voltage follower on output of reference zener - Page 1
Forum section: electronics / projects

author: 5065AGuru
author_rank: Frequent Contributor
timestamp: February 28, 2018, 05:20:41 am
content: Hi, Would a voltage follower work to buffer a 9V reference zener ? Main consideration is that the output still reflects the temperature coefficient (approx. 1PPM/deg C)of the zener.thanks,Corby

author: danadak
author_rank: Super Contributor
timestamp: March 01, 2018, 10:50:28 am
content: Basically look at / select offset drift of buffer and select on that to maintainyour ref as dominant limitation on drift. Of course the offset at fixed tempalso contributes error, so that should be considered as well. And youhave to examine noise, PSRR in system to do an overall error budget.Regards, Dana.

author: David Hess
author_rank: Super Contributor
timestamp: March 01, 2018, 03:32:39 pm
content: Quote from: 5065AGuru on February 28, 2018, 05:20:41 amSure, a voltage follower will work. Usually a non-inverting amplifier is used to scale and trim the output voltage without loading the zener diode but a follower is just a non-inverting amplifier with a gain of +1. 1ppm/C at 9 volts is 9 microvolts/C which is not particularly demanding of a precision operational amplifier configured as a voltage follower. Zener diode noise is pretty high so additional noise from the follower will not be a problem. Typical inexpensive precision parts to use would be the OP-07 or LT1013. A 741 or 324 is not going to cut it unless graded for low drift.Often the buffer amplifier for the zener diode is configured as a low pass filter to remove as much of the zener diode's noise as possible. In order to get a low cutoff frequency, this requires a low input bias current operational amplifier to avoid loading the high resistance of the RC filter network. Filter capacitor leakage becomes a significant problem in this sort of design so special techniques are required.

author: 5065AGuru
author_rank: Frequent Contributor
timestamp: March 05, 2018, 02:26:41 am
content: Thanks for the inputs!I'm going to try using an ADA4522-1It will allow me to use the existing +-20V power supplies.Might be overkill but at $2.68 each who cares!Cheers,Corby

author: babysitter
author_rank: Frequent Contributor
timestamp: March 05, 2018, 08:22:06 am
content: Preparing a even-superer-rubidium, hm ?

author: Zero999
author_rank: Super Contributor
timestamp: March 05, 2018, 10:53:41 am
content: If you're bothered about precision, why are you even considering a zener diode and an expensive op-amp? There are far superior options, which are much cheaper.Use the TL431, with an emitter follower in the feedback loop.http://www.ti.com/product/TL431/datasheet/applications_and_implementation#SLVS5436731E24 values:Top resistor = 47kBottom resistor = 18kE96 values:Top resistor = 2k37Bottom resistor = 909REDIT:Here's a more practical circuit, which includes current limiting and the resistor values have been chosen to take the reference pin's 2µA bias current into account.

author: David Hess
author_rank: Super Contributor
timestamp: March 05, 2018, 11:56:25 am
content: I am not sure what zener diode he is using but temperature compensated zener diodes are lower noise and can have lower drift than a TL431. The best cheap bandgap references are good to about 25 ppm/C.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 05, 2018, 04:42:06 pm
content: There are quite good zener refs (e.g. 1N829), however they are more like around 6.2 to 7 V. 9 V is a kind of odd value for a zener reference: one might get a low TC with a 7.6 V Zener and 2 diodes for compensation, but this would be rather high noise.Without much filtering the simple OP07 or similar is usually good enough for a zener. There is no need to use the negative supply for buffering 7 or 9 V.

author: 5065AGuru
author_rank: Frequent Contributor
timestamp: March 06, 2018, 12:15:51 am
content: babysitter, You winkled me out! "Preparing a even-superer-rubidium, hm ?"The 5065A uses a 1n938 reference diode. If you disconnect the load going to the C-field circuit you can trim the diodes current to give close to zero TC for the +20V power supply. Mine came out less than 1PPM/deg C.I then replaced the C-field circuit with a purpose built one with 1PPM/ degree C TC.However some don't want to hack their unit to that degree. Buffering the diode output to the C-field circuit allows you to use a tiny piggyback board for the buffer that mounts on the A15 board. Since all you do is lift the ends of two resistors and mount the buffer board it's an easily removable mod!Will post more details when done on the time-nuts site.Cheers,Corby

author: edpalmer42
author_rank: Super Contributor
timestamp: May 15, 2018, 04:11:47 am
content: Hi Corby,I just found this message thread and wanted to ask you a question related to your idea. I'm considering a slightly more invasive mod to improve the performance of the power supply.CR5 (the 1N938 Zener) provides a reference voltage for the C-field and the base of Q5B which is part of the 20V regulator. I was thinking of using a dual op-amp to isolate CR5 from any other current source/sink and buffer/isolate the two areas from each other. Then provide a low temperature coefficient circuit to set the diode current. Because of the opamps, the current could be set in isolation from anything else.Does this sound like something that would be worthwhile?Ed