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It has nothing to do with the feedback loop, as it's neither inside, nor parallel to the feedback loop.
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Oh, it most definitely is across the feedback loop. Read the circuit diagram very carefully, paying attention to all the common terminal symbols marked 'B'. You'll find that 'B' is connected to the output of the class B output stage (at the same place that all the other feedback connections start from at a star point), one end of that 250G resistor (via an insignificant 2.2k, where it also connects to that trim circuit) and the centre rail of the 5V bootstrap supply. The other end of the 250G resistor is attached to the amp's summing point. -
I have both Keithley 610C and 616 electrometers. I have a triaxial test cable to use with the 616. Earlier this year I built a current to voltage converter using an ADA4530-1 IC. It has switch selectable feedback resistors which included every decade between 1k and 1T ohms. It has 100 zeptoamp resolution--none of this coarse attoamp stuff!
I searched for the switch with the best insulation resistance specification. I didn't find anything with the desired figure significantly above 1T ohms. I used Johnson/Cinch banana jacks as a first trial and found them adequate for at least attoamp measurements. I used Teflon insulated wires and standard insulation and cleanliness precautions.
Attached are pictures of my ADA4530-1 A-V converter. The second one shows it with my calibration box connected (before I labeled the ADA4530-1 switch). That's the only way which I found to make extremely low current measurements without having noise contamination problems. Inside of it is a 1.5 volt N cell battery which feeds a megaohm voltage divider resistors which has a 150 uV output feeding a 1T ohm 10% accuracy resistor (it has an actual resistance of about 950G ohms). That produced the 160 aA current which I measured with 0.1 aA resolution. That was to the point where I needed to take the ADA4530-1's bias current into account.
Attached is a view of my similar measurement of 1.6 fA. No correction to the IC's bias current was made for this. Longer tests had what's shown on the attachment continue. I kept the recorded measurement short due to the precautions required to minimize noise.
While my Keithley meters can be handier to use, at other times the ADA4530-1 converter can provide better results.
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It has nothing to do with the feedback loop, as it's neither inside, nor parallel to the feedback loop.
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Oh, it most definitely is across the feedback loop. Read the circuit diagram very carefully, paying attention to all the common terminal symbols marked 'B'. You'll find that 'B' is connected to the output of the class B output stage (at the same place that all the other feedback connections start from at a star point), one end of that 250G resistor (via an insignificant 2.2k, where it also connects to that trim circuit) and the centre rail of the 5V bootstrap supply. The other end of the 250G resistor is attached to the amp's summing point.
Nope, in Ampere mode, Ground "B" is 'bootstrapped ground', not the signal output!
Signal output is created as explained on page 6-6.
It's really a bit tricky, to understand, how this is accomplished, as the whole driver circuit is intended also for volt and ohm circuit which may float +/- 210V. Carefully check the relays settings for the different modes.
In U, R modes, this amplifier output works differently, than in current and charge mode.
But we're discussing Ampere mode, anyhow.
The feedback loop, or in other words the shunt resistors, are R312, R322, R331, or R330, i.e. 100 Ohm, 100k, 100M or 100G. These create a voltage, proportional to the input current.
This amplifier output is at last created over RL, see figure 6-7 on page 6-6!
The Ground B you're referring to, is left to RL, and is signal output for Volt and Ohm mode only, like in figure 6-6. This bootstrap circuit accounts for 200TOhm input resistance in Volt mode, for all ranges up to 200V, which is very special.
R332, the 250G resistor, has absolutely nothing to do with with this current measurement / feedback loop.
You can check this, if you carefully read the bias calibration paragraph 7.4.9 on page 7-5, and following the schematic in parallel.
I have to admit, that this part of the circuit is not well explained, like the whole "Theory of Operation" chapter is very bad. No fun to read, in contrast to other (HP) manuals.
Frank -
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It has nothing to do with the feedback loop, as it's neither inside, nor parallel to the feedback loop.
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Oh, it most definitely is across the feedback loop. Read the circuit diagram very carefully, paying attention to all the common terminal symbols marked 'B'. You'll find that 'B' is connected to the output of the class B output stage (at the same place that all the other feedback connections start from at a star point), one end of that 250G resistor (via an insignificant 2.2k, where it also connects to that trim circuit) and the centre rail of the 5V bootstrap supply. The other end of the 250G resistor is attached to the amp's summing point.
Nope, in Ampere mode, Ground "B" is 'bootstrapped ground', not the signal output!
Signal output is created as explained on page 6-6.
It's really a bit tricky, to understand, how this is accomplished, as the whole driver circuit is intended also for volt and ohm circuit which may float +/- 210V. Carefully check the relays settings for the different modes.
