26.Ck Compensation Part Three

Compensation for the DAC bipolar output configuration is more difficult than for unipolar.

Description how to get a flat response anyway.

Again I did many experiments for some hours to find the right parts and another compensation method, after these experiments I was sure that I'am able to compensate every tested opamp for a flat response under bipolar and unipolar configuration. 

Mounting the air-capacitor on a copper PCB. The small carrier epoxy PCB made with small dimensions. Capacitor body has rotor potential.

Removed copper on the epoxy holder.

Copper grounded (white wire) .

Amplifier xxxx with DAC xxxx for a 0V to -10V Full Scale Step Unfortunately the response is no more flat, remember before the capacitor was hanging on air wires only and the response was more flat.

Touching the epoxy is a sensitive part of the capacitor.
Touching adjusts the response, also differs if the other hand touchs ground or not.

Still busy finding the right compensation

Another air-capacitor holder

Capacitor with plastic holders.

Copper box

Second Air Capacitor Compensation 

Bipolar Output Compensation required in my assembly a second capacitor for a very flat response. This is a full metal, new old stock VALVO capacitor in highest air capacitor quality. Made for the German military, had the typical long part numbers on the packaging. 

Ball bearings, rotor turns very soft and easy. It's a wonderful air capacitor.

Assembling the Main Compensation Capacitor

Right side - extension will carry the capacitor, box remains removeable by screws.

Bottom view - space for a future electronic?

Capacitor mounted with srews - (my first srews on this project).

First measurements with the double capacitor compensation

Precision Air Capacitor for an extra compensation.

Measurement Action - today using the 7D20 digital oscilloscope Plug-In with an averaging capability. Left there are two 500 series Plug-In's in a power supply compartment, it's a TG501 Time Mark Generator and a PG506 Calibration Generator, I used them before to calibrate the 7D20 horizontal and vertical gain in the used 7904 mainframe.

Amplifier xxxx with DAC xxxx for a 0V to -10 Volt Full Scale Step. Note, the oscilloscope vertical gain set for 5mV/DIV (similar to 125µV/DIV at the Settle-Node and 250µV/DIV at the OP Output).

1 LSB = 152µV
Measurement Bandwidth approx. 10 to 15 MHz.
Displayed Noise approx. 350µV peak
Lower Trace, time corrected LDAC signal (2V/DIV)

Amplifier xxxx with DAC xxxx for a 0V to -10 Volt Full Scale Step.

Same measurement as above, but the 7D20 Plug-In operates in it's averaging mode - 256 samples averaged

250µV/DIV (Op Output) - 152µV (1LSB)

Averaging reduce unwanted nonperiodic noise signals, only periodical signals remaining.

The double capacitor compensation is very flat adjusted, it can be said after 2.6µs the DAC reaches a stable tolerance band of 1/5 Division (50µV).

2.6µs Settling Time to 5ppm (approx. 18 Bit) for an aperiodic adjusted falling 0V to -10V full scale step.

Such a flat precision compensation takes about 15 minutes adjustment.

Setting the measurement for a 0V to +10V Full Scale Step

Positive going step.

Same measurement like above.

2.6µs Settling Time to 5ppm (approx. 18 Bit) for an aperiodic adjusted rising 0V to +10V full scale step.

Setting the measurement for a -10V to +10V Full Scale Step

5mV/DIV corrosponds to 250µV/DIV on the DAC output.

1 LSB = 305µV  ===>   1/5 Division = 50µV = 2.5ppm

2.9µs Settling Time to 2.5 ppm (approx. 19 Bit) for an aperiodic adjusted rising -10V to +10V full scale step.

Setting the measurement for a +10V to -10V step

Again changing the step direction while leaving the compensation unchanged.

If you think in the 5mV/DIV the small overshooting looked dramatically, have a look in the 20mV/DIV view without averaging - here it looks almost flat - remember the 20mV/DIV setting was OK for an 16 Bit adjustment and we are talking about an 16 Bit device and not about an 18 Bit device, anyway we will recompensate for the 18 Bit now:

Recompensate for a flat +10V to -10Volt Full Scale Step.

2.6µs Settling Time to 2.5 ppm (approx. 19 Bit) for an aperiodic adjusted falling +10V to -10V full scale step.

