yes, I have the input grounded, that is how I have been testing this thing the whole time. I think someone just assumed that it would work better with 68 and 130 ohms (when they tried it with a higher value in the lab). The 68:130 ratio is right, but it just does not work. I did nothing with my circuit but replace the resistors with higher values and it magically started working. With the 68 and 130 ohms it made no difference if I connected the output of the chopper amplifier.

author: blackdog
author_rank: Frequent Contributor
timestamp: December 24, 2013, 07:12:44 pm
content: Hi SArepairman,What is the offset voltage whitout the Chopper correction amp.Is the LT1028 in spec?Kind regarts,Blackdog

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 24, 2013, 07:35:30 pm
content: Quote from: blackdog on December 24, 2013, 07:12:44 pmyea the LT1028 is pretty much in spec, the offset is like a few milivolts with a gain of 100, I have like 6 LT1028's they all pretty much do the same thing (and I recieved them at different times, its not from the same batch). I can't tell you the specific offset without gain, but with gain its like 2-3 mV (without any offset resistors) , which makes sense given the max offset of 40uV * roughly a gain of 100. If its important to you I can make a bare bones offset test circuit, but I'd rather not bother with it now. I am using 1% tolerance resistors but I don't remember if I used a 10 and a 1000 or a 10 and a 1100.

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 25, 2013, 08:01:40 am
content: OK, I put a 10nF instead of the 100nF, I put a 10k instead of the 30k and a 1300 ohm instead of the 1kohm. The circuit works well now, it pretty much is focused around 3-8uV offset in its deadbugged and still unwashed state.I found that it does not like to work with a 30k ohm on the output, if I put a 30k there, along with 680 and 1300 ohms the offset sets around 400uV, I believe that a 30K is not letting enough current through.Do you think that better performance will be offered by increasing the 1300 and 680 ohm resistors by some multiplier in order to achieve the 30k ohm current/clock feedthrough resistor or to stick with the 1300 and 680 ohm resistor and use a 10-15K clockfeedthrough/current limiting resistor?Another theory, other the LT engineers being careless, is that the LTC1052 has more current output capability then the LTC1151 due to its design, which is why it works with 68-130 ohms.Do you guys think that maybe the 10x increased resistors on the offset pins degrades the performance of the LT1028 significantly? Perhaps I should just stick to the LTC1052, as it is a known working design. I don't really have the setup to measure the performance of this "hack" that I made..blackdog, can you be more specific as to why the resistor should not be made lower then 10k? just wondering about your reasoning here.

author: blackdog
author_rank: Frequent Contributor
timestamp: December 25, 2013, 10:39:24 am
content: Hi SArepairman, :-)The extra 130 Ohm and 68 Ohm on the outside of the LT1028 is to make the offset controle range smaller.There is a deliberate imbalance between pin 1 and 8, the LTC1151 corrected this imbalance by looking at the voltage difference between the inputs of the LT1028.Because you use 1300 and 680 Ohm, there is a smaler controle range for the opamp.If you lower the 30K to 10K you wil get a bigger controle range, but also there is a change that the chopper frequentie wil be at the output of the LT1028.You inject more and more noise out of the chopper opamp in to the LT1028 if you make the controle rage bigger.I hope if you use a LTC1151 you connected the "spare" the wright way? output to - and + to ground?OK,If you use the LTC1151, remove the zeners from it's powersupply.If you have a scope, use a probe to measure the DC level on the output of the LTC1151 (pin-7 or 1)Optimum is the scoop line near the center of the screen (bottom is say -15V and top of the screen is +15V)The scoop line wil move if you move above the circuit :-)This level of measurements @ uV levels is a bitch :-)Light, draft, moving measuring cables, mechanical stress all make you crazyYou have to screen the circuit to make good measurements.Change the 130 and 68 Ohm resistor so that the scope line stay's reasonable in the middle of the oscilloscope.Maybe you need a 130 and a 100 Ohm resistor...The trick is to find out how big the controle range must be for your circuit.If you make the rage to big than there wil be some chopper noise on the output of the LT1028.The output of the chopper opamp must not be near the powersupply voltage!Be aware that the pins 1 and 8 are verry sensitive, see them as to extra inputs of the opamp, ofcourse with a lower gain than pin 2 and 3.Ih hope this helps.Kind regarts,Blackdog

