Author Topic: Ultra Precision Reference LTZ1000  (Read 990657 times)

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Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1425 on: May 15, 2016, 07:00:06 pm »
I'm placing final photos and the schematic diagram of my version.

Still waiting to finish better temperature chamber.

Hello,

thanks for the schematics.
There are some ideas where I still have not thought of:
- guarding
- using ferrites

What do you plan for the temperature chamber?
I think you already know my low cost chamber from the T.C. measurements thread.
https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg462298/#msg462298

I simply use a peltier cooling box for cooling. (which is running continously for the cooling phase).
And 2 heater foils mounted on a aluminium sheet which is used as heat spreader.
Controlling is done by a FET power stage via RS232-signals (RTS/CTS) for the heater.
And by a SCR-Relays via RS232-signals for the 230V peltier cooling box.
The controller runs on PC. Simply switching on/off  the heater every second.
(with a small p-controller part generating some kind of PWM within 0.3 deg C of the setpoint).

With best regards

Andreas


 

Offline TiN

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Re: Ultra Precision Reference LTZ1000
« Reply #1426 on: May 17, 2016, 05:02:32 am »
I'm parsing data and got retested HP 3458A's reference module (A9 PCBA) in thermal box.



Having hard time to find out tempco of it, no visible change over 15°C delta.
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Online doktor pyta

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Re: Ultra Precision Reference LTZ1000
« Reply #1427 on: May 25, 2016, 09:54:12 pm »
Below  mesurements of first assembled unit.
I used Fluke 732A + Fluke 720A + Fluke 845AR. The test setup is quite good, but I think I'm observing also the tempco of my measuring setup (for example 720A KVD has max. +/-0.1ppm/K tempco according to the manual). Any ideas how to solve this ? Maybe comparing results at different room temperatures would be OK, but it would take very long time.
« Last Edit: May 25, 2016, 10:01:11 pm by doktor pyta »
 

Offline TiN

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Re: Ultra Precision Reference LTZ1000
« Reply #1428 on: May 26, 2016, 04:37:38 am »
Have the reference with constant temperature and vary room temp with all gear, taking samples.
Best go both ramps, up and down in temperature. Then you can get linear TC of setup, and approximate result using it as multiplier to actual measurement.
If TC is linear, you can get pretty close.
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Offline BU508A

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Re: Ultra Precision Reference LTZ1000
« Reply #1429 on: May 26, 2016, 05:12:49 am »
Hello TiN,

nice article on xdevs.  :clap: :-+

https://xdevs.com/article/kx-ref/
“Chaos is found in greatest abundance wherever order is being sought. It always defeats order, because it is better organized.”            - Terry Pratchett -
 

Offline Alex Nikitin

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Re: Ultra Precision Reference LTZ1000
« Reply #1430 on: May 26, 2016, 10:05:43 am »
Hello TiN,

nice article on xdevs.  :clap: :-+

https://xdevs.com/article/kx-ref/

Nice article! Could somebody tell me what is the purpose of CR1 diode in the LTZ1000 circuit there (the one between the heater and ground)?

Cheers

Alex
 

Offline d-smes

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Re: Ultra Precision Reference LTZ1000
« Reply #1431 on: May 26, 2016, 10:20:21 am »
Could somebody tell me what is the purpose of CR1 diode in the LTZ1000 circuit there (the one between the heater and ground)?
Read data sheet "Pin Functions" section and reference "Block Diagram".  CR1 diode prevents parasitic substrate diodes within LTZ1000 structure from becoming forward biased.
 

Offline Alex Nikitin

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Re: Ultra Precision Reference LTZ1000
« Reply #1432 on: May 26, 2016, 10:28:53 am »
Could somebody tell me what is the purpose of CR1 diode in the LTZ1000 circuit there (the one between the heater and ground)?
Read data sheet "Pin Functions" section and reference "Block Diagram".  CR1 diode prevents parasitic substrate diodes within LTZ1000 structure from becoming forward biased.

1) No, it does not. It can not protect against a negative polarity voltage on the heater (pins 1 and 2) relative to pin 4. Only if that diode is in series with pin 1 than it would be useful.

2) In a circuit with a single supply as in this article, this protection is irrelevant in any case.

Cheers

Alex

« Last Edit: May 26, 2016, 12:47:14 pm by Alex Nikitin »
 

Offline d-smes

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Re: Ultra Precision Reference LTZ1000
« Reply #1433 on: May 26, 2016, 07:38:35 pm »
Could somebody tell me what is the purpose of CR1 diode in the LTZ1000 circuit there (the one between the heater and ground)?
Read data sheet "Pin Functions" section and reference "Block Diagram".  CR1 diode prevents parasitic substrate diodes within LTZ1000 structure from becoming forward biased.

1) No, it does not. It can not protect against a negative polarity voltage on the heater (pins 1 and 2) relative to pin 4. Only if that diode is in series with pin 1 than it would be useful.

