PX Reference

[hwj-d]

This is TEKO - USA

[hwj-d]

Another idea is, to perform the additional resistors of the voltage divider, according to the procedure described, as soldering posts/terminals too. Anyone can then decide for himself, whether someone only wants the buffer, or what values to use there for 10V.

DigiKey ED1087-ND or ED10992-ND, for example

[cellularmitosis]

Drat, I have just realized that I made a mistake on the board which will cause a few degrees of self-heating in the LTC2057.

This op amp has a shutdown pin.  The datasheet says the max voltage at this pin is 5.3V, but I have tied it to the 15V supply (actually, 14.7V after the 1N5819).

The 2057 block diagram shows the SD pin goes through a 10k resistor to a 5.25V zener, so I'll be burning about 15mW total across these components.  If the thermal resistance of an SIOC-8 package is 100 to 200C/W, this will cause 1.5 to 3C of self heating.  If the thermal EMF of copper to solder is 1.5 to 3uV/C, that's a worst-case error of up to 9uV (but will be much smaller than that in practice, as the thermal conductivity of the joint will keep the thermal gradient far below 3C).

I'll post a new version of the board soon.  If you have already purchased a copy of the board and would like to perform a bodge fix, you can lift pin 1 of the 2057 and use a 1meg resistor to connect it either to pin 7 of the 2057, or to the positive pin of C33 (the pin which is closer to the edge of the board).

[mimmus78]

Drat, I have just realized that I made a mistake on the board which will cause a few degrees of self-heating in the LTC2057.

This op amp has a shutdown pin.  The datasheet says the max voltage at this pin is 5.3V, but I have tied it to the 15V supply (actually, 14.7V after the 1N5819).

The 2057 block diagram shows the SD pin goes through a 10k resistor to a 5.25V zener, so I'll be burning about 15mW total across these components.  If the thermal resistance of an SIOC-8 package is 100 to 200C/W, this will cause 1.5 to 3C of self heating.  If the thermal EMF of copper to solder is 1.5 to 3uV/C, that's a worst-case error of up to 9uV (but will be much smaller than that in practice, as the thermal conductivity of the joint will keep the thermal gradient far below 3C).

I'll post a new version of the board soon.  If you have already purchased a copy of the board and would like to perform a bodge fix, you can lift pin 1 of the 2057 and use a 1meg resistor to connect it either to pin 7 of the 2057, or to the positive pin of C33 (the pin which is closer to the edge of the board).

Why you used shutdown pins?

As by datasheet: if the shutdown feature is not required, SD and SDCOM may be left floating. Internal circuitry will automatically keep the amplifier in the ON state.

Inviato dal mio ONEPLUS A5010 utilizzando Tapatalk

[mimmus78]

Drat, I have just realized that I made a mistake on the board which will cause a few degrees of self-heating in the LTC2057.

This op amp has a shutdown pin.  The datasheet says the max voltage at this pin is 5.3V, but I have tied it to the 15V supply (actually, 14.7V after the 1N5819).

The 2057 block diagram shows the SD pin goes through a 10k resistor to a 5.25V zener, so I'll be burning about 15mW total across these components.  If the thermal resistance of an SIOC-8 package is 100 to 200C/W, this will cause 1.5 to 3C of self heating.  If the thermal EMF of copper to solder is 1.5 to 3uV/C, that's a worst-case error of up to 9uV (but will be much smaller than that in practice, as the thermal conductivity of the joint will keep the thermal gradient far below 3C).

I'll post a new version of the board soon.  If you have already purchased a copy of the board and would like to perform a bodge fix, you can lift pin 1 of the 2057 and use a 1meg resistor to connect it either to pin 7 of the 2057, or to the positive pin of C33 (the pin which is closer to the edge of the board).

Why you used shutdown pins?

As by datasheet: if the shutdown feature is not required, SD and SDCOM may be left floating. Internal circuitry will automatically keep the amplifier in the ON state.

Inviato dal mio ONEPLUS A5010 utilizzando Tapatalk

Maybe is better to just cut the two traces near the opamp pins.

Inviato dal mio ONEPLUS A5010 utilizzando Tapatalk

[hwj-d]

Page 22, Applications information, shutdown mode:

Table 1. Shutdown Control Logic
SHUTDOWN PIN CONDITION
SD = Float, SDCOM = Float: AMPLIFIER STATE = ON

So, cut trace to Pin 1 and Pin 8.