In U, R modes, this amplifier output works differently, than in current and charge mode.
But we're discussing Ampere mode, anyhow.
The feedback loop, or in other words the shunt resistors, are R312, R322, R331, or R330, i.e. 100 Ohm, 100k, 100M or 100G. These create a voltage, proportional to the input current.
This amplifier output is at last created over RL, see figure 6-7 on page 6-6!
The Ground B you're referring to, is left to RL, and is signal output for Volt and Ohm mode only, like in figure 6-6. This bootstrap circuit accounts for 200TOhm input resistance in Volt mode, for all ranges up to 200V, which is very special.
R332, the 250G resistor, has absolutely nothing to do with with this current measurement / feedback loop.
You can check this, if you carefully read the bias calibration paragraph 7.4.9 on page 7-5, and following the schematic in parallel.
I have to admit, that this part of the circuit is not well explained, like the whole "Theory of Operation" chapter is very bad. No fun to read, in contrast to other (HP) manuals.
Frank
I think we're going to have to agree to disagree because I can see the diagram in front of me and I can quite clearly see the (unswitched) path from the output of the preamp's class B stage, back through that 250G resistor to the summing point. We can hope that it's perhaps a difference in terminology rather than understanding.
Talking of terminology, I don't think it's helpful to keep using the word 'ground' for several things that aren't - the "bootstrap ground" isn't a ground, it's just the common voltage of the preamp's bootstrapped +/- 5V supply, which is at the same potential as the preamp output. There's a time you can play fast and loose with the the terminology of grounds and common points, but that time is not in describing a circuit like this, with a handful of separate common points, none of which is ground in the proper sense.
Just to add to the confusion, the published circuit diagram has failed to label the JFET pair sources as powered by -5V(B), and the +/-5V labels on the offset trim circuit ought to say +/-5V(B). Given these, there are probably other errors lurking waiting to be discovered. -
Oh, my favourite electrometer
. I have one at work and one in my home lab (modified with an opamp in place of the input FETs) , as well as it's perfect partner in the Keithley 263 Calibrator/Source.
250G resistor is only there to trim the input current as close to zero as possible, with some thermal compensation as well. The circuit will work just fine without that resistor, with a somewhat increased input bias current.
Cheers
Alex -
Hmmm...Interesting. I never knew we made chips in the Philippines.
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Oh, by the way..Is this a triax-bnc adapter or an adapter and a feedthrough terminator at the same time?
The triax is the other way around though.
Tektronix Adapter/Terminator -
I think we're going to have to agree to disagree because I can see the diagram in front of me and I can quite clearly see the (unswitched) path from the output of the preamp's class B stage, back through that 250G resistor to the summing point. We can hope that it's perhaps a difference in terminology rather than understanding.
Talking of terminology, I don't think it's helpful to keep using the word 'ground' for several things that aren't - the "bootstrap ground" isn't a ground, it's just the common voltage of the preamp's bootstrapped +/- 5V supply, which is at the same potential as the preamp output. There's a time you can play fast and loose with the the terminology of grounds and common points, but that time is not in describing a circuit like this, with a handful of separate common points, none of which is ground in the proper sense.
Just to add to the confusion, the published circuit diagram has failed to label the JFET pair sources as powered by -5V(B), and the +/-5V labels on the offset trim circuit ought to say +/-5V(B). Given these, there are probably other errors lurking waiting to be discovered.
Sorry, that's not a case of different views or terminologies.
It took me a while to fully understand the current vs. volt circuit, but in the end, I'm right.
Alex Nikitin just confirmed my understanding about this 250GOhm resistor..
Also, think about the conversion factors for current shunts..
If you have a current of 10mA, 1mA, 100µA, 10µA, down to 1 pA , the A/D converter (which is obviously a 2V F.S. type) needs a conversion factor of 1 * 10^X, or a voltage of 1V for each of the above mentioned ranges and measurements.
If this 250GOhm would be used in the feedback loop, as a current shunt, it would give 2.5 * 10^11 as a conversion factor, or 0.25V for 1pA input, which would be amplified additionally by a factor of 10. A f.s. input current of 2pA would give 5V for the A/D, and overrange it.
That makes absolutely no sense, as it would be usable for this single range only.
Please, check if your view makes any sense under these arguments.
Frank
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Here is a somewhat simplified circuit for the voltage and current modes of operation. R7 in the current mode circuit is the feedback resistor.
Cheers
Alex
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Perfect !
If anyone is interested how works modern, low-end K6485 I attach simplified diagram of main loop.
If anyone is looking for cheaper triax -> koax adaptor, it is Pomona 5300, mouser:
http://cz.mouser.com/ProductDetail/Pomona-Electronics/5300/?qs=sGAEpiMZZMv8kklI404QlX%2fxYBqdL8C9
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chips were not made in the phillipines but assembled ( injection molded ) in the phillipines
as for that fet int to99 : that's the protection diode pair (two matched npn's used as diode).
the actual fet sits elsewhere.
And yes, if you flurp that fet, it is game over. i got the last 3 remaining new ones (new old stock) in the world >
2N59-something and yes, they are hand made , on demand only.
Last time i needed those to repaired these machines they only had 5 in stock. i bough all 5 of em. I've use two so far. They are (were, they are gone off their website now) made by a small company called Linear systems ( not linear technology ! )
an other note : if you solder in that area : it needs to be cleaned with freon and you need to wait a few hours for the surface charge to dissipate .. BEFORE you attempt calibration.
don't touch anything with pokenfingersptizen! the grease on us bipedals is deadly for this things precision. And when it is running : don't touch it either. that top board makes +120 and -120 volts for the floating amp...
During calibration : the shield must be installed. There is a small 2 pin jumper for calibration : after flicking that jumper you need to wait a few hours before attempting anything.
Another nice note : current does not really flow through a sense resistor. this machine has no burden voltage ! the machine creates an output voltage , sent through a very high ohmic resistor , into it's output. all the amplifier does is keep the balance between in and out equal to zero. so for every electron sent in to this thing , the high voltage amplifier forces an electron out ( direction doesn't matter, hence the+120 to -120v drive capability.
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chips were not made in the phillipines but assembled ( injection molded ) in the phillipines
as for that fet int to99 : that's the protection diode pair (two matched npn's used as diode).
the actual fet sits elsewhere.
And yes, if you flurp that fet, it is game over. i got the last 3 remaining new ones (new old stock) in the world > 2N59-something and yes, they are hand made , on demand only.
Last time i needed those to repaired these machines they only had 5 in stock. i bough all 5 of em. I've use two so far. They are (were, they are gone off their website now) made by a small company called Linear systems ( not linear technology ! )
an other note : if you solder in that area : it needs to be cleaned with freon and you need to wait a few hours for the surface charge to dissipate .. BEFORE you attempt calibration.
don't touch anything with pokenfingersptizen! the grease on us bipedals is deadly for this things precision. And when it is running : don't touch it either. that top board makes +120 and -120 volts for the floating amp...
During calibration : the shield must be installed. There is a small 2 pin jumper for calibration : after flicking that jumper you need to wait a few hours before attempting anything.
Another nice note : current does not really flow through a sense resistor. this machine has no burden voltage ! the machine creates an output voltage , sent through a very high ohmic resistor , into it's output. all the amplifier does is keep the balance between in and out equal to zero. so for every electron sent in to this thing , the high voltage amplifier forces an electron out ( direction doesn't matter, hence the+120 to -120v drive capability.
1) The special FET can be replaced by a cheap modern opamp, with good results.
2) The supply range for the input amp is +/-210V, so the preamp output can be up to that value (actually in Amps mode it is limited to about 20V, but in the Volts and Ohms it could be the full 200+ Volts)
Cheers
Alex -
Dave:
I spent about an hour trying to find what TG-162 is.
Some facts:- TG- is the internal Keithley designation for transistors
- These designations are universal among all Keithley products and don't change over time (with one or two exceptions)
- Some TG- designators are actually selected versions of others (e.g. TG-161 might be a selected TG-135)
- Keithley products one or two generations earlier than the 617 will list complete manufacturer details of all parts (but this generation does not)
Unfortunately, I scanned as many instrument manuals of that era as I could find, and none of them anywhere use TG-162. There is one false positive in searching which is a TG-182 that OCRs as TG-162 for some reason.
So unless some insane person is going to decap one of those, or unless someone knows one of the instrument designers, we'll never know!
Oh well, I tried.
-tg -
In case one would need a replacement for the input FETs, there is no real need for the part number. It is a low current JFETs. So look for a pair of JFETs with suitable low input currents. One might need to select some, or if the parts is not that good allow for more compensation (e.g. change the divider at the 250 G resistor).
It is a little odd they use a 10 M series resistor just for protection. So don't expect really low voltage noise. I would have more expected 2 or 3 lower value resistors in series, to make sure not to exceed the voltage limits. -
like i said : it is a hand-selected matching pair 2N5912 from Linear systems.
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Sorry, that's not a case of different views or terminologies.
It took me a while to fully understand the current vs. volt circuit, but in the end, I'm right.
