LTZ1000 10v buffer

[kj7e]

After trying the LTC1050 + LT1010 combo and a few other ideas I ended up with a LTC1250 driving a 2N5088 and I'm really happy with it.  Short circuit current is 18mA, drives a 2.5K load without issue.  Noise appears very low, but wont be able to test it for sure until final assembly.  The 350 and 1K resistors in the trimming section are Vishay Z202's, VR1 is a Burns Trimpot 3290 Vishay Accutrim 1240W,  the 10K Rf and 25.2K Rg is a Vishay VHD divider network.  Appears to be very stable even in the breadboard and I'm happy with the trimmer setup, especially for using parts I have.

Edit - updated the trimmer section to correct VR1 placement and the wiper orientation (cw increases the voltage output).

Sharable link to the Scheme-It file and app;
https://www.digikey.com/schemeit/project/ltz1000a-buffer-P8VAILG3010G/

[kj7e]

I like the 2057, but SMD wont work well for this particular project.  The LTC1250 is less noisy than the LTC1050 and seems to about the best option for old school through hole.

[cellularmitosis]

kj7e, what did you use to design that circuit?

[kj7e]

There are DIP adapters available...

Ill take a look.

[kj7e]

kj7e, what did you use to design that circuit?

Grid paper and a HP35 calculator... but mostly trial and error haha.  I drew it up on Digikeys Scheme-it on-line app.
https://www.digikey.com/schemeit/project/

Just to make it look more professional.

[cellularmitosis]

I fooled around a bit in LTSpice.

Here's a 7V-10V with digital calibration.  R1+R2 would be a VHD200, and V3 would be a 0-5V DAC.  The 100meg resistor turns 12 bits into 0.1uV per LSB, yielding about 41ppm total adjustment range.

[hwj-d]

Hello together,

cellular, from where i can get LTSpice without spend an arm and a leg?

[eurofox]

Hello together,

cellular, from where i can get LTSpice without spend an arm and a leg?

Here free of charge 

http://www.linear.com/designtools/software/

[hwj-d]

Hello together,

cellular, from where i can get LTSpice without spend an arm and a leg?

Here free of charge 

http://www.linear.com/designtools/software/

Big thanks. Don't know that.

[cellularmitosis]

Just posted this link in another thread for the idea of using a copper resistor for tempco compensation, but the same link is also useful to this thread for the spreadsheet!

https://www.eevblog.com/forum/metrology/spread-sheet-aided-design-of-compensation-for-7v-to-10v-step-up-resistor-set/

[MisterDiodes]

I like the 2057, but SMD wont work well for this particular project.  The LTC1250 is less noisy than the LTC1050 and seems to about the best option for old school through hole.

You're on the right track!

Yes, thru hole is better if you can get it - at ppm level smd parts will pick up vibration all too well, from anywhere.

The other thing to consider is higher freq chopper amps like '2057 are notorious for chopper beating + demodulating any nearby noise from a CPU - or worse yet an FPGA - right onto its output signal.  On any chopper amp keep vigilant for that - The chopper demod beating with a nearby CPU/ FPGA/SMPS clock will show up somewhere downstream in your data and you'll go nuts wondering where that cyclical data artifact comes from.

Chopper amps can work fine just keep an eye on all noise into and out of that amp and surrounding area.  That means inputs / outputs / power rails and the die itself, especially the demod section.  Keep an eye on the switch charge injection on the chopper inputs too.

Just curious - why do you need a 10V cardinal point?  Older equipment??  If you can use a 5V Vref it's usually easier / less noisy / more stable to attenuate down to 5V if you need that.

Dr Frank also has a 10V boost circuit here on the forum where he trims the LTZ down to 7V, and then gains up from there on a matched resistor set.  That's another way to do it.


 

[kj7e]

Just curious - why do you need a 10V cardinal point?  Older equipment??  If you can use a 5V Vref it's usually easier / less noisy / more stable to attenuate down to 5V if you need that.

Only one reason, I wanted to learn how to do it. The entire LTZ1000 project and buffer was just a learning project, something I wanted to do.

[MisterDiodes]

Yes - that's fun.

You'll find that the LTZ circuit is really drop dead easy (if you don't fuss with the resistors and useless voodoo slots and crop circles too much) compared to a 10V booster.

In your calcs, don't forget you're gaining up noise too...suggest you stay with PWW since Edwin will be happy to make whatever values you need (and tempco heading same direction) for no real extra cost, which is nice.

Use good PWW trimmer pots wisely.

Remember - you normally don't need to hit 10V ON THE DOT...You just want to get close to 10V to whatever you think is within reason - say 10 or 50ppm is a practical goal, and then see how -stable- you can get it.  Having a 732 on hand is a handy reference guide if you have one.

Realize you'll never build a 732a/b but you'll build something that is handy for use around the lab that is a lower noise source.

[montemcguire]

It's the LF noise (0.1Hz to 10Hz band) that gets you.  You might try one of the newer high-voltage (works at >5V) chopper amps like the LTC2057, or an OPA189 (and I think ADI has one too-- but can't remember the part number right now).

The Analog part is the ADA4522 series, available in singles, duals, and quads. The 0.1Hz to 10Hz noise is specified at 117nV p-p typical, which is about half of the LTC2057 at 200nV p-p. The voltage noise is also about half of the LTC2057 as well, so it can be a useful amplifier.