 

[cellularmitosis]

Thanks guys, I should have read the datasheet more closely 

So, the work-around for existing board is either to cut the traces to pins 1 and 8 of LTC2057, or lift pins 1 and 8.

[mimmus78]

Thanks guys, I should have read the datasheet more closely 

So, the work-around for existing board is either to cut the traces to pins 1 and 8 of LTC2057, or lift pins 1 and 8.

In my opinion is better cut trace, it may it help to keep the other pins at the same temperature (but this is all to demonstrate).

Inviato dal mio ONEPLUS A5010 utilizzando Tapatalk

[cellularmitosis]

Here's v2.3, with the shutdown pins left floating:

https://oshpark.com/shared_projects/6puqHG9j

https://github.com/pepaslabs/px-ref/tree/master/kicad/releases/v2.3

Also, I've combined the two variants of the board into one (it has footprints for either a divider for R4/R5, or discrete resistors).

[hwj-d]

Hello cellular,

thanks so much for your very quick actions developing this nice little board. The performance ist outstanding but same time by its through-hole construction very uncomplicated to build. Also thanks to Andreas, and all the others, who are involved in the underlying work.

Some thoughts for this todos, to make the board slightly smaller in one dimension to fit within a TEKO case.

These cases are also available in different versions:

• TEKO 371:   54 x 50 x 26 mm
• TEKO 372:   83 x 50 x 26 mm
• TEKO 373: 106 x 50 x 26 mm

(the inner width of these is only 47 mm!)

but also for example:

• TEKO 392:   83 x 68 x 28 mm
• TEKO 393: 122 x 68 x 28 mm

I used the TEKO 372, because to test the effects of better magnetig shielding from his steel sheet, and as a compact alternative to the Hammond 1590B. But for a standalone, maybe TEKO 393 is better, not only to have space for the pomona's, but also to use the pcbs as they are to save your extra work and service.

So, actually, an extra slim pcb is only necessary, if one plan to put a TEKO 371 or 372 without binding posts or banana jacks inside an other housing for additional magnetic shielding.

Thanks for your good work 

[eurofox]

Here's v2.3, with the shutdown pins left floating:

https://oshpark.com/shared_projects/6puqHG9j

https://github.com/pepaslabs/px-ref/tree/master/kicad/releases/v2.3

Also, I've combined the two variants of the board into one (it has footprints for either a divider for R4/R5, or discrete resistors).

Just placed an order for the PCB's and start to collect all components. 

[hwj-d]

Drat, I have just realized that I made a mistake on the board which will cause a few degrees of self-heating in the LTC2057.

This op amp has a shutdown pin.  The datasheet says the max voltage at this pin is 5.3V, but I have tied it to the 15V supply (actually, 14.7V after the 1N5819).

The 2057 block diagram shows the SD pin goes through a 10k resistor to a 5.25V zener, so I'll be burning about 15mW total across these components.  If the thermal resistance of an SIOC-8 package is 100 to 200C/W, this will cause 1.5 to 3C of self heating.  If the thermal EMF of copper to solder is 1.5 to 3uV/C, that's a worst-case error of up to 9uV (but will be much smaller than that in practice, as the thermal conductivity of the joint will keep the thermal gradient far below 3C).

I'll post a new version of the board soon.  If you have already purchased a copy of the board and would like to perform a bodge fix, you can lift pin 1 of the 2057 and use a 1meg resistor to connect it either to pin 7 of the 2057, or to the positive pin of C33 (the pin which is closer to the edge of the board).

Leave one of my finished boards without this modification in continuous operation.
So far absolutely no negative effects are noticeable.

€: Ordered next three boards v2.3   

[cellularmitosis]

I'm on vacation -- time to futz with this board some more! 

v2.4 is up:

https://github.com/pepaslabs/px-ref/tree/master/kicad/releases/v2.4

https://oshpark.com/shared_projects/uh2iEJZp

Changes:

Board dimensions should now be small enough to squeeze into a TEKO 371.16 case.

The "shutdown" pins of the LTC2057 have been exposed via break-out pins (for implementing off-board control, e.g. if you want to disable output until an enclosure oven stabilizes).

The center terminal of the R4 / R5 divider has been exposed via a breakout pin (for off-board manipulation of the temperature set-point, for e.g. Pickering-patent shenanigans).

The chassis-ground jumper has been removed and instead the pad of the mounting hole adjacent to ground has been enlarged (which facilitates soldering a wire from chassis to ground).

