[volt-nuts] LTZ1000A low frequency noise & tempco

[Dick Moore]

Andreas, please, when you have sorted this out, post us a complete schematic and some pictures -- I would like to do what you're doing too.

Dick Moore

> Message: 1
> Date: Sun, 31 Oct 2010 21:23:40 +0100
> From: "Andreas Jahn" <Andreas_-_Jahn at t-online.de>
> Subject: Re: [volt-nuts] Low Frequency noise on LTZ1000A and tempco of
> 	unheatedLTZ
> To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
> Message-ID: <213A047360A2436B9D5AF34142818477 at laptop>
> Content-Type: text/plain; format=flowed; charset="iso-8859-1";
> 	reply-type=response
> 
> 
>> Andreas,
>> 
> Hello Frank,
> 
>> I can confirm, that the unstabilized LTZ1000 has a TC of around 50ppm/K.
> 
> Ok on the one side i'm relieved to hear that. On the other it makes me
> hardly to believe that a tempco of 0.05ppm/K can be reached just by heating
> the reference. In this case the temperature regulator should have a 
> stability of 0.001K???
> 
>> I did not try to use the 200Ohm resistor for tempco trimming, as this has 
>> other disadvantages.
>> I think, that the tempco depends on the absolute value of the Zener diode, 
>> and therefore a higher or lower total ref voltage may give lower TC.
>> But this would be too expensive to selct...
> 
> Yes at least the total output voltage will increase by about 800mV, since 
> the
> Zener voltage will flow through this resistor too. On the other side the 
> tempco
> of the external resistor and the heated reference have to be adjusted too.
> But anyway, if I have the room in my next layout I will at least put one 
> resistor as option.
> 
>> I also found a sensitivity / unstability due to grounding problems, 
>> especially the Temp.-stabilization circuit seems to be volatile.
> Yes thats exactly what I'm observing here: Pin 6 of the LTZ1000(A) is the 
> most
> sensitive pin in the whole cirquit. Any induced voltage will give a large 
> drift
> over a relative large time. Since the the error voltage is stored as 
> temperature change
> within the LTZ1000(A).
> 
>> When I made a 24h measurement, I have seen many glitches, jumps of 
>> about -5ppm which last for 10-30s, and then returning to the original 
>> value.
> 
> On my noise measurements I had around up to +/- 15uV which give around 
> 4-5ppm peak-peak
> But I did not have a 24h measurement.Only relative short times looking on 
> the oscilloscope.
> 
>> Perhaps those glitches are caused by the MUX switch of the DVM (34401A or 
>> 3458A).
> In my case I do not believe that it has something to do with the ADC. I have 
> seen some pattern
> with repeating rate all 0,8 seconds or 2,8 seconds then switching off for a 
> time and coming back again.
> So for my side it looks like a pulse package heated device like a laser 
> printer or something like that
> causing trouble on the mains line. Perhaps comming in to my cirquit by the 
> oscilloscope.
> 
>> If I ground the metal case of the unit, I see a permanent change of the 
>> output in the same order of magnitude.
> Thats one reason why I have a battery on my LTZ1000. Now in battery mode 
> after all cirquit changes
> a single earth ground in the cirquit influences the output voltage below my 
> noise floor of 1uV.
> On the other side: when using a switchmode power supply to charge the 
> battery and using earth ground
> on the negative reference output the voltage shifts up to 2mV !!!
> 
>> 
>> I will test your hint implementing the 200nF cap, and will see, if this 
>> described sensitivity will be reduced.
> 
> Perhaps I did not write clearly enough what I have done: (I think I should 
> make some photos).
> I have one 100nF between Pin 6 and Pin 7 of the LTZ1000A (necessary).
> 
> The other 100nF is between Pin 4 and Pin 7 in my cirquit.
> But be carefuly: this capacitor may lead to heavy oscillations in zener 
> current
> without further changes in the cirquitry. I have additionally a resistor of 
> 10K
> between Pin 4 of the LTZ and Pin 2 of the LT1013. And a 100nF between
> Pin 1 and Pin 2 of the LT1013. Both parts are not in the original cirquit of
> the datasheet. Further against your cirquit  I have a FET in series with a 
