LCR Meter Problem with Second Input offset adjustment

[BenjaminKe]

Hi!

I just build my LCR Meter and tried to do the first time calibration.

The first Input Offset adjustment worked without any problems.

But on the Second Input offset adjustment I always get measurements between 1.5V and 2V (the target is 0.00V).

Do you have any suggestion where the problem might be?

On TP5 I always measure 1.2mV during the adjustment.
On TP10 I have 106mV.
Difference between TP9 and TP11 after the failed adjustment is 1.35V (the AU2011 Program shows 1.535V).

Thanks
Best Regards
Benjamin

[snoopy_94]

Bonjour Benjamin,

Ce forum étant en Français, c'est pour moi plus facile de continuer dans cette langue! Ok?
Pouvez-vous me préciser la version du Firmware et celle du programme AU2011 utilisé ainsi que le langage (français?) de ce dernier.
Aussi qu'elle est la Révision du PCB principal. Si c'est la Rev3 avez vous effectué la modification matérielle décrite dans le numéro d'octobre 2013 d'Elektor (édition française)?

Cordialement,
Jean-Jacques

[BenjaminKe]

Bonjour Jean-Jacques!

Sorry my french is not so good, but I understand a little bit so no problem for me if you continue like you prefer

I have firmware version V300 and AU2011 version V300.
I also have a PCB with all the changes from October 2013.

Thanks
Best Regards
Benjamin

[snoopy_94]

Hi Benjamin,

The first input offset adjustment ("Input_Offset_U adjustment...") is with a short-circuit on the input, but other input adjustments are with open input circuit. All right?
Where is your problem? For the "Sinus_Offset_Adjustment..." or for the "Input_Offset_I adjustment..."?

Best Regards
Jean-Jacques

[BenjaminKe]

Hi Jean-Jacques,

the first one with short-circuit worked without any problems.

The second one (the one with open input) is the problem (the program only writes "Second input offset adjustment needed" so I guess this is the "Input_Offset_I adjustment").

I already tried to measure on TP5 and have stable 1.2mV
On TP10 it is 106mV (I guess this is the gain of 100 from the first OpAmp)
Between TP9 and TP11 the voltages change during the calibration but it is around 1.35V

Thanks
Best Regards
Benjamin

[snoopy_94]

Hi Benjamin,

At the first power-on after installing firmware V3.0.0. the program checks are, in order:
-The "Buffer_Offset".
-The "Input_Offset_V" if you have the 10 k resistor wired on J16-9 (not J17 as erroneously indicated in the original article). Short circuit at the input.
-The "Input_Offset_I". No cable connected.
-The "Sinust_Offset". No cable connected.

Have you performed successfully the "Buffer_Offset" adjustment?
The two settings "Input_Offset" use the DAC0 of the MCU. At the beginning of the setting, voltage on R47 (pin 17 of the MCU) is 1.25V then varies more or less to search for the correct setting.
The conditions at the "Input_Offset_I" setting are:
- Max gain for U6 (R22 = 100k selected with R_SEL_A0 and R_SEL_A1 high)
- Gain U7 max (= 100 with GAIN_A0 low and GAIN_A1 high)
- Mode "Measuring current" selected (UI_SEL / J14 high)

While adjusting, the voltage in TP5 and TP6 must change. Target (TP6) is near 0mV.

Attention to external disturbances (main...)

Have you the Display board?

Best regards,
Jean-Jacques

[BenjaminKe]

Hi Jean-Jacques,

The "Buffer_Offset" worked and was ok.
The "Input_Offset_V" also worked and was ok (I have a 4K7 resistor on J16-9 (LCD_SI) so this should work).
The "Input_Offset_I" is not working (I have no cable connected).

R_SEL_A0 and R_SEL_A1 is high so R22 should be selected correctly.
GAIN_A0 is also low and GAIN_A1 and UI_SEL are high so this should be ok.
The real difference on the input (TP3 - TP4 - which is the input of U7) is 0.24mV which should be ok I think (this needs to be calibrated to measure 0mV at the end).

So let's go to TP5 (which is the output of U5 and the input of U7):
When the LCR Meter is switched of I measure 0mV (which is normal because there is no power supply).
When I switch it on I measure 17mV.
When I start the PC program this offset jumps to -10mV (seems like mikrocontroller waits in the bootloader and jumps to the main program and loads the V offset setting).
When the PC program starts the adjusting it jumps to 3mV, then changes to 1.5mV quite fast and sticks at about 1.5mV without changing much.

On TP6 I measure 130mV (which are the 1.5mV with a gain of 100).
I guess this is too high here?

The PC program shows a target of +/- 5mV.
Where do I need to achieve this target? On TP5 or TP6?
What point should the measurements on the PC reflect (TP5 or TP6 or something else)?

Yes I also have the display board.

Thanks
Best Regards
Benjamin

[snoopy_94]

Hi Benjamin,

The compensation is done by applying a (low) voltage on pin 5 of U5.

At the start of the LCR-Meter, the "bootloader":
- Initializes the MCU ports (P1 outputs are 0, except DAC_CS/ which is to 1), so gain U7 = 1 and R18 (10 k) selected.
- UART0
DAC0 output is high impedance.

Without "Display board" and with the USB connection detected, it turns on the LED D4 and expects that the AU2011 program connects to port COMx to then launch the "firmware".

Pin U5-5 voltage (the one at the terminals of R46) is therefore approximately -1.3mV at startup (boot loader), can then evolve to approximately +1.3mV when the output of the DAC0 is +2.5V.
The target voltage of +/-5mV is between TP9 and TP11. It is the voltage displayed on the PC.
Check if the voltage in pin 17 of the MCU (DAC0) evolves between 0 to 2.5V

On TP6 I measure 130mV (which are the 1.5mV with a gain of 100).

There is something wrong, because TP6 voltage is the output of U6 and must be <= +/-8mV with no cable connected (OPA354 is with gain of 1, and it's the max voltage input offset).

Best Regards,
Jean-Jacques

[BenjaminKe]

Sorry Jean-Jacques,

I just saw I had a typo - I meant TP10 with the 130mV and NOT TP6.
The Output of U7 shows the 130mV (the 1.5mV with a gain of 100).

What's the maximum allowed offset here?
+/- 5mV?

Between TP9 and TP11 I measure around 1.5V so definetely too much I guess.

Thanks
Best Regards
Benjamin

[snoopy_94]

Hi Benjamin,

I modified the firmware V300 (-> V000) for a period of 2 seconds (instead of 500 ms) between each step of the "Input_Offset_I" function.
I hope that will help!

The voltage on TP5 is: (V_TP3 - V_TP4) * 2 + V_U5-5
The voltage on TP10 is: V_TP5 * 100
The voltage between TP9 and TP11 is V_TP10 * 8.66 * 2
Target voltage on TP5 is 0!

Best regards,
Jean-Jacques