Why do 1.65V voltage reference ICs not exist? - Page 1
Forum section: electronics / projects

author: shanekent
author_rank: Contributor
timestamp: October 18, 2022, 09:22:11 pm
content: This is something that I've seen time and time again...I am working on a design at the moment that is based on a 3.3V rail and I am using some bidirectional current sense ICs that require a reference voltage to be applied that sets the common-mode voltage. I am planning on setting the CM to 1.65V and thus I need to generate a 1.65V rail for those ICs.I can easily use a low impedance voltage divider as a crude method. I could use a buffering op-amp on that network to benefit from the low-impedance output... I could use a shunt and deal with the excess heat generation (small, yes, but not necessarily negligible). Ideally I'd like to use a small voltage reference IC that would be perfect for this application (https://www.digikey.com/en/products/filter/power-management-pmic/voltage-reference/693), but there don't seem to be any 1.65V parts on the market other than a one off from TI. This leads me to use an LDO which is fine and I'm going to do that, but it makes me wonder why does a 1.65V voltage reference part not exist?? How is this not something that other folks have needed in the past?-Shane

author: Kleinstein
author_rank: Super Contributor
timestamp: October 18, 2022, 10:01:23 pm
content: The common mode voltage / virtual ground usually does not have to be accurate. It is more like about in the center is good enough - a well designed circuit should not mind if it is off or not very stable. There are a few special chips for a virtual ground / divide by 2 and buffer.

author: alm
author_rank: Super Contributor
timestamp: October 18, 2022, 10:14:51 pm
content: Some ideashere. Unfortunately the TLE2426 only goes down to 4V. A solution like shownherewith the BUF634 replaced by an op-amp that goes down to 3.3 V might work. If it's very low current and you're not so concerned about quiescent current, even a low-impedance (compared to the load) resistive divider bypassed with caps without the buffer could work.Or if you don't care about exactly splitting the rail, but just roughly in the middle, just a 1.65V LDO. There are some in tiny packages. A voltage reference is very similar to a linear regulator, except generally better stability and less output current.

author: Someone
author_rank: Super Contributor
timestamp: October 18, 2022, 10:27:17 pm
content: Quote from: alm on October 18, 2022, 10:14:51 pmNot all are suited for rail splitting, asymmetric +/- current quadrant capacity can be a non-starter. Usually well described/documented for references, regulators not so much (most expecting to be single quadrant).

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: October 19, 2022, 12:57:55 am
content: This one's 1.3V, is that close enough?https://www.ti.com/product/UC19432-SP</troll>I don't see why you'd need a low impedance divider, or a buffer, reference inputs typically only take a handful of microamps. My go-to if I wanted "half rail" would be a high impedance voltage divider and a small cap to reduce noise.MAX40056 also uses a fixed 1.5V reference for 3.3V supply, sure you have a slightly asymmetric measurement range but a fixed reference might be useful.If you've seen it time and time again you should have a functional block you can just drag and drop by now

author: tszaboo
author_rank: Super Contributor
timestamp: October 19, 2022, 09:09:15 am
content: Your 3.3V is not going to be accurate anyway, so why would you need a reference voltage in the first place? It's just to set the common mode voltage of signals then you can create it with two resistors and an opamp. You are in the realms of "few % accuracy" with 3.3V voltage rails.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 19, 2022, 09:54:29 am
content: A red LED could be used as a reasonable low impedance "reference" - depending on the current / model it should be between 1.4 and 2 V. Compared to just a divider the diode is relatively low impedance (some 30 Ohms for 1 mA or diode current). Another option can be just 2 resistors and plenty of capacitance.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 19, 2022, 01:59:02 pm
content: I don't see a common use.If and when you need exact Vdd/2 - for example, ADC uses Vdd as reference -, then you need to derive it from Vdd so that it tracks variations of Vdd. Resistor divider (with precision resistors) and opamp follower does fine.If you just need similar measurement range in either polarity, then it doesn't need to be exactly 1.65V.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 19, 2022, 04:18:18 pm
content: $3 REF1933AIDDCT (Vbias = Vref / 2)

author: Benta
author_rank: Super Contributor
timestamp: October 19, 2022, 09:58:49 pm
content: Seems a bit like a "Karen" question to me.A good engineer works with what's available, not what's "desirable".

author: ajb
author_rank: Super Contributor
timestamp: October 19, 2022, 10:10:04 pm
content: Quote from: Kleinstein on October 18, 2022, 10:01:23 pmThe trick here, though, is that all of your other signals must remain referenced to that common mode voltage. If you're feeding the output of the current sense ICs into a single-ended ADC, for example, and expecting to subtract the midpoint voltage post-conversion in software, then the exact common mode voltage must be known, and furthermore must be stable--at least over such a timescale that you can calibrate it out, perhaps by taking a measurement of the common mode reference directly. In such a situation maybe you do need to use a decent voltage reference to set the common mode point. But differential ADCs are quite common, even for the built-in ones in MCUs these days, and would be a simpler solution given the option.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: October 20, 2022, 09:28:22 am
content: I think it is a valid question,you are right, no little cheap part exist for this, ask your self why ?i think we find out why :if the demand is there, business potential ? a part to solve a task will be invented and manufactured,since this did not happen, this means the solution two resistors high value and an opamp, or simply two resistors low value and no opamp, will solve the problem.you see creating a vcc/2 voltage is what you want, not another fixed ref voltage with its own accuracy and temperature drift, it is much better to track the vcc, since the cpu / adc or whatver, it using vccby using vcc/2 you removed one accuracy problem, out of your equationbut choose the resistors / opamp parts wisely.