LCR Meter Problem with Second Input offset adjustment

Re: LCR Meter Problem with Second Input offset adjustment

Postby BenjaminKe » Fri Mar 25, 2016 5:30 pm

Hi Jean-Jacques,

I tried this firmware, but the results are the same - but it helps a little bit to see the steps.

1 big question:
Where does these +/- 5mV limit (which is shown on the PC program) count?
Since this is from the measured value I guess it's between TP9 and TP11 right?
so the maximum allowed offset on TP10 is around +/-0,288mV.
and thus the maximum allowed offset on TP5 is around +/-3uV right?

Currently the "best" offset after calibration I get on TP5 is 1.4mV.
on TP10 I have 130mV and between TP9 and TP11 I have around 2V.
So it looks like this OPV chain is ok but I guess this 1.4mV offset is not good enough correct?

So I have to look for the problem on the INPUT_TRIM of U5?

Befor starting the calibration my offset on TP5 is -10mV.
When it start adjusting the steps are jumping first to 3,8mV then to 2,6mV, 2mV and finally 1,4mV.
Then the offset is not chaning anymore although the adjusting is still running.

Thanks
Best Regards
Benjamin
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Re: LCR Meter Problem with Second Input offset adjustment

Postby snoopy_94 » Sat Mar 26, 2016 11:40 am

Hi Benjamin,
Where does these +/- 5mV limit (which is shown on the PC program) count?
Since this is from the measured value I guess it's between TP9 and TP11 right?
Yes!
so the maximum allowed offset on TP10 is around +/-0,288mV.
and thus the maximum allowed offset on TP5 is around +/-3uV right?
Yes and No, because it is also necessary to take into account the offset of U7 (+/-100uV typ. G = 100)
So it looks like this OPV chain is ok but I guess this 1.4mV offset is not good enough correct?
Yes

Look at the DAC0 output of MCU and its evolution during adjustment.
If the voltage is 0 or 2.5V try to change the value of R46 (15 ohms instead of 10). (If this is good, need to redo the setting "Input_Offset_V").

Best Regards,
Jean-Jacques
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Posts: 90
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Re: LCR Meter Problem with Second Input offset adjustment

Postby BenjaminKe » Fri Apr 01, 2016 2:43 pm

Hi Jean-Jacques,

Thanks for all the hints - I also think there is a problem with the DAC0 adjustment.

When I measure the voltage on the DAC0 pin of the microcontroller the voltage jumps to 1.2V and than slowly changes to almost 2.5V.
So this looks ok.

But I found 1 strange thing on my PCB:
When I tried to measure R45 (the 39K resistor) I measured 200R.
So I tried to remove the resistor and it had 39K.
With the resistor removed I also measured 200R between the 2 pins where R45 should be.
I guess this is not normal?
Or is this a normal influence of the U5 offset input pin?

Best Regards
Benjamin
BenjaminKe
 
Posts: 12
Joined: Sat Mar 19, 2016 1:33 pm

Re: LCR Meter Problem with Second Input offset adjustment

Postby snoopy_94 » Fri Apr 01, 2016 4:13 pm

Hi Benjamin,
It is normal, because on one side there are R46 (10 ohms to ground) and on the other (through L1) the diode D7 (cathode grounded) of - 5V PS. The measured value is depending on the polarity of the DVM (ohm-meter mode).

The "Input_Offset_I" is made in the output of U5 : pin 5 voltage is added to that of the input multiplied by the gain of U5 ( x 2).
Did the problem on output of U5?
- Connect TP5 to ground and short-circuit R46. Without launching the procedure, measure the tensions in TP10 then between TP9 and TP11.

Best Regards,
Jean-Jacques
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Posts: 90
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Re: LCR Meter Problem with Second Input offset adjustment

Postby BenjaminKe » Tue Apr 05, 2016 5:38 pm

Hi Jean-Jacques,

When I short R46 and connect TP5 to GND I measure around 100uV on TP10 and around 400uV between TP9 and TP11 (but I don't know if my multimeter is really accurate here with such a small voltage).

I tried now also to short R46 and removed the short on TP5.
When I start now the calibration (which is looping around nowhere of course) I measure around 2.2mV on TP5, 240mV on TP10 and around 2.45V between TP9 and TP11.
So this looks all ok.

When I look at my DAC output it looks a little bit strange - the steps are:
2.43V, 1.21V, 0.6V, 2.2mV - and here it is stuck - it's not changing away from the 2.2mV anymore.

I removed now also R47 to test only the voltage divider.
With R46 shorted I have a offset on TP5 of around 2.2mV.
With R46 normal 10R I have a offset on TP5 of around 1.1mV.
I tried to change R46 to 22R and got a offset on TP5 of around 0.5mV.
With this setting my calibration already goes down to 280mV - so much better than hanging around at around 2.4V.
Would it be better to increase R46 to let's say 27R or something or would it be better to make R45 smaller?

EDIT:
with R46 = 27R my offset adjustment works.