I'm not certain of this, as it's not directly specified, but it seems likely that all amplifiers in a dual or quad will run on the same chopper clock, making it easier to use two or more amplifiers in the same circuit without any chopper frequency beating artifacts. I'm using an ADA4522-2 in a DC servo design, and have found that when used as an integrator, it puts out no detectable switching noise whatsoever. Sure, my integrators have a corner of around 24Hz, and this should lowpass the 800kHz and 4.8MHz artifacts into oblivion, but still, no artifacts larger than around 200nV could be found up to 1.2MHz using an Audio Precision test set, and it's more likely that these were leakage from broadcast AM stations into the test circuit and cabling, since the spur frequencies were not related to those used in the ADA4522, and lined up with local AM broadcast stations.

So, it's a nice amplifier, and it's worth considering for any sorts of low noise, low offset and low drift uses. The specifics might favor the 2057 over the 4522, but it's worth evaluating IMHO, especially if you need to use more than one channel at a time.

[cellularmitosis]

Looks like it is also pin-compatible with the 2057, assuming you aren’t using the shutdown pins.

[MisterDiodes]

The other heads up is (and the OP has correctly addressed this with an output amp):  Remember to keep your chopper output current low - or at least well under 2mA for '2057 or similar.  That part isn't explained well in the datasheets.  The same applies to AD parts.

When you draw current then suddenly the output demod can't suppress the "chop" part of the chopper, and you'll get increased noise and offset error on the output signal.  No, the datasheet spec of Output Short Circuit current is meaningless here (output signal is zero volts at that point), please don't get that confused with "Nominal Useful Max Output Supply Current"...which is usually never on the datasheet.  Keep it under 2mA for best result.

So chopper amps when correctly used will almost -always- be paired up with some sort of another "quiet" amp or transistor on the output.   And if you need lower noise that does not mean an LT1010 either, like that show up on the examples.  Yes that big ole' noisy bitch is in the feedback loop, but also notice that it's isolated from the chopper inputs too via 10k/.1uF RC - so the end result is a quite a bit more output noise than you were planning on.  Just a head's up.

As I said - chopper amps camp be useful, and also they are far from perfect.  Just use wisely and learn the limits and the various noise traps.

A 10V buffer is great learning example!  Have fun!

[kj7e]

Buffer has been built, tried to keep all the leads as short as possible;




Installing;







This LTZ1000A reference had right at 1uV P-P noise, now through the 10v buffer I see 1.8uV of P-P noise.  Not too bad since the amplification is 1.4x which would account for 1.4 to 1.5uV of the noise alone;


Edit: Also tested the effect of various output loads on the noise, from 10G, 10M, 1M, 100K, and 10K.  Found only a very slight decrease in noise with 100K load, maybe 100nV or so.

No noticeable voltage change with loads over 1M Ohm.  I see a 2uV (0.2 ppm) drop with a 100K load on the binding posts due to lead resistance in the return side.  About 20uV (2 ppm) drop with a 10K load and I run out of headroom at about 2K.  Short circuit testing shows no overshoot or delay on the recovery.

Doing some TC measurements now.  Initial results are no noticeable change in TC beyond that of the LTZ ref itself from 20-35 C.

[kj7e]

Tempco of 10v output from 15 deg C to 35 deg C, relative set to 10.000000v



Starts off at 15 deg C then warms in the ambient room temp to about 20 deg C by 1500 seconds then I placed the reference enclosure on a hot water bottle to continue warming (upward jump from 1500 seconds to 2350).  If the reference temp rises too quickly it gives a false positive tempco, from here the temp started to stabilize internally and the voltage started to come back down.  At 3890 seconds the internal temp was about 30 deg C and I started to cycle the oven heater to warm it the rest of the way to 35 deg C, you can see the momentary heating in the vertical rises until the nominal temp of 35 C was reached around the 6000 second mark.  Again as the internal temp stabilized the voltage came back down to exactly 10.000000v.  Im sure if I was to use a slower and more stable ramp graph would correlate, but the end result is, minimal tempco of the 10v buffer, 10.000000v at 15deg C, 10.000000v at 35deg C and only a 0.3ppm shift during heating.



Ive been preparing a transport case to send if off, now its time for a few other guys to measure it;

[cellularmitosis]

Dude, congratulations!  Very nice result.

[AG7CK]

Very clever. I knew it would take a practicing ham to make a  Volt-a-Go-Box.

[kj7e]

Very clever. I knew it would take a practicing ham to make a  Volt-a-Go-Box.

The HAM part in me is responsible for the front panel meters, switches and LED's.  Also all the full proof'nes with short circuit and reverse polarity, over charge and over discharge LVD protections.

[kj7e]

Short video;

[AG7CK]

Very clever. I knew it would take a practicing ham to make a  Volt-a-Go-Box.

The HAM part in me is responsible for the front panel meters, switches and LED's.  Also all the full proof'nes with short circuit and reverse polarity, over charge and over discharge LVD protections.

Yep. And that is the difference between an LT1000-On-a-PCB and an apparatus you can take out hiking or risk being dropped to the floor by some postal service droid ...

Anyway - looks like a piece of finished equipment. I guess that's why I like it ...

[TiN]

  Nice!

Did you get final tempco numbers on let's say 18-28C range? How long is the battery life?

Now it's time to calibrate it and check for long-term drift by 2020 year  .

[kj7e]

Nice!

Did you get final tempco numbers on let's say 18-28C range? How long is the battery life?

Now it's time to calibrate it and check for long-term drift by 2020 year  .

Battery life is more than 72 hours with the oven on in 20-25 deg ambient.  Oven off the battery life is more than a week.  Ill dig out the temp chamber and do some more tempco testing tomorrow, but it seems very near zero from 20-30C.