suggestions for stable 3.3v reference from 3.3v or other options - Page 1
Forum section: electronics / projects

author: jones1403
author_rank: Newbie
timestamp: August 22, 2024, 12:32:57 pm
content: Hello,For a mobile aka battery powered application, I need a stable 3.3V precision reference (LM4128AMF-3.3/NOPB) with only 3.3V from an LDO available. Is it bether to use either a voltage doubler or boost converter to power the 3.3V reference or would it be preferrable to redesign the circuit to use a lower voltage reference, or any other option? Below is some mor info about the project.I have used the circuit below on wired applictions (USB) to scale the signal from a 3950K 100k NTC thermistor to a within the 3.3V range of an ADC. Thus I've always had 5.0V available to power a precision 3.3V reference. The reference is used in conjuntion with some resistors and trimmers to shift the output of the OPAMP up or down in order to calibrate multiple NTC's to the same range. now I'm using a Feather ESP32-S3 reverse as microcontroller for the project instead of the usual ESP32 Devkit V1 for wired application.The feather board, when powered by battery does not supply 5.0V or anything higher than the 3.3V supplied by the LDO.Thus the question arises, how to make the stable 3.3V reference for the analog circuit from the 3.3V supplied by the feather board. I simulated that the opamps can be powered directly from the LDO 3.3V without hurting the application, so those are not the problem.One option I can think of is to add a boost converter to the daughterboard to create something like 5.0v to supply the reference voltage IC with. However the converter would only need to provide for a few mA. So it feels a bit like overkill.Are there any other suggestions on how to make this application mobile?Thanks,

author: ledtester
author_rank: Super Contributor
timestamp: August 22, 2024, 02:02:57 pm
content: How about using the 2.5V version of the LM4128? The LM4128 only requires 400mV of headroom so it is usable with a 3.3V supply.If I've done the circuit analysis correctly (and assuming you're using Vref to power the NTC/R23 voltage divider) the op-amp's output is proportional to Vref. With all of the trimmers in the circuit I'm sure you can adjust things to get a full 3.1V measurement range.* 3.1V according to this page:https://docs.espressif.com/projects/esp-idf/en/v4.4/esp32s3/api-reference/peripherals/adc.html#adc-attenuationUpdate:Also see the docs for the adc1_config_channel_atten() call where it is suggested that only a subrange of the full measurement range be used:https://docs.espressif.com/projects/esp-idf/en/v4.4/esp32s3/api-reference/peripherals/adc.html#_CPPv425adc1_config_channel_atten14adc1_channel_t11adc_atten_t

author: Kleinstein
author_rank: Super Contributor
timestamp: August 22, 2024, 02:42:09 pm
content: A good resistor measurement circuit should not rely on a stable reference. For this all voltages, including the ADC ref. should be from the same source and it's stability would not matter. So this could be the "normal" 3.3 V LDO. If really a stable reference is wanted (e.g. also have other sensors), a lower voltage ref. would be the obvious choice. Charge pumps are rather noisy, from the current spikes.Depening on the range needed I don't think one would really need to trim the offset in the analog domain, but could all the trim and adjustment digitally. At least I see no need for 3 trimmers, 1 timmer would be plenty. The resistor values are a bit power hungry for battery operation. So more like 10 x or 50 x the resistors at the amplifier. There are also lower power options (e.g. OPA333) for the amplifier.

author: peter-h
author_rank: Super Contributor
timestamp: August 22, 2024, 02:45:09 pm
content: If you have a square wave available and need ~1mA or less, then the cheapest way to pump up the +V rail is with a diode pump. 2 diodes, 2 capacitors, and you are done.If you can do some factory cal then use a TL431 and you can get any voltage below +V, with it as a shunt regulator. I use these in production and they are super stable and super accurate, for a device which costs pennies.Yes resistor measurement should be ratiometric but you needtworeadings for that.

author: jones1403
author_rank: Newbie
timestamp: August 25, 2024, 11:54:41 am
content: @peter-h. Unfortunately it's a bit more than 1mA, but thanks for the suggestion@Kleinstein: The VREF is also used for the ADC and other sensors thus it is rather critical. Yet I can redesign it using a lower voltage reference as also suggested by @ledtesterNot all of the 3 trimmers are populated though, only the middle one (RP3). RP1 and RP2 are bridged. They are in the design to give the option for some more wiggle room in terms of gain and offset if needed. But in practice I have never needed them.The range is rather critical, it is designed to be able to measure the temperature between -10 and 45 degrees with an a precision of 0.1 degree and an accuracy of +-0.1 degree. Therefore I try to get the maximum range of the ADC (3.1v) and thus all signal conditioning is done in the analog domain. I'm aware that the 100k NTC's are not linear and that the tolerance on them is rather large, yet with proper calibration it has been possible to achieve this.Thanks for the suggestion to reduce the power consumption of the amplifier. This would definitely be needed for the mobile application.@ledtester: Yes you did the analysis correctly. Initially we used the ESP's ADC so the range is set to 3.1V however we switched to another ADC with better specs in terms of resolution and linearity. Thanks for suggesting the 2.5V version of the LM4128