[volt-nuts] Fw: Some questions to zeners (1N823-1N829)

[Andreas Jahn]

Unfortunately the original message has been filtered out by the large 
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So sorry for the bad quality of the reszized picture.

----- Original Message ----- 

Hello Mickle,

ADC #13 is just my "serial number" of ADCs
Up to now all (#1-#14) are based on the 24 bit sigma delta LTC2400.

On the photo the left pcb has:
lower left photocoupler to isolate from PC (RS232 either direct or via USB
serial cable).
upper left PIC12F675 for reading ADC  + storage of all calibration constants
like
nominal reference voltage, 3rd order temperature correction,
2nd order linearity correction, serial number etc.
upper right: AD586LQ. Note that only Pin 4 is soldered directly to the pcb.
Other pins with Vero wire to remove PCB stress (humidity) from the
reference.
The orange part at the AGND pin is the temperature NTC.
lower right: LTC2400.

Foreground: voltage stabilisation with low noise LT1763 to 14V (minimum
needed for AD586)

right PCB:
Capacitive 2:1 voltage divider with LTC1043 + LT1050 buffer.
The noise from ADC input is isolated from LT1050 buffer
by a  R/C low pass. Otherwise the switching from ADC input will
increase noise level well above the datasheet spec.

When measuring all (ADCs + other references) is mounted on a large metal
ground plane.
Clothes over the ADCs + connections keep air currents away.

Since the noise level of LTC2400 is rather high, I have to use large
integration times
(averaging several hundred measurements over 1 or 5 minutes) to reduce
the noise from a single measurement of 10uVpp (0.3ppm RMS in the datasheet)
to below 1uVpp.

For further experiments I will probably have to change to a other ADC with
less noise
like LTC2440 or AD7190.

But the LTC2400 has some advantages that I'm missing on other ADCs.
first is the +/-12.5% (or at least some millivolts when regarding linearity)
overrange without clipping of the ADC-value.
This makes offset + full scale calibration more easy than on other devices
which are clipping the reading at zero + full scale.
The next is the very low gain (0.02ppm/K) + offset drift (0.01ppm/K)
compared to other sigma delta converters.
And finally the 4 sub-bits below the 24 bits (giving a readout of totally 28
bits) would help
on a low noise ADC like AD7190 to get a more gaussian distribution of the
values.

With best regards

Andreas


----- Original Message ----- 
From: "Mickle" <timka2k at yandex.ru>
To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
Sent: Tuesday, January 29, 2013 8:31 AM
Subject: Re: [volt-nuts] Some questions to zeners (1N823-1N829)


> Hi, Andreas!
>
> AJ> But anyway ADC #13 was the first ADC which I can use for ageing
> AJ> measurements.
>
> Where to find information about this ADC? It is very interesting thing. Is
> it
> Multi-slope III, PWM, or another type?
>
> Regards,
> Mickle T.
>
>
>
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