Alex Nikitin just confirmed my understanding about this 250GOhm resistor..
And when I look at Alex's diagram I see exactly what I described. I see a 250G resistor, from the output of the preamp feeding back to the input of the preamp, in every circuit configuration. You appear to see something else. Those two views appear to me to be irreconcilable as I'm just describing what I see, literally the physical topology of the circuit.
That's why I suggested we agree to disagree, but you seem to be determined to insist that someone's 'right' and someone's 'wrong' and refuse to accept that we are probably both essentially saying the same thing but misunderstand what the other is saying. I really do see no point in me repeating variants of my description ad-infinitum in the hopes that you'll 'get' what I'm saying; that generates no enlightenment for anybody and I do not enjoy mud wrestling*. Hence this is my last word on the subject.
* Anybody who doesn't get this, just Google "engineer argue pig mud". -
chips were not made in the phillipines but assembled ( injection molded ) in the phillipines
I think his point was that everybody else spells it "Philippines"... -
The 250 GOhms resistor is in a kind of feedback path. However up to the point from where the 250 G resistor feeds back to the input the voltage gain is very close to 1 (I would estimate something like 0.99999). So the 250 G resistor is not very effective in setting the input impedance or the amplifiers gain. So despite of the resistor, the input impedance can be significant larger than 250 G Ohms. The propose of the resistor is to compensate for a possible small bias current of the FETs and protection diodes.
The 2 pA (FS) range has a separate resistor for the feedback. -
Analog Devices has the ADA4530-1 Electrometer OpAmp
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And when I look at Alex's diagram I see exactly what I described. I see a 250G resistor, from the output of the preamp feeding back to the input of the preamp, in every circuit configuration. You appear to see something else. Those two views appear to me to be irreconcilable as I'm just describing what I see, literally the physical topology of the circuit.
That's why I suggested we agree to disagree, but you seem to be determined to insist that someone's 'right' and someone's 'wrong' and refuse to accept that we are probably both essentially saying the same thing but misunderstand what the other is saying. I really do see no point in me repeating variants of my description ad-infinitum in the hopes that you'll 'get' what I'm saying; that generates no enlightenment for anybody and I do not enjoy mud wrestling*. Hence this is my last word on the subject.
* Anybody who doesn't get this, just Google "engineer argue pig mud".
I have to point out that this resistor is connected from the output to the non-inverting input, hence it only can be considered as a positive feedback. Fortunately, it is connected with the feedback coefficient just under 1, so no oscillations, and it is used to provide a DC bias to the input while following the output exactly, so the effective value of this resistor referred to the input is hundreds of Teraohms!
Cheers
Alex -
If anyone is looking for cheaper triax -> koax adaptor, it is Pomona 5300, mouser:
Be careful using adapters that short the inner and outer shield with equipment that uses a driven guard.
http://cz.mouser.com/ProductDetail/Pomona-Electronics/5300/?qs=sGAEpiMZZMv8kklI404QlX%2fxYBqdL8C9 -
Definitely. One must exactly know what he is doing.
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If anyone is looking for cheaper triax -> koax adaptor, it is Pomona 5300, mouser:
Be careful using adapters that short the inner and outer shield with equipment that uses a driven guard.
http://cz.mouser.com/ProductDetail/Pomona-Electronics/5300/?qs=sGAEpiMZZMv8kklI404QlX%2fxYBqdL8C9
In the Keithley 617 the guard output is driven through 150K resistor and it is relatively safe to short it to the case ground (that happens if you try to engage the guard with the adapter) . The problem could be if you connect anything but the case ground to the "COM" terminal with this adapter in place. There is a fuse in line with the COM terminal but it is better be careful in that respect. You need to remember that with the adapter you lose the isolated analogue common and measure anything in relation to the case ( = mains) ground. If the shorting link is connected externally anyway (as shown in Dave's video) it doesn't matter.
Cheers
Alex -
I prefer adapters that either connect outer shield to BNC shield and leave the inner shield (often guard) floating, or connect inner shield to BNC shield and leave the outer shield floating. The former is good for higher current measurements when leakage current is not an issue. The second is good if the inner shield is close to ground or if the leads are inside an interlocked case.
I do not have part numbers handy, but Trompeter makes them, and Keithley also sells them at probably a substantial mark up. -
I built a 617 into a cyclotron mass spectrometer system in the 1980's. The electrometer was used to measure ion beam current injected into a superconducting magnet. I started out with a 614 and upgraded to the 617 because GPIB current read-out and range changing were needed to implement auto tuning for the ion optics.
Memories of the brown boxes.... The system also used a Keithley switching chassis.
Jon