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 25, 2013, 12:11:01 pm
content: Oh, your oscilloscope visualization method is wonderful, I was using a multimeter prior. I did not realize how narrow the range for success was. I guess there are two stability points, if one output is fixed at 130 ohms then you can either make the output greater or smaller, so the op amp can either source or sink current in order to stay within bounds, like the solution of a quadratic equation.A ten turn 200 ohm trim pot cleared this issue up. I wish I realized the merit of using a scope in this situation, it would have saved me tons of fustration....Howver, there is still a problem. With this resistor change, to 130 and 116, the offset seems to center at 20uV, while, with the 1300/600/10000 configuration, the offset settled at like 4uV.How come the circuit is having troubles honing down to 0 now, I figured that it should also go down to 4, like the previous circuit.I am not sure if this offset is because I am adjusting the potentiometer in such a way that works with the oscilloscope bias current injection but then steps out of bounds when the scope is disconnected, leading to the 20uV offset. I will wire in a electrometer in voltmeter mode to measure the voltage output of the chopper and adjust the trimpot then, as it has a very small bias current, much smaller then my other multimeters.

author: blackdog
author_rank: Frequent Contributor
timestamp: December 25, 2013, 12:31:57 pm
content: Hi SArepairman, :-)Download the datasheet of the LTC2057 ( thats my Girl ;-) )Start reading page 18 to 22 just some basic info how to build at micro signal levels.Also in the datasheet of the LTC1051 from page 6.If you don't comply, you burn your signal ash.The 130 and 68 ohm resistors are to make a deliberate imbalance, and the chopper opamp compensates for this,by removing too much current from the 68 Ohm resistor via the 30K resistor, so that a balance is created.Maybe start, by connecting the 130 and 68 resistors, and then make the 30K resistor 3K or 3K3.This must give you enough controle range, but don't forget the fist four lines in this replaySucces!Blackdog

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 26, 2013, 07:41:46 am
content: I am just playing with it now, but I found out that using an electrometer to measure voltage on the chopper output is ideal. The reading is much much much more stable on the electrometer (while the multimeters cause the circuit to oscillate, at least some what).... good reason to have a nice cheapo 1970's electrometer in the lab.Out of curiosity what kind of effect does an oscilloscope hookup have for a low level circuit like this?I noticed that now even connecting a multimeter @ 10mega ohm to the output of the lt1028 causes a 10V shift (still in bounds because I zeroed the potentiometer nicely) while a >10 Gigaohm causes a 3V shift the output of the chopper (while it is ~0.3 volts output with no multimeter connected) in order to get the output offset ~5uV.what a treacherous beast! luckily I have another electrometer in the attic....i am going to add a device to the output of the LT1028 and see how things behave then....at least its getting exciting.and then I am sure it will behave totally differently when I add guard rings and dummy thermal loads on the real pcb....lmfao connecting an alligator clip to this thing causes it to go bonkers... an alligator clip not connected to anything...this is not a circuit is a circusi guess thermocouple...i think that everyone interested in electrons should try to build some kind of circuit like this at least once in their life.....

author: blackdog
author_rank: Frequent Contributor
timestamp: December 26, 2013, 09:45:59 am
content: Hi SArepairman, :-)Just some remarks...Say a measuring kabel of less dan 1M is not just a cable.Its a resistor ~ 60 mOhmIts a inductor 1 a 2 uHIts a capacitor 10 a 150pF (it is one site off a capacitor, thats wy its difficut to be precisely )Its a antennaBe always aware of this!Older types of chopper opamps don't like capacitive load's, the modern types are a lot better, but not as good as say a OP07 and its family.If you want to measure the outputpin of a chopper amp, first connect say a 1K resistor directly on the output pin and connect your multimeter on the other side.Normaly this wil help to keep the chopper opamp stable.If you use a scoop probe in the 1:10 Mode, most of the time its a ~15pF capasitor in parallel with a 10M resistor.If you use a scoop probe in the 1:1 Mode its about 1M and 100 - 200pF <- this is a NO NO Mode!!!You can youse the 1:1 mode only on verry slow circuits, say a output of an powersupply, there is already a big capacitor on the output.Your composit amplifier is NOT a slow device...Just some 2 cent of opinionKind regartsBlackdog