Author Topic: Ultra Precision Reference LTZ1000  (Read 453452 times)

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Offline d-smes

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Re: Ultra Precision Reference LTZ1000
« Reply #1950 on: April 21, 2017, 02:42:10 AM »
Going back to grounding, isn't Figure I2 from AN-86 the WRONG way to do Kelvin Sensing?   You see, Zener (and other) currents create a voltage drop across the Zener-Force trace resistance (red marks in attached).  This makes current want to flow in the Kelvin sense connection (green arrow) which defeats its purpose.


So, what's the proper way to implement Kelvin sensing?  One way would be to buffer the Kelvin point, but then you'd need a negative supply.  A more practical implementation might be to treat the junction of R5, R1 and Pin 7 as a Kelvin star node and use that as your Kelvin Sense and as a star point for return currents from e.g. resistive divider of a 7V to 10V output converter as shown in second attachment.   Comments?  How do you all do it?

« Last Edit: April 21, 2017, 02:43:43 AM by d-smes »
 

Offline 2N3055

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Re: Ultra Precision Reference LTZ1000
« Reply #1951 on: April 21, 2017, 03:01:13 AM »
Output of LZ1000 is taken from pins 3 and 7.. It is combination of zener and transistor that is used...
 

Offline Galaxyrise

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Re: Ultra Precision Reference LTZ1000
« Reply #1952 on: April 21, 2017, 03:29:31 AM »
I'm not sure why AN-86 appears to suggest creating a ground loop.  The "7V Positive Reference Circuit" in the data sheet is clearer.
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Online pitagoras

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Re: Ultra Precision Reference LTZ1000
« Reply #1953 on: April 21, 2017, 04:12:24 AM »
Is there a reference design PCB for this "normal" positive 7V reference?
 

Online Micke

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Re: Ultra Precision Reference LTZ1000
« Reply #1954 on: April 21, 2017, 04:58:22 PM »
On the Eval. board for AD5781, they connect ground to the resistive divider directly to pin 7 on LTZ1000.
I have done like this on my first LTZ1000 design, I have not experienced any drawbacks yet...  :)
They also have a RC filter between unbuffered 7V and OP-amps with 1.5k and 10µF, quite big capacitance! I thought of doing like this but reduced the cap to 100nF instead, I was afraid that tiniest leakage current of the cap would introduce errors. I was thinking of though if having 10µF use a solid wet tantalum.
Schematic AD5781 Eval board with LTZ1000: https://ez.analog.com/servlet/JiveServlet/download/13219-2-41351/EVAL-AD5791SDZ_LTZ1000A.pdf
 

Offline ap

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Re: Ultra Precision Reference LTZ1000
« Reply #1955 on: April 21, 2017, 07:22:45 PM »
Well, the true ground point should be at the output of the unit. Now for a PCB, one could have that at pin 7, for a unit in a box with e.g. binding posts, things look different. It should be at the negative output binding post. It needs to be set in a way that the change in current through the related wire from pin 7 to the output (load effects, temperature changes / related copper wire drifts and current accross) is sufficently low. The output impedance of a 732B e.g. is 0.1mOhm, as a side comment. So 10mA load at the output causes 1ppm change in voltage. It musst be assured, if that e.g. is the goal, that any return current from that binding post to pin 7 is not generating a ground shift.
The filter capacitor should be a lowest leakage foil capacitor, and the input resistor could be higher, provided the bias current (change again) of the opamp is low, the capacitor will anyway filter resistor noise.
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Online Micke

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Re: Ultra Precision Reference LTZ1000
« Reply #1956 on: April 21, 2017, 09:02:41 PM »
I did at least not drive the output current via pin 7 of LTZ1000... I do have built in the PCB in a metal box with 2 pair of binding posts (Buffered 7V with LTC2057 and 10V with LTC2057) , the output negatives are done with separate 0,75mm² cables directly to incoming ground on the PCB. I have done load tests, for example loading the 10V output with 10mA, the output drops 7,6ppm... not great result, but most likely I will not load the outputs that much   :)
But I realize now I should have done like your good explanation, have the output ground as the common ground point... 
 

Offline mimmus78

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Re: Ultra Precision Reference LTZ1000
« Reply #1957 on: April 23, 2017, 02:50:00 AM »
What you think about using LT1037 to buffer LTZ1000 output with a LTC2057 to zero out offset (same circuit as shown in LTC2057 datasheet).
I'd like to use it in my 4 TLZ1000 reference as "Ultralow Noise Composite Amplifier"

The problem I see is that LT1037 has a little much input bias current than just LTC2057.
This should be not much a problem as this input bias/offset current seems to be constant from 20°C and up ... I think you will get some nA difference from 25° to 45°c that is the internal temperature of the reference.
What will be the influence on the LTZ1000 of this nA bias current offset?

Also LT1028 is interesting, but input bias current is not linear over the temperature range.
« Last Edit: April 23, 2017, 04:58:36 AM by mimmus78 »
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Offline Kleinstein

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Re: Ultra Precision Reference LTZ1000
« Reply #1958 on: April 23, 2017, 04:56:48 AM »
The standard LTZ1000 circuit can deliver quite some current. The OP is delivering the current and the zener connection is used for sensing. The variants with an extra transistor as emitter follower can even deliver higher currents if needed. This goes up to the point of providing force/sense connections to compensate wire resistance.