The only thing sombody could complain, there is a small 20µV ringing between division 3 and 5 (approximately 1.25 MHz). I will do a future trial to remove it.

Conclusion for these first measurements

Now I understand how to compensate.

Assembling the Special Compensation

Your eyes don't lie what you see next

In many of my scope photos the compensation is always flat, that's because I give my best for a good adjustment. When seeing always a flat compensation may be the reader guess "it is easy to do flat compensation" - no it's not - I want to show in one example how critical the adjustment for a flat compensation really is. After some hours of doing experiments, this work changed to a "compensation-sport".

Sorry text not for puplic.

The xxxxxxxxxxxxxx allowed an excellent flat adjustment of a +10V to -10V step using xxxxxxxxxxxxxxx.

Measuring the Capacitance

Air Capacitor measured with a Rohde & Schwarz KARU bridge.

Closed package - scale written with a pencil and painted with clear varnish.

Second Air Capacitor - Capacitance in Picofarad.

Measurement results with extreme averaging

Using a LeCroy 9304A Quad 200 MHz Oscilloscope in it's math averaging mode under a limited 30 MHz bandwidth. The 9304A can do near infinite averaging, the old 7D20 plug-in reaches the limit after 256 sweeps.

The following compensation is very flat and took me almost one half an hour to adjust. Such small voltages buried within the noise getting hard to observe. Every little change on the compensation take time to wait for at least 50 to 100 samples to see a new change. Also the offset bias needs many times a readjustment.

Congratulation Circuit Works !

  • Averaged over 2049 sweeps
  • Load DAC triggered on first Division
  • Oscilloscope vertical resolution with Math-Zoom function expanded from 5mV/DIV to 1mV/DIV
  • Deflection divided by a residue amplifier gain of 40 results in a deflection of 25µV/DIV at the settle-node and 50µV/DIV on the DAC-output.
  • 1 LSB (15ppm) for a 20V range = 305µV.

20µV = 1ppm = 20 Bit Resolution
10µV = 0.5ppm = 21 Bit Resolution
Don't start discussion if the result reaches 20 or 21 Bit, it doesn't matter.

The oscilloscope amplifier is not overdriven under the used 5mV/DIV vertical amplifier gain (additional expanded by internal math to 1mV/Div), because the vertical amplifier sees 5mV/Div. Measurements of such small levels are many times questionable. I can tell you, reaching a flat response under a high gain, it's really a hard job to do. Again, it took me 30 minutes playing with the compensation settings.

Try to reach the same results using the "paper development" methods nowadays - haha.

Same measurement as the last one, averaging increased to 9938 sweeps - no further improvement compared to 2049 sweeps. Increasing the average makes no further sense, the averaging stability limit has already reached. 

Rerun the measurement with a Math-Zoom function expanded to 0.5mV/DIV and 7854 sweeps.
DAC settles in 3µs within the 20µV tolerance band.

Results without internal scope math vertical expansion

Aperiodic settled waveform, measurement with a 5mV/DIV deflection factor, corrosponding to 250µV/DIV on the DAC output.
  • Averaged 2072 sweeps
  • Capacitive Diode Bridge blow-by distortion (at Division 1 to 2)
  • Window ON (at Division 2.3)
  • Settling Time for a +/-1LSB tolerance band 2.5µs (at Division 2.5)
  • Window OFF (at Division 9.6)
  • No overdriven oscilloscope !

Main Compensation Capacitor

Ball Bearing with a 6mm shaft, fits exactly in a epoxy PCB.

Ball bearing glued with 2K and lubrificated with Molykote grease.

Ball bearing adjustment-wheel, transmission with a twisted yarn, old radio style.

A thick strong yarn is ideal for a sensitive adjustment in both directions with less drive-mechanism-play - 3:1 transmission ratio.

Soldering a copper wire on the brass shaft.

Screwed capacitor package.

Upper Scale 17 pF to 155 pF
Bottom Scale 14.2 pF to 19 pF

Now the instrument looks very different compared to the beginning - that's a brute force development method what I am doing here in this project.

Keep your mind free ----- Do only what's technically necessary ----- Don't care for the technical rules of people who can't help you in the project.  -----

With the development methods common used nowadys you would measure the Settling Time with luck for a 14 Bit accuracy but never succesfully for up to 20 Bit vertical resolution.

Compensation Schematic

Sorry not for puplic


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