Fixed bipolar precision voltage reference - Page 1
Forum section: electronics / general

author: PartialDischarge
author_rank: Super Contributor
timestamp: August 29, 2022, 11:30:17 am
content: For some internal tests I need a bipolar voltage source, nothing quite special about this project, still I wanted to post it.Some of the requirements are:- Battery powered by 9V battery- Low output impedance (50 Ohm too high)- 4 or 5 fixed outputs, low output voltages- 0.5% or better precision with high stability over time.- Bipolar outputsI wanted to have separate adjustments for each output, not the typical decade division, thus the parallel configuration of resistors and switches. I looked at whats available commercially but nothing similar, having fixed outputs changeable by a jumper makes tests much faster.Board is 2oz copper + gold plating just to lower somewhat the resistances.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 29, 2022, 12:54:49 pm
content: Many DCDC converters produce quite a bit of EMI problems. So if possible I would avoid them and more go for a virtual ground. That is split the 9 V battery to something like +-4.5 V or +5 V and -4 V.Many DCDC converters also have quite some minimum load.The OPA189 OPs are low noise and low drift, but that can have some spikes at the input and output. Ideally there would be some filtering at the output and provisions that the ouputs stay stable at least with some capacitive load.The way the voltage switching is done is a bit tricky, with the switch resistance effecting the output. It may still work for the modest requirements.

author: PartialDischarge
author_rank: Super Contributor
timestamp: August 29, 2022, 03:17:16 pm
content: Quote from: Kleinstein on August 29, 2022, 12:54:49 pmYes, I also don't like DCDC converters but would like to keep a +-5V supply constant rather than a decaying +-4.5V, so that's why I'm using a DCDC conv. and EMI shield. I think that EMI is not as a bad problem as changing Vcc in precision circuits.As for the quiescent current , it is 25mA for this one and considering it is going to be powered maybe 10-30 minutes a day I consider this acceptable.As for the OPA, I cannot have any filtering that increases the output resistance, maybe an LC filter, I'll test and include a small capacitor when I get the prototype.The output loading is mostly determined by the 5k resistor (0.5mA max) and the REF5025 shows negligible change for this range.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: August 30, 2022, 01:34:55 pm
content: FWIW, I also like jumpers. They're cheap and reliable. Long ago I built a Kelvin-Varley divider using that technique and it worked great.

author: PartialDischarge
author_rank: Super Contributor
timestamp: August 30, 2022, 01:47:23 pm
content: Quote from: Conrad Hoffman on August 30, 2022, 01:34:55 pmGreat, and I'm using both gold plated jumpers and contacts.

author: iMo
author_rank: Super Contributor
timestamp: August 30, 2022, 02:02:17 pm
content: REF5025 - is in epoxy and smd, so do not expect miracles (like "high stability") out of it. Switchers are not good, imho, even though ADI is recently using them in their high end dev kits.. Trimmpots - also not good when talking high stability.You also (certainly) would need to protect the outputs of those opamps, with a transistor buffer and current limiter, for example, that helps with the capacitive loads too..

author: PartialDischarge
author_rank: Super Contributor
timestamp: August 30, 2022, 03:09:10 pm
content: Quote from: imo on August 30, 2022, 02:02:17 pmI just need something better than 0.5% and with this setup I think I can do better. Temperature seems the biggest factor in the whole setup, but for lab use it will not hurt my objective.As I mentioned I prefer to have noise and EMI than a varying voltage from a 9V battery. This is strictly for DC.Those trimpots are 12 turn and the range they have to change the voltage is low.The outputs I need them like that, raw outputs, this is not a general purpose voltage source, it is designed for a very specific purpose.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 30, 2022, 03:27:56 pm
content: There is a way to make the output more resistant to capacitive loading by sperating the low frequency feedback and higher frequency feedback. This allows to add some 1 nF or so to have some filtering and still low output impedance at least for DC and low frequencies.The OPA189 has not real problem with a slightly changing voltage - EMI is possibly a larger issue. The nasty point is that it is not very well predictable, as there may be unintentional resonances in the external circuit if one is not careful. EMI may also effect the external circuitry. Instead of a unregulated +-4.5 V one could also keep the regulator and have something like a stable +-4 V or +-3.5 V, possibly down to +-2.5 V so that one would also be free to use lower voltage OPs (e.g. OPA388) instead of the OPA189.A possible configuration could have the Ref5025 ref from the negative side, powered between the +2.5 V and -2.5 V.25 mA is quite some load and also heat source.If only used for a small effect, the trimmers can be OK.

