Author Topic: Discrete Capacitive Divider-Circuits  (Read 998 times)

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Online Echo88

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Discrete Capacitive Divider-Circuits
« on: December 11, 2018, 04:15:39 am »
Since the LTC1043 has its limitations in reality and as a model in LTSpice, i begun to simulate discrete capacitive dividers.
My discrete capacitive-divider simulations, which you can find attached, work quite well. Attachment contains /2, /3, /4 and /10-dividers. However: i cant get the newest circuit on the right side to work. Maybe someone can help?
Description:
/2, /3 and /4-Dividers are copied from the LTC1043-examples. The /10-Divider contains 10 caps, after charging and equalizing they are sequentially paralleled to the cap C13. The nonworking /10-divider should do the same more elegant, but it seems i built it wrong.
 

Offline Andreas

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Re: Discrete Capacitive Divider-Circuits
« Reply #1 on: December 11, 2018, 05:34:41 am »
The nonworking /10-divider should do the same more elegant, but it seems i built it wrong.

Its not more elegant.
The first capacitor sees 9 switches (RDS,ON) when paralleled to the last capacitor.
the pre-Last sees only one Switch.
So the time constant / settling time for each capacitor is different -> reduced accuracy or longer settling time.

I guess that the "working" problem has to do with the threshold voltages of the switches and the wiring of the clock signals.

with best regards

Andreas




 

Online Echo88

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Re: Discrete Capacitive Divider-Circuits
« Reply #2 on: December 11, 2018, 07:31:01 am »
Thanks for the hint. The settling-time approach sounds correct, however in the simulation the newer circuit has way faster settling. Maybe the simulation-comparison isnt correct, because the paralleling switches in the old circuit could run faster. Anyway: Found the problem and corrected the circuit, the 10V-input should of course be switched away when the caps are equalized. Otherwise it will disturb the process. Attached the new corrected version.
Old approach: 32switches/11caps New approach: 28 Switches/10caps  :)
 

Offline SilverSolder

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Re: Discrete Capacitive Divider-Circuits
« Reply #3 on: December 11, 2018, 02:48:01 pm »
What makes a capacitive divider better than a resistive one?
 

Offline Andreas

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Re: Discrete Capacitive Divider-Circuits
« Reply #4 on: December 11, 2018, 04:29:01 pm »
Hello,

See here a comparison of ratio stability of a DSMZ (Vishay Z-Foil) resistive divider and a LTC1043 2:1 divider over temperature:

https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg521240/#msg521240

and another here:

https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg528192/#msg528192

with best regards

Andreas
 

Online Echo88

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Re: Discrete Capacitive Divider-Circuits
« Reply #5 on: December 11, 2018, 09:04:39 pm »
I found your temperature measurements of the LTC1043 very interesting Andreas.
Did you also try to compensate the offset error of a few ten µV, which is typical for the LTC1043, for example by shielding the capacitors or varying the Vcc?
I havent shielded the caps in my LTC1043-/2-Testdivider yet, but varying the Vcc couldnt compensate the about -40µV offset which i had.
 

Offline branadic

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Re: Discrete Capacitive Divider-Circuits
« Reply #6 on: December 11, 2018, 09:17:38 pm »
Even though I'm not Andreas, you can find a picture of the divider on the web, which shows one of the caps being shielded.



-branadic-
Fluke 8050A | Prema 5000 | Prema 5017 SC | Advantest R6581D | GenRad 1434-G | Datron 4000A | Tek 2465A | VNWA2.x with TCXO upgrade and access to: Keysight 3458A, Keithley 2002, Prema 5017 SC, 34401A, 34410A, Keithley 2182A, HDO6054, Keysight 53230A and other goodies at work
 

Online Echo88

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Re: Discrete Capacitive Divider-Circuits
« Reply #7 on: December 11, 2018, 09:36:00 pm »
Yeah, question is: Can the shielding and/or varying Vcc completely compensate the offset-error?
 

Offline Andreas

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Re: Discrete Capacitive Divider-Circuits
« Reply #8 on: December 12, 2018, 12:45:07 am »
I havent shielded the caps in my LTC1043-/2-Testdivider yet, but varying the Vcc couldnt compensate the about -40µV offset which i had.

Hello,

Shielding of the capacitors has only minor effects. (In my newer designs I only have a copper area under the floating capacitor.)

40uV offset is neither the LTC1043 nor the buffer op-amp.
I guess you are having either a common conductor (star grounding) problem or a EMI-problem.

In my early designs I had problems by the used ADC (LTC2400).
The switching noise from the input of the ADC has been rectified on the output stage (ESD protection diodes?) of the buffer amplifier. 
This gave around 20-30 uV negative "offset".
With a adapted R/C low pass between Buffer and ADC you should have less than 10 uV "offset".   (typically -4 .. +4 uV)
(be carefully with the R/C to keep gain and linearity errors low see Data Sheet).

Of course the whole is also dependant on good decoupling of the LTC2400.

And the LTC1043 also needs at least 1 nF at the input.

with best regards

Andreas


 

Online Echo88

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Re: Discrete Capacitive Divider-Circuits
« Reply #9 on: December 12, 2018, 01:34:41 am »
Thanks for the detailed description. I miss such details in the LTC1043-Datasheet.
Since i use the LTC2057 as the output buffer, it may be the culprit.
Makes sense that the input current peaks of the buffer influence the rather small 1µF-caps without the RC-filter.
I will change the LTC1043/LTC2057-circuit, measure it again with my 5440B/3458A and report back.  :-+
 

Offline Andreas

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Re: Discrete Capacitive Divider-Circuits
« Reply #10 on: December 12, 2018, 02:59:41 am »
Hello,

The LTC2057 is better than the LTC1050 together with the LT1043 + LTC2400.
With the LT1043 / LTC1050 you have temperature dependant clock interferences which increase output noise.
https://www.eevblog.com/forum/metrology/ltc2508-32-bit-adc/msg1728077/#msg1728077

The LTC2057 shows much lower interference in combination.

With best regards

Andreas
 


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