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

Andreas Jahn Andreas_-_Jahn at t-online.de
Wed Jan 30 17:04:34 EST 2013

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----- 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
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
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

With best regards


----- 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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