author: PartialDischarge
author_rank: Super Contributor
timestamp: August 30, 2022, 04:20:56 pm
content: Quote from: Kleinstein on August 30, 2022, 03:27:56 pmActually I made a mistake in the schematic, the part # for the converter is for the 1W version but the one I bought is the 0.25W which is the correct, and it has 12mA quiescent current.

author: iMo
author_rank: Super Contributor
timestamp: August 30, 2022, 06:21:29 pm
content: FYI - REF5025 - Last week I spent couple of days messing with the ADuC845 (24bit SD + MCU) board where I replaced the ADR431 with the REF5025. Thanks Jaromir I had the chance to use his LTZ1000 calibrator as the source (2V here). Below a measurement, the temperature comes from the ADuC chip (aprox 4cm off the REF, pcb in a box). The noise in data is 1.3uV rms (EMA w/ alfa=0.2, ADuC 5.5samples/sec), the tempco around 1.7ppm/C. The 48uV offset there is matter of luck, as the REF5025 has pretty hysteresis after soldering and initial precision cannot be so great too, imho.

author: PartialDischarge
author_rank: Super Contributor
timestamp: September 08, 2022, 10:06:14 am
content: I have the board with only the REF5025 missing, but will adjust and test soon. I'm interested in seeing what noise comes out of the OPA189

author: strawberry
author_rank: Super Contributor
timestamp: September 08, 2022, 05:34:42 pm
content: above 1ppm it is not a reference but a temperature sensor

author: PartialDischarge
author_rank: Super Contributor
timestamp: September 09, 2022, 09:16:15 am
content: So first results, adjusting network with calculated resistors is working very well, trim range is very narrow and within limits. The positive output is very clean and can't find any peaks. I added 1nF right at the output and noise increased.Inverting output however has negative going peaks at 310KHz. Added 330pF at the output and they reduce by about 40%, but going up to 4nF creates oscillations.I'll do more testing after I read this interesting threadhttps://www.eevblog.com/forum/metrology/low-frequency-noise-of-zero-drift-amplifiers/

author: iMo
author_rank: Super Contributor
timestamp: September 10, 2022, 07:48:56 am
content: Loading outputs of an opamp with capacitive load directly is not a good idea. As Kleinstein and me wrote above you would need something there to improve the situation..

author: PartialDischarge
author_rank: Super Contributor
timestamp: September 10, 2022, 08:39:43 am
content: Quote from: imo on September 10, 2022, 07:48:56 amYes I know that capacitive loading creates oscillations but was just testing anyway how much it could tolerate directly.The OPA189 is working great in the follower but not on the inverting configuration, and since I can’t have an R at the output I’m going to look for another opamp that is not chopping at the input for the inverting configuration, probably an opa192, but will test a couple.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 10, 2022, 09:05:50 am
content: Usually the inverter configuration should be the less critical one, when it comes to capacitive loading as the amplifier sees a noise gain of 2. A litte bit of capactiance (e.g. 10s of pF range) in parallel to the feedback capacitor can help with the stability.There is no need for a rather fast amplifier and things with output capacitance gets more relaxed with a slower amplifier. This also helps to keep the power requirement low. The OPA202 would be a reasonable candidate. One can still isolate the output by separate feedback for the low and high frequency part, but this needs a few extra parts and this a different PCB.A point that sometimes helps as an afterthough is to add some series RC element (e.g. 100 Ohms and 10 nF range) to load the output for higher frequencies.

author: PartialDischarge
author_rank: Super Contributor
timestamp: September 10, 2022, 10:45:57 am
content: I think you are right about the advantage of using a slower op-amp, also for the lower current draw. Also I tested a 100Ohm+33nF at the output and it does indeed reduce hf noise.The offset of the OPA202 is a bit high for the 15mV output, I may try the LT6010 seems to have all I need.

author: PartialDischarge
author_rank: Super Contributor
timestamp: September 11, 2022, 09:07:23 am
content: The trim ranges I'm getting are:1V: 1.00158 - 0.99932150mV: 152.03 - 149.81mV50mV: 50.34 - 49.82mV15mV: 15.166 - 14.74mVI may tweak a bit the 15mV and 150mV range.Also I put the circuit in an enclosure and glued 2 jumpers together and attached them to the box with a string just not to lose them.