The problem why sometimes buffering for the output is desired is, that if the load to the output is so high that the voltage drops, this shifts the temperature set point to a higher value and thus can easily cause the heater to turn on all the way. This thermal stress can cause a shift in observed voltage, e.g. due to thermal hysteresis. In theory one could add an extra protection against this: so limit or turn of the heater, if the reference voltage drops by more than something in the mV range  (enough to allow a cold start).

So there is no really problem for the ref circuit to drive even a much higher input current in the mA range, if this internal. Current spikes from the LTC2057 would be the greater concern. So it is a good idea to have an RC (maybe LC) filter in series, at least for that OP. Without extra filtering, the white noise of the LTZ1000 is way higher than that of the LT1037 - so if would make sense to have at least some filtering here as well.

The LT1037 is not unity stable, the LT1007 is the unity gain stable version.
 

Offline mimmus78

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Re: Ultra Precision Reference LTZ1000
« Reply #1959 on: April 23, 2017, 10:28:38 AM »
Thanks Kleinstein, I understand this now.
By this buffer I want to obtain short circuit proof circuit, and a more easier way to sum up the four cells without much interferences and current loops one each other.
LTC2057 will be only used to zero out the offset. His spark noise should not came out directly and I was considering RC filtering both at the inputs that at the output of it.

What op amp to use as current cancelling?
For this I'm considering again LTC2057, this time it will be tied up to a very low impedance to ground node, I was thinking in this case I can ignore those sparks.
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Offline Kleinstein

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Re: Ultra Precision Reference LTZ1000
« Reply #1960 on: April 23, 2017, 05:38:12 PM »
Current canceling usually should not use an AZ OP - definitely not a high current noise one like the LT2057. More like a normal precision one, like LT1013 (if single supply is needed), OP07, LT1097 or maybe OPA141. In some areas even an TLC271 or LM358 might do the job. Having a small DC error here is not that critical. More of the error will likely be due to the resistors anyway.

Just for averaging, there is no need to have a buffer. The LTZ1000 circuit has no problem driving the rather constant maybe 10-100 µA needed for a resistive combiner. An important principle in a precision circuit is not to use many precision parts, but to have a circuit that does not critically depend on many parts. So it is more like having a minimum number of amplifiers in the signal path. So one would rarely have something like only a buffer - so it is a little like going back and learning from old times when an OP was expensive and not that good. No OP can beat a piece of copper wire when it comes to drift and noise. There can be still quite a few parts in not so critical areas, like protection, current canceling and the power supply or monitoring.
 

Offline ap

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Re: Ultra Precision Reference LTZ1000
« Reply #1961 on: April 23, 2017, 07:45:47 PM »
Why would the current cancelling Opamp's Vos drift not have the same impact on output drift as in the buffer amp (amplification factor, if any, left aside)?
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Offline Kleinstein

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Re: Ultra Precision Reference LTZ1000
« Reply #1962 on: April 23, 2017, 09:43:36 PM »
There are different ways of doing current canceling / combining the negative side. It depends on the solution chosen of cause:
Using a kind of buffer or virtual ground for the whole circuit would have a large impact of the OPs offset. In this case a really good OP is needed, and the LT2057 might be acceptable.

A second, similar option is using resistors to combine the low side voltage too and than use a buffer with optional force/sense function. Here too a good OP is needed.

If one uses classical current canceling - thus sending a constant current of the right size to the GND point, the OPs offset is even less important. It is attenuated by a factor set by the resistor used to set the current and the resistance of the ground loop - so usually something around 100-1000, maybe more. However the resistors used in the compensation circuit also contribute, likely more than the OP.

An other option is using a virtual ground only for the main current ("120 Ohms " resistor) and keep the transistor to sense the ground. In this case the drift / noise of the OP used is attenuated by a factor of about 10. So drift is less important by that factor. One can combine this with current canceling for the 100 µA at the transistor if really needed.

So my idea was about using the last option of current canceling by current sources, as this should give the best results.
 

Offline d-smes

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Re: Ultra Precision Reference LTZ1000
« Reply #1963 on: Yesterday at 01:59:40 AM »
The standard LTZ1000 circuit can deliver quite some current. The OP is delivering the current and the zener connection is used for sensing. The variants with an extra transistor as emitter follower can even deliver higher currents if needed. This goes up to the point of providing force/sense connections to compensate wire resistance.  <clip>
If you pull several mA from the Pos output, where/how do you set up a return current path such that it does not corrupt the output with a I*R voltage drop?  Do you guys buffer the ground and use force/sense connections?  Or just have one massively heavy-duty ground connection at R5, R1, Pin 7 super-node?
 

Offline Kleinstein

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Re: Ultra Precision Reference LTZ1000
« Reply #1964 on: Yesterday at 04:42:48 AM »
With an additional current source to provide the approximate current needed by LTZ1000 Zener and the divider for the temperature set point, the positive side of the zener can act like a low input-current sense connection. The OP (usually LT1013) will act as the forcing output.

This is similar to return current compensation on the negative side. However the negative side might be a little easier with only about 100 µA from the transistor used for sensing.

However the more normal situation is to have an only very moderate load current, so that just a single ground point and low impedance connection. The really troublesome part is only the possible change in load current. The more predictable part of the load current could be added locally at the load.
 


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