Looking for a better way to create a split rail - Page 1
Forum section: electronics / projects

author: foetusmachine
author_rank: Contributor
timestamp: November 29, 2023, 09:15:01 am
content: I've been posting a little about parts of a project I'm working through to create a millivolt meter. The last part I worked on was limiting the input voltage to an InAmp to ~230mV.The circuit is working and shared here. The other post is here:https://www.eevblog.com/forum/projