Measuring nanoamps and below like a Ninja

[Vgkid]

Nice.
Looking forward to the updates.

[MegaVolt]

Looks like sweep error into range +10V...-10V(20V span) with programmable speed 1mV/s is less 100nV/s

What were the measurements taken in relation to?

[julian1]

Looks nice!. What is the purpose of the blue (film caps?) on the toroid? To reduce diode reverse recovery ripple on the rectifier?

[Kleinstein]

The capacitors at the transformer are a bit unconventional, but they are a good idea, filtering some of the mains and in addition providing some of the magnetization current from the secondary side too.

The usual way are capacitors (usually smaller ones) in parallel to all diodes at the rectifier. With the capacitors at the supply the shown caps are mostly equivalent.

With a low voltage they don't have to be X2 type, but it does not hurt if they are.

[Kleinstein]

There may be a tricky point with some of the pA meters: some of them want a minimum resistance for the current source and they may not be stable with too much capacitance at the input. The fault is on the side of the meter, but still a point to check and if needed add some series resistance.

[MacIntoshCZ]

I ve just seen that video and truly experienced RUSIA POWEER
Palpatine would be proud of you.

[HighVoltage]

On top - P597 100pF air capacitance standard and Micron-GLIN.

You have interesting things !

[miro123]

The capacitors at the transformer are a bit unconventional, but they are a good idea, filtering some of the mains and in addition providing some of the magnetization current from the secondary side too.

The usual way are capacitors (usually smaller ones) in parallel to all diodes at the rectifier. With the capacitors at the supply the shown caps are mostly equivalent.

With a low voltage they don't have to be X2 type, but it does not hurt if they are.

Thanks for sharing - I enjoyed to follow your post. I even tried to read the Russian text without translator.
1. Do you try to follow the datasheets e.g. LM317 chapter 9 all figures has 0,1u capacitor. Chapter 11 gives layout example
https://www.ti.com/lit/ds/symlink/lm317.pdf

2. LM750l05 is LDO - I cannot find the datasheet. Generally speaking LDO don't like low ESR caps at output. normally LDOs datasheets shows safe operation area Capacitance vs ESR vs Load current
3. X or Y caps are used at line inputs they have special safety properties.  The art of electronics fig.9.49-> Damping circuit has Rs and Cs, you hac only simple  low ESR Cap like #1

[SilverSolder]

I thought the capacitors are supposed to be the same value in a Sallen-Key section?

Edit:  I guess you can do it either way!

[David Hess]

I thought the capacitors are supposed to be the same value in a Sallen-Key section?

The capacitor values are only equal for a specific gain of the amplifier.

[Kleinstein]

Is it sure the nonlinearity is from the GLN and not from the DMMs ?
A simple test to do for this would be to repeat a test with the DMM reversed. This would at least change some of the DMM caused INL, especially the even powers.
The rather wide curve from the 6500 looks a bit like this could be an effect of the meter, maybe an effect of a limited number of digits or internal resolution limitiation.

I would expect the time base errors to be very small, so no worries there.

Another possible error source may be DA of capacitors in the filter. There is a large 10 µF MKS cap on the board. Is that doing ouput filtering (could cause trouble) or is it more reference filtering (would be OK) ?
10 µV are still quite a bit and usually more than to expect from thermal EMF,  and the loading error to an OP like OPA140. For the OP error I would be more afraid of output cross over, that may cause some 1 µV step when the polarity of the current changes, though still a relatively small error.

[KT88]

My preferred idea for today: op-amp SHE(self heating effect) and load problems of OPA 4140 which is used as R2R matrix power driver. Op-amp error may expand DAC errors.

An easy litmus test would be an additional load resistor...

[KT88]

I was thinking of a parallel load for the opamp just to increase the self heating of the opamp die...
If it changes you identified the culprit - if not the root cause is elsewhere.

[Kleinstein]

With the given INL problem, the first point would be to identify the cause of the INL.  Chances are high that this is not the clock reference or the votlage reference. A new version may be needed to fix the INL problem, but not for a better voltage or clock reference.

The candidates for the INL are more the DAC,  self heating and maybe a routing problem (e.g. coupling the variable reference current to the DAC to the signal or reference).  The exact timing when the DMM actually does the measurement could at least be a factor for the DMM6500, causing the wide band with moire like patterns.

[ramon]

Yes, that is a 1W heater. Three years ago there was someone on the action site selling 'Ultra Low Power OCXO' (150mW).

[Kleinstein]

To make full use of the DDS part one should have at least some filtering between the DDS chip and the comparator for the sine to square wave conversion. So the minimum would be some capacitor in parallel to R2. With a relatively low ouput frequency to look at a relatively simple filter can be sufficient.

I think there should be additional decoupling / filtering (e.g. a seires ferrite and / or resistor) at the supplies, espeically for the oscillator. Without a clean supply a OCXO may not help very much and chould show off jitter and FM effects.

The auxiliary clock output may want a series resistor to get an approximately 50 ohm output impedance.

Edit:
For the sine to square wave conversion I would prefer a true comparator or maybe just a schmidt trigger logic chip like LVC1G14. The OP would have a relatively slow transition.

[MegaVolt]

Colleagues advise: NC7SV04, inverter, with a resistor about 10K between the entrance and exit. Noise at -175DB / Hz.

[Kleinstein]

In the first curve with the DMM6500 there is a broad band, but it does not really look like noise, more like a moire pattern and thus more like a systematic error. I could imagine a problem with the time stamps. The keithley meters also tend to show some odd low frequency extra noise that seems to be related on how the AZ mode is done and this can also effect the time stamps. This can especially occur with higher PLC values that are averages over multiple shorter readings and thus a bit tricky with the time when the measurement is actually done.

The newer curves show some odd periodic pattern with some 0.2 V "period". This looks odd and may indicate some rounding problem with the corrections.

[Kleinstein]

I totally agree that the  I = C * dV/dt way is very nice for low currents, lets say below some 100 pA or 1 nA.

I think one would definitely need to have some low pass filtering - othewise the DAC step would result in current spikes - in the extreme case the current is more like a series of charge packets from the steps in the DAC. This may be OK for some ampmeters that measure the current with the integrator method, but could be a problem for other, especially faster meters, that can resolve the spikes or get to a nonlinear range.

In some cases one may need some series resistance anyway (some pA meters need it) and this resistor may do part of the filtering.

I have not used the capacitor way to generate small currents, but I like it as a convenient way to measure small currents (pA range). Compared to resistors in the Gohms range small capacitors (e.g. 100 pF range) are much easier to get and also quite stable and there is less noise.

[Kleinstein]

The LTC2057 may not be that happy with an input impedance of some 660 K. Its voltage / current noise is matched for some 70 K and it would thus be dominated by current noise. Chances are it can still be good enough, at least with a higher voltage.

R16 and R17 look like they could run quite hot. I don't think the Zener diodes at teh protection would need that much current and the resistors may be better a bit larger than 0603.

R19 is quite low in values: this would be quite some load (up to some 2.5 mA) to the output driver and thus quite some power due to the relatively high voltage.

I think the gain switching would interfere with the low pass filter function at the input. With gain the SK filter changes and in some cases can become unstable.

The rather high voltage could be a problem for the LT5400, as it would run rather hot. Ideally one would want higher resistance, though the 100 K version would be a bit on the high side.

[Kleinstein]

With there nominal load and normal PCB layout SMD resistors get uncompftably hot. With leaded solder there is even a chance they may melt the solder. So it is a good idea to stay well below it.  The power rating is valid if used with quite some copper or thick traces. Not sure the 1206 size when used with 8 mil traces would be OK with 100 mW. Also power from neighboring parts add up.

For R17 at the force output one could consider a polymer fuse / PTC instead of a normal resistor. This would also limit the worst case load to the OP in case of a short and reduce the chance for overheating there. I am not sure the LTC2057 is safe with a near short with a +-30 V supply.

Anyway for a precision circuit it helps to keep the overall power consumption low to keep the whole circuit cool.

The load due to R19 is laoding the LTC2057. With 15 V ouput this are 1.5 mA and thus 22.5 mW extra for the LTC2057. With a thermal resistance of some 100 K/W for a SO8 case this would be an extra 20 K temperature rise for the already warm OP due to the high supply voltage.
For best precision chopper OPs should normally be used with light loads and for higher loads one may consider an extra driver (e.g. transistors as emitter followers).

The filter with gain can be still stable in the used configuration. The change in gain would still effect the cross over characteristic. So with changing the gain the cross over frequency and Q values would change. This may not be a big problem, but one should be aware of this effect.

The input current to a chopper is mainly as current spikes up and down from the chopper action. Here asymmetry gives an average input bias and much of the energy is at high frequencies, but there are also variations in the chopper spikes that result in an
about white current noise part that also extends down to low frequencies.

The capacitors help with the chopper spikes - though they still load the output. However they help little with the lower frequency part of the current noise. Due to the low bandwidth this noise may still be acceptable. Changing the resistor values to a lower value is still a reasonably easy change if needed, though this means a higher filter cross over.

A 1 Hz cross over for the fitler may anyway be relatively low for some cases as it means that one would have to wait several seconds from the start of the slope before getting the final slope (in theory SW could speed this up).
On the other side the 766 Hz filter at the amplifier may not have much effect with an update frequency for the DAC in the 100 Hz range. It can still be useful to dampen the DAC switching spikes. I think the relevant frequencies for filtering would be about 5 Hz to 200 Hz.

[SilverSolder]

My dear friend, please don't worry about load of precision circuits... a few days ago i try use my new precision resistors as led current driver, they works fine!


 

I am beginning to like the Russian approach to precision components! 

Can a precision circuit be powered by one of those cute little nuclear reactors that were used to power lighthouses? 

[TimFox]

LED driver:  32 mA will make most LEDs bright.

[Kleinstein]

The BAV99 diode is a fast diode and thus relatively high leakage.  If lower leakage is needed one could use a BAV199, that should come in the same package. It is a slower diode and also higher forward voltage, but I think this should not be a problem.

[RandallMcRee]

The mkp4 seems to be “bad”. DA sb around 0.0002

My 10 all measure similarly.  Also my Russian K71-7 is lower ( but larger ) as you demonstrate.

Randall

Edit
My bad—Shodan said DA not D, dissipation! Ignore me!