I welcome the bombardment.
The small feedback caps are silver mica. I chose those because they should be good at RF frequencies. To the best of my knowledge, they are also low leakage, but I can easily remove them and check the offset.
Yes, the input resistor is truly 100k. Checked many times. I will check the soldering again.
I have been pretty careful about flux removal, but will check again.
Jac
Now You are exagerating.. 
)
Just trying to help.
And no, I'm not 'strongly' preferring the ADA.. right now I had been listening to the OPA627 - equipped FE modules..
And revisiting my past judgement. Yes the ADA4627 is better at the end but it isn't a bad choice at all, the good old 627..
)Just trying to help.
And no, I'm not 'strongly' preferring the ADA.. right now I had been listening to the OPA627 - equipped FE modules..
And revisiting my past judgement. Yes the ADA4627 is better at the end but it isn't a bad choice at all, the good old 627..
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Thinking over again: could you try to short not only the input, but also C13.. only for these tests.
If the offset goes away, that means that definitely pin2 is seeing some extra current leakage.
If not, zhan it could be: current into the other input;
Defctive input offset voltage - - that is, ESD problem..
And many other things which do not come in mind.
Like the real cause..l
If the offset goes away, that means that definitely pin2 is seeing some extra current leakage.
If not, zhan it could be: current into the other input;
Defctive input offset voltage - - that is, ESD problem..
And many other things which do not come in mind.
Like the real cause..
l
Condition;
input pins - shorted
C9 jumper
C13 jumper
DC offset L/R = 9/42 mV
some drift over time. Left ranges from 6 to 10mV, Right ranges from 41 to 44 mV.
Much lower than 265/1360 mV
You said that it means pin 2 is seeing some current leakage. The question is from where. With C13 shorted, pin 2 should be seeing 0 V. My thought is that the source is either from another pin externally, for example opamp power, or somehow we have leakage current in the feedback loop and the DC is created by R11 to ground. Also, might be internal, but I can't imagine how to check.
Jac
input pins - shorted
C9 jumper
C13 jumper
DC offset L/R = 9/42 mV
some drift over time. Left ranges from 6 to 10mV, Right ranges from 41 to 44 mV.
Much lower than 265/1360 mV
You said that it means pin 2 is seeing some current leakage. The question is from where. With C13 shorted, pin 2 should be seeing 0 V. My thought is that the source is either from another pin externally, for example opamp power, or somehow we have leakage current in the feedback loop and the DC is created by R11 to ground. Also, might be internal, but I can't imagine how to check.
Jac
All the resistors are soldered in. Two caps are on faston tabs, C9 and C13. Everything else is soldered close to the board, clean, and with good electrical connection. In fact, I went back to compare my old boards and they are not as clean as the new boards.
A few new experiments. An opamp's mission is to make the '+' and '-' inputs equal. So by measuring voltage drop on resistors in the loop, I should get an idea of currents and if the opamp is working.
First, I measured with input, C13, and C9 shorted. The current going through R10 and R12 was an order of magnitude different, R10 higher, so I realized that some current was escaping to ground through the input short. Here are some numbers anyway.
R11 - 2.3 uV
R12 - 0.98 mV
R10 - 33 mV
I removed the input short, keeping C13 and C9 shorted. Of course, with input open, C13 short doesn't matter, but it doesn't hurt anything. Here are the VDC/amps for this configuration.
Output - 1.1V
R11 - 0.1 mV or 100 uA
R12 - 0.9 mV or 0.3 uA
R10 - 27 mV or 69 uA Hmmmnnn....
opamp pin3 to pin6 - 1.1V (measured across the open holes of C10)
pin2 to pin3 - 0V
pin2 to pin6 - 42 mV and rising
First of all, current in R10 should be equal to current in R11 + R12. It might be a measurement problem, but it doesn't look right. The input impedance of my multimeter in the uV to mV range is quoted as >10G Ohms.
0V between opamp inputs is right, how/why are the pin 2-6 different from pin 3-6?
Jac
Jac.. first of all, it seams that You are using a benchtop multimeter for these tests. I would be a bit afraid of doing so - they are usually not floating input.
Though it can be well used for ground-referenced test points. But tests like measuring across not ground referenced resistors could give strange results.
So for this I can not really trust the test on the/ accross the opamp pins.
But you constantly measure again and again dozens of mV -s across R10, which is ground-referenced so should be ok. Not the mV -s though.. it should be practically zero dropping accross R10... the other pin, pin 2 is seeing ground, so pin 3 should seat close to ground..
Something is fishy.
Though it can be well used for ground-referenced test points. But tests like measuring across not ground referenced resistors could give strange results.
So for this I can not really trust the test on the/ accross the opamp pins.