Minor routing tweaks.

[hwj-d]

This damn little thing goes to be better and better ... 
Ordered them once more 

Big thanks again, cellular! 

[cellularmitosis]

I’m planning on making a small voltage regulator board which would fit along side it in a Hammond 1590B case.

Andreas has already found a great low noise part — the LT1763, so I’ll go with that as well.

As far as operating voltage, it would be nice to support a variety of battery powered options:

A lead acid battery can be considered discharged around 11.6 volts.

It looks like 10 NiMH cells can be considered discharged somewhere between 11.5 and 11 volts.

A 3S Lipo can be considered discharged at 11.1 volts.

So, if we can operate the 1763 on 11 volts minimum, that would be ideal.

That means we will be operating the LTZ around 10 or 10.5 volts.  Are there any disadvantages to this?  Andreas has shown that the voltage coefficient of the LTZ circuit is slightly negative, but more importantly, is well behaved at least down to 9V, so I think this should be fine: https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg842662/#msg842662

[Andreas]

Hello Jason,

be aware that I do not use the standard cirquit.
I have a FET (BF245C/BF545C) at the output of the LT1013 / LTC2057 which does some kind of level shifting.
So with this modification I need less headroom than the standard cirquit.

I never tested this but you will probably need at least 3 V headroom for the standard cirquit.
(2V for the LT1013 and 1V for the diode). But 10.5-11 V should be fine also for the standard cirquit with not too high temperature setpoint.

I also have a battery monitor for the NiMH cells. It does not only switch off when the minimum voltage (I calculate 1.15V / cell) is reached (then already one of the cells could have reverse polarity) but also when a certain discharge gradient (e.g. 0.6V in 2 minutes) is reached.

with best regards

Andreas

[hwj-d]

With LT1763 firmly installed, we don't need additionally the shottky - voltage drop anymore, right?

[cellularmitosis]

Thanks Andreas!

hwj-d yes I think you are right.

I wonder also if some of the 1N4148 could be replaced with schottkys.  I found the 1N4148 forward drop to be high nearly 0.8V with ~20mA)

[Andreas]

Hello,

With LT1763 firmly installed, we don't need additionally the shottky - voltage drop anymore, right?

Thats right.

but be aware that if you put a variable heat source (depending on battery voltage)
near the LTZ you will have some (bad) thermal influence on PSRR.

I wonder also if some of the 1N4148 could be replaced with schottkys.  I found the 1N4148 forward drop to be high nearly 0.8V with ~20mA)

D2 (5mA) in your cirquit (at the output of the LT1013 current regulator) can be replaced by a LL103A or BAT48 (with larger holes also the 1N5819)
D1 (20mA) in the heater return must not be changed (otherwise you risk to forward bias the substrate diode, which is a no no).

with best regards

Andreas

[TiN]

I can probably share my old design around LT3042/3045 if one is up to little fine-pitch microscope work with 0805 and 0402's

[Echo88]

That would be nice! 

[hwj-d]

TiN, really cool things 

But, what advantages would these have, except the size?

[Andreas]

They are more nice (golden pins) than my LT1763-design with 0805 and SO-8 as smallest parts.

with best regards

Andreas

Edit: the PCB is a single sided design

[hwj-d]

Hello,

With LT1763 firmly installed, we don't need additionally the shottky - voltage drop anymore, right?

Thats right.

but be aware that if you put a variable heat source (depending on battery voltage)
near the LTZ you will have some (bad) thermal influence on PSRR.
...

Question,

is it better to use, maybe 4s 18650 unregulated (shurely observing the discharge level) as to use this variable heatsource regulation down to 11V? In my subjektive practise, I see no negative psrr impact with unregulated 4s over the whole entire discharge voltage range. But maybe I miss something?

[cellularmitosis]

Well, an oven circuit which adheres to the underbelly of the Hammond 1590B case is already in the works for me, so I’m not worried about the variable heat.

However, Andreas did measure a tempco on his LTZ circuit which he did attribute to variable regulator heat, and added another PCB slot to confirm this.  So in his case, both the variable voltage and the variable heat were a concern.

If you’ve measured no voltage coefficient, then you are correct that unregulated cells would be better.  Hmm, I should see if I can measure a voltage coefficient!

Edit: link https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg875274/#msg875274


Edit2: and I have just the tool to perform this test  https://oshpark.com/shared_projects/39B3GV50
 https://github.com/pepaslabs/ProgrammableLM317