Thanks
Best Regards
Benjamin
BenjaminKe
 
Posts: 12
Joined: Sat Mar 19, 2016 1:33 pm

Re: LCR Meter Problem with Second Input offset adjustment

Postby snoopy_94 » Wed Apr 06, 2016 10:17 am

Hi Benjamin,
When I short R46 and connect TP5 to GND I measure around 100uV on TP10 and around 400uV between TP9 and TP11 (but I don't know if my multimeter is really accurate here with such a small voltage).
It's ok for this part of the circuit.
I tried now also to short R46 and removed the short on TP5.
When I start now the calibration (which is looping around nowhere of course) I measure around 2.2mV on TP5, 240mV on TP10 and around 2.45V between TP9 and TP11.
So this looks all ok.
Yes, but it is the value on TP5 (2.2mV) that is too high, which explains the problem. Indeed, as I said in a previous message, the R46 offset voltage is maximum of +/-1.3mV.
Putting a resistor of 27 ohm (R46) increased by 2.7 times the voltage compensation and allows the correct setting.

Stupid question: is resistance R34 removed?

If yes, this is your copy of U6 (OPA354) that comes out of the usual, with an offset voltage too hight, even if it remains in the specification. Put 27 ohms for R46 is virtually unimportant (it is the common-mode rejection which is degraded, but as an input of U5 has the potential of GND, not matter.). You can also change R45 and R47; the best is: R46 = 15 ohms, R45 = 22k and R47 = 5,6k.

After completion of all offset, don't forget to perform again "Input_Offset_V", "Input_Offset_I" and "Sinus_Offset".

Best Regards,
Jean-Jacques
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Posts: 90
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Re: LCR Meter Problem with Second Input offset adjustment

Postby BenjaminKe » Sat Apr 30, 2016 4:29 pm

Hi Jean-Jacques,

First of all: thanks for all your help - sorry for the quite late reply but I finally had some time to try around much more detailed.

I have modified R46 and the offset adjustment worked.

I tried around and measured some resistors and it seems to be also ok.

But below the field with the results I see in red "Short Circuit Trim" and "Open Circuit Trim".
So I click the "Trim" button to calibrate this but it failed (100Hz can be seen, then 1kHz and then "failure").
I tried also to do measurements with 100Hz, 1kHz and 10kHz.
100Hz and 10kHz works but when I select 1kHz as frequency nothing happens.
I guess there is a problem with the 1kHz and that's also why the trim fails.
Do you have an idea?

EDIT:
I checked also the serial port.
With 100Hz and 10kHz I get measurements.
With 1kHz I only get the message: "OVERFLOW".

Thanks
Best Regards
Benjamin
BenjaminKe
 
Posts: 12
Joined: Sat Mar 19, 2016 1:33 pm

Re: LCR Meter Problem with Second Input offset adjustment

Postby snoopy_94 » Sat Apr 30, 2016 4:55 pm

Hi Benjamin,

I shall not be at my home before next Monday, and cannot thus help you before.
Have you an oscilloscope?

Best Regards,
Jean-Jacques
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Posts: 90
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Re: LCR Meter Problem with Second Input offset adjustment

Postby BenjaminKe » Sat Apr 30, 2016 5:03 pm

Hi Jean-Jacques,

no problem.

Yes I have an oscilloscope.

Best Regards
Benjamin
BenjaminKe
 
Posts: 12
Joined: Sat Mar 19, 2016 1:33 pm

Re: LCR Meter Problem with Second Input offset adjustment

Postby snoopy_94 » Mon May 02, 2016 1:29 pm

Hi Benjamin,

Ok, I'm at home and can make measures with my oscilloscope! Please check the following voltages:

On pin 2 of U2 (MAX7404) - filter input : rectangular, about 0.4V / 2.1V (100Hz, 1kHz, 10kHz).
On pin 8 of U2 (MAX7404) - filter clock : rectangular, about 0V / 3V (10kHz, 100kHz, 1MHz).
On TP2 - filter output : sinusoïdal voltage (with offset of about 1.5V) --> 2.7Vpp (100Hz, 1kHz, 10kHz).

On pin 1 of U20 (TLC2274) - output of 11kHz Low Pass Filter
Freq. 100Hz - sinusoïdal voltage (with offset of about 1.5V) --> 2.7Vpp
Freq. 1kHz - sinusoïdal voltage (with offset of about 1.5V) --> 2.7Vpp
Freq. 10kHz - sinusoïdal voltage (with offset of about 1.5V) --> 2.7Vpp

On pin 14 of U20 (TLC2274) - output of 1.1kHz Low Pass Filter
Freq. 100Hz - sinusoïdal voltage (with offset of about 1.5V) --> 2.7Vpp
Freq. 1kHz - sinusoïdal voltage (with offset of about 1.5V) --> 2.7Vpp
Freq. 10kHz - DC about 1.5V

On TP1: sinusoïdal voltage of about 1.2Vpp (100Hz, 1kHz, 10kHz).

Best Regards,
Jean-Jacques
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Posts: 90
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