But you constantly measure again and again dozens of mV -s across R10, which is ground-referenced so should be ok. Not the mV -s though.. it should be practically zero dropping accross R10... the other pin, pin 2 is seeing ground, so pin 3 should seat close to ground..
Something is fishy.
Thanks. I have earlier, but not since I sorted the power supply issue. May be worth another look.
Fishy, I agree.
The meter is a Keithley 2015P. I couldn't find out if the meter is floating or ground referenced from the owner manual, but I think it is floating because it also tries to do distortion measurements which I think are best floating. Let me put it this way, the leads connect to "High" and "Low", not GND. I can compare a few numbers with my portable.
It funny because I trust the bench meter more than my two battery powered portable meters. I have noticed some drift in the portables after doing a number of measurements.
At lunch, I was thinking about my numbers from this morning and decided that I must have a mistake there and remeasured in more detail. I am working on the 'right' module because it's the worst offset. Condition is input = open, C9 and C13 = short
To start, I rechecked the power supplies +/- 31.5V and +/- 14.5V for the regulated voltage.
Output terminals - 1.1VDC - relay stays on
pin 3 to pin 6 measured at C10 - 1.1 VDC - relay turns off
pin 2 to pin 6 measured at R12 and C10 - 5.2 mVDC relay on
pin 2 to pin3 measured at R12 and C10 - 3 uV relay on
Output + to pin 3 (@C10) - 1.1 VDC relay on
Output + to pin 6 (@C10) - 1.1 VDC relay on
across R13 (@ R12 and C9) - 9.5 mV
across R11 directly - 0.12 mV
across C32 (the same as R7) - 1.1 VDC
across R10 directly - 37.5 mV
The reason for the difference in the pin 3 to pin 6 and the pin 2 to pin 6 is because the loading of the meter causes the relay to turn off. At least, that's my theory.
I am surprised that pin 2 to pin 6 is only 5 mV. I expected all of the voltage gain to have happened here. Is this right?
9.5 mV across R13 (100k) is a very small current as expected from an FET opamp input.
C32 which is also R7 is showing the output voltage or very close to it. Looking at R10 and R7, the voltage gain for the feedback seems correct.
R11 is running 0.12 mV or the same mA to ground. I am guessing that this is the current that is creating the DC offset, but I haven't figured out where the current is coming from.
My current thought is to check the diode bridge to see if it is installed correctly or leaking current. Or maybe something in the LM3886?
I too am guessing.
Jac
At George's suggestion, I dug out my old Fluke 75. Not a lot of precision, but it has seemed more repeatable than my other portable. I will list voltages as desk top/Fluke.
Output terminals - 1.1/1.2V
pin 3 to pin 6 - 1.1/1.1V
pin 2 to pin 6 - 5.2 mV/30 mV
pin 2 to pin 3 - 3 uV/0
Output to pin 3 - 1.1/1.2V
Output to pin 6 - 1.1/1.2V
R13 - 9.2/35 mV
R11 - 0.12/0.1 mV
R10 - 37.8/35.4 mV
Naturally, the amp is changing temperature over time, so some of these are just temp related drift.
Jac
Output terminals - 1.1/1.2V
pin 3 to pin 6 - 1.1/1.1V
pin 2 to pin 6 - 5.2 mV/30 mV
pin 2 to pin 3 - 3 uV/0
Output to pin 3 - 1.1/1.2V
Output to pin 6 - 1.1/1.2V
R13 - 9.2/35 mV
R11 - 0.12/0.1 mV
R10 - 37.8/35.4 mV
Naturally, the amp is changing temperature over time, so some of these are just temp related drift.
Jac
Jack,
In this latest test the 'fishy' part is the drop on R13(100k), for me.
9.4mV =~10mV.
10mV drop on 100kohm >> is caused by 100nA current flow.
It looks like if 100nA bias current is flowing. Typical modern BJT input opamp value.
The datasheet for the ADA4627 declares 1pA typical.
100000 less than measured...
And yes, in this particular test the drop on R10 instead is quite in agreement with the measured output.
It was not in agreement in your 'shorted input ' + C9 +C13 test..
In this latest test the 'fishy' part is the drop on R13(100k), for me.
9.4mV =~10mV.
10mV drop on 100kohm >> is caused by 100nA current flow.
It looks like if 100nA bias current is flowing. Typical modern BJT input opamp value.
The datasheet for the ADA4627 declares 1pA typical.
100000 less than measured...
And yes, in this particular test the drop on R10 instead is quite in agreement with the measured output.
It was not in agreement in your 'shorted input ' + C9 +C13 test..
for the comp cap, are you referring to the pin 2 to 6 cap or a different one?
I might as well try removing the pin 2- 6 cap and remeasure.
Jac