> 220R
> resistor instead of your bipolar transistor. So the maximum current through
> the zener will be around 14 mA in case of oscillations. With the bipolar
> output transistor the zener will be probably destroyed.
> The 10K and the 100nF over Pin 1+2 of the LTZ I have needed agaist
> large capacitive loads on the output.
> In my case I have a 100nF between Ref + and Ref - within the cirquit.
> 
> So when in doubt do not use the capacitor between Pin 4 and 7 in your case.
> Im not shure if this capacitor will really help.
> 
> Both capacitors together reduced the voltage shift of capacitive influence
> on the cirquit from 8-10uV to about 4 uV (a factor of 2 .. 2.5).
> The heater output voltage on Pin 1 of the LTZ does no longer change
> when connecting the switchmode charger. The low frequency
> voltage fluctuations in the noise measurement were still there.
> 
> In the meantime I did further changes to the cirquitry.
> (It's now more like a capacitor grave yard ;-)
> 
> I measured the output voltage with a Oscilloscope on the heater (pin 1 of 
> LTZ) and
> recognized a relative high frequent noise with up to 15mVpp in AC coupling.
> Since I have learned that 2mV Heater voltage change will result in 4.1uV 
> change
> in output voltage I decided to calm down the heater regulator.
> When adapting the LT1013 pins with a steel needle I found out that Pins 1,2 
> and 6
> are sensitive.
> So the resulting measures are:
> A 10nF capacitor between Pin 6 + 7 of the LT1013 (necessary).
> After this the heater voltage noise is below 2mV.
> 
> For the sensitive pins I added further 100nF capacitors:
> 
> One from Pin 5 on Pin 6 on the LT1013.
> (I had preferred connecting between Pin 6 and Pin 4 of the LT1013 but this 
> leads
> to heavy oscillations on the heater output voltage).
> 
> The other from Pin 2 to Pin 3 on the LT1013. But this change will probably 
> only
> work with all the other changes aroud the current regulator with FET.
> So I would not recommend this capacitor in your case.
> 
> With all these further changes it seems that the low frequency noise
> is reduced from +/- 15uV to around +/- 3uV.
> Most of it will be due to the 10nF capacitor.
> 
> I had still some 1-2uV relative fast spikes on the output voltage which
> never could be from the reference cirquit.
> On the other side I recogized that the 0.1Hz .. 10Hz amplifier had
> only a high-pass filter on the input coupling directly on the OP-Amp
> pin. With a additional 4K7 + 1uF Low pass filter (33 Hz) in front of
> the first high pass these fast spikes disappeared.
> 
> What is left is a 4uV sensitivity to the switchmode charger
> and about +/-3uV ( = 6uV or 1 ppm) low frequency noise in total.
> I will have to observe the low frequency noise. Maybe it will
> be lesser on week ends than within the week.
> 
> In the next layout I will have to pay more attention to the routing
> of the power supply lines with regard to the output lines.
> 
> With best regards
> 
> Andreas
> 
> 
> 
> 
> ------------------------------
> 
> Message: 2
> Date: Mon, 01 Nov 2010 00:21:08 +0100
> From: "Dr. Frank Stellmach" <drfrank.stellmach at freenet.de>
> Subject: Re: [volt-nuts] Low Frequency noise on LTZ1000A and tempco of
> 	unheatedLTZ
> To: volt-nuts at febo.com
> Message-ID: <4CCDF9E4.3000900 at freenet.de>
> Content-Type: text/plain; charset=ISO-8859-15; format=flowed
> 
>> Ok on the one side i'm relieved to hear that. On the other it makes me
>> hardly to believe that a tempco of 0.05ppm/K can be reached just by heating
>> the reference. In this case the temperature regulator should have a
>> stability of 0.001K???
> 
> Obviously, that's exactly the case.
> 
> In the Datron 4910 manual, p. 1-4, "principles of operation", it's described that the chips temperature is controlled to within 1mK.
> The spec is 0.05ppm/K. Although both values do not have to do directly with each other, that seems to be the description of a 50ppm/K TC of the unheated element.
> 
> Well, do not add too many capacitance, otherwise the temp regulation may get stuck, heating the element to over 100?C, which may cause drift.
> 
> regards Frank