2) In a circuit with a single supply as in this article, this protection is irrelevant in any case.

Cheers

Alex
I answered assuming the referenced circuit "Schematics 2: xDevs.com KX voltage reference (Rev.B00) reference" was operating in steady-state conditions.  Therefore, it is assumed heater current is flowing which will forward bias CR1 into conduction.  In doing so, pin 2 is now at roughly the same voltage as pin 4 (referenced to pin 7, ground).  In reality, it's slightly positive since the Vf of CR1 (at room temperature) is greater than the Vbe of Q1 (pin 4 to pin 7) which is heated above room temperature (owing to the roughly -2mV/C temperature coefficient of Vbe).  Thus, the substrate diode between pin 2 (cathode) and pin 4 (anode) is reverse biased.  This satisfies the requirement that the substrate diode(s) not be forward biased.
 

Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1434 on: May 29, 2016, 04:25:47 pm »
Hello,

Shi(f)t happens ...

I accidently shorted the unbuffered output of my best aged LTZ (LTZ#1) which had a annual drift of about -1 ppm.
This resulted in a low setpoint voltage for the heater -> infinite temperature -> stress + hysteresis to the zener.
After curing a part of the hysteresis by power cycling several times
the remaining shift is about -1.9 ppm.   :-//   |O

By the way: I read in a german forum that precision OP-Amps are not always a perfect means to prevent shorting the output.
The reason is that most of the precision amplifiers have protection diodes between both inputs.
So if a unity gain follower without any series resistors is shorted to GND on the output, the positive input is clamped to 0.7 V.
see figure 3 of OP27 datasheet:
http://www.analog.com/media/en/technical-documentation/data-sheets/OP27.pdf

On my new references I tested short cirquits on the buffered output having around 1 mV influence on the zener voltage.

With best regards

Andreas
 

Offline TiN

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Re: Ultra Precision Reference LTZ1000
« Reply #1435 on: May 29, 2016, 04:46:57 pm »
Ouch. How long was it shorted? I'm pretty sure that was how my first LTZ chip died, after short temp loop went out of bounds and shorted the zener.
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Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1436 on: May 29, 2016, 05:34:45 pm »
Hello,

Not really long.

It was a unpowered ADC (LTC2400 with LTC1043) connected to LTZ#1
I had a DMM as difference voltage to LTZ#2.
And I soon recognized during connection that the DMM had not a few mV as expected
but 7V as measurement value.

So perhaps 5 seconds short cirquit time.

With best regards

Andreas
 

Offline zlymex

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Re: Ultra Precision Reference LTZ1000
« Reply #1437 on: May 30, 2016, 12:38:43 am »
Hello Andreas,

I cannot find the idea 'switched capacitor idea for the 7V->10V boost' of yours mentioned by DiligentMinds.com at the other thread, can you give me the reference?

I had this ideal as well by using a LTC1043 or ADG1236 to multiply 7V by 1.5 times to 10.5V, and use resistor networks to divided down to 10V, I wonder if they are similar. I bought some LTC1043 and ADG1236 long time ago but cannot find time for the implementation. I made some simulation and functionally it's working. I had two LTZ1000 with voltages slightly below 7V that particularly suitable for this kind of application. Also, there are some low voltage compensated zeners(namely 2DW232, 6.3V) which good for this as well.
 

Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1438 on: May 30, 2016, 04:57:54 am »
Hello,

one suggestion is here (5.2V out of 7V by *3/4 transfer)

https://www.eevblog.com/forum/projects/building-a-7-decade-voltage-calibrator/msg304819/#msg304819

the other here (* 3/2 transfer to get 10 V out of 7V)

https://www.eevblog.com/forum/projects/building-a-7-decade-voltage-calibrator/msg298350/#msg298350

both cirquits only simulated by now (just had no time to build a complete calibrator).

Edit: and of course you will need a high impedant/low leakage buffer at the output.

with best regards

Andreas
« Last Edit: May 30, 2016, 05:41:08 am by Andreas »
 

Offline zlymex

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Re: Ultra Precision Reference LTZ1000
« Reply #1439 on: May 30, 2016, 06:50:24 am »
Thanks Andreas, here is my simulation.
« Last Edit: May 30, 2016, 07:10:25 am by zlymex »
 

Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1440 on: May 30, 2016, 07:30:14 pm »
Hello,

the cirquit looks good.
But in the diagram the voltage ripple is rather large. (and average voltage too low).
Is the measurement instrument simulated as a pull down resistor ? (which value ?)

So round about estimation is 200ms time constant (50mV ripple in 1 ms).
Which would give a 1 Meg resistor as load.

With best regards

Andreas
 

Offline zlymex

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Re: Ultra Precision Reference LTZ1000
« Reply #1441 on: May 31, 2016, 01:29:45 am »
That's right, I suspect the simulation program(Multisim) is not very good, and it had difficulty running the simulation(errors, adjustment, roll-back).
There are many times that Multisim gives contradictory results. For instance, how can a Vpp be 221uV but Vrms is 21.1V?
There is no apparent source of leakage from the circuit, there must be some equivalent leakage resistors in the capacitor or in the switch.
 

Offline necessaryevil

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Re: Ultra Precision Reference LTZ1000
« Reply #1442 on: June 02, 2016, 01:35:08 pm »
Hmm... how about a Weston Cell from ebay instead of an LTZ1000? I know, a Josephson voltage standard would be more accurate, but  I don't think they fit on my bench nicely. Also, they are not available on ebay for a price which agrees with my budget.

Quote
. For instance, how can a Vpp be 221uV but Vrms is 21.1V?
DC offset?
 

Offline zlymex

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Re: Ultra Precision Reference LTZ1000
« Reply #1443 on: June 02, 2016, 02:26:03 pm »
Hmm... how about a Weston Cell from ebay instead of an LTZ1000? I know, a Josephson voltage standard would be more accurate, but  I don't think they fit on my bench nicely. Also, they are not available on ebay for a price which agrees with my budget.

Quote
. For instance, how can a Vpp be 221uV but Vrms is 21.1V?
DC offset?
Poor ones, not worth considering. Good ones, often suffered from transport effect.

There is no DC offset apart from 10V, which is much smaller than 21.1V. I do come across several instances where the output of an opamp gone up to 10kV :palm:
 

Online Kleinstein

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Re: Ultra Precision Reference LTZ1000
« Reply #1444 on: June 02, 2016, 03:05:55 pm »
Sometimes simulations have trouble with convergence and start with crazy values like 10 kV at an OP.  This can also happen with LTspice, not a Multisim exclusive. The 21 V RMS at 220 µV_pp look faulty - but at least the is obvious.

The ripple could be real, because if internal effects in the CMOS switches. So switches have significant charge injection. Though I don't think the effect should be that large - could be a problem with the model. Another point could be not so perfect capacitors used in the simulation. As perfect parts sometimes cause trouble in simulations, the program might use more realistic models by default.
 

Offline zlymex

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Re: Ultra Precision Reference LTZ1000
« Reply #1445 on: June 02, 2016, 04:39:32 pm »
Yes, large capacitor should be chosen to weaken the charge injection effect. They use 1uF most of the time in the datasheet examples. I use smaller values(and 100kHz frequency) in the simulation to speed up the process.
 

Offline necessaryevil

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Re: Ultra Precision Reference LTZ1000
« Reply #1446 on: June 02, 2016, 05:02:21 pm »
I'm sure circuit simulators are great tools, but they are no panacea. Creating a good simulation model is probably more difficult than breadbording (and perhaps than flying to the moon).
 

Online Kleinstein

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Re: Ultra Precision Reference LTZ1000
« Reply #1447 on: July 20, 2016, 09:14:19 pm »
The LT1677 is not such a good choice, the LT1013 is better in at least three parameters:
The current noise of the LT1677 is rather high (1.2 pA/Sqrt(Hz) at 10 Hz - compared to 0.07 pA/Sqrt(Hz) for the LT1013.
As the OPs sense the rather high output signals at the collectors (e.g. 70 K impedance), current noise in the pA range is more important than low voltage noise.  So the LT1677 is lower noise only with low source impedance (< about 30 K), but not with the typical LTZ1000 circuit.

At low input voltage, the bias current is huge (e.g. 400 nA) and thus a negative supply is needed. Otherwise the LT1677 would be worse than an LM358.

The other point is current consumption - the LT1677 take quite some extra power (e.g. 6 times more).

Bias current drift could also be worse with the LT1677, though the data-sheet does not specify it.
 

Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1448 on: July 20, 2016, 09:21:27 pm »
then the LT1677 is superior to the LT1013 in almost every important spec,

Hello Ken,

really?

I personally believe that for the current regulation loop the amplification should be as high as possible.

At 5 mA (roughly 2K-Ohms) load the LT1013 has a factor 10 higher amplification,
with a power consumption of 1/10 th of the LT1677.

With best regards

Andreas

 

Online Kleinstein

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Re: Ultra Precision Reference LTZ1000
« Reply #1449 on: July 20, 2016, 10:52:10 pm »
The dynamic impedance at the transistors collector is not given by voltage divided by current. The transistor is working essentially as a constant current source and thus has a very high output impedance. The typical data-sheets don't give it for such a small U_CE. A simple simulation for the 2N3904 in LTspice is about 500 K. So this node is really about 70 K impedance.

The noise of the LTC1677 (or that of most OPs) would be still swamped be the LTZ1000. But at more than about 30 K source impedance the noise of the LTC1677 is higher than that of the LT1013.

The current regulation loop has the additional gain of the transistor and it's working at an essentially fixed condition. So a limited gain by itself would not be a problem at all. Only when changing with time or temperature it might be a small problem. But with the additional factor of about 200-250 from the transistor, anything above 50.000 should be enough to cause an contribution of less than 1 µV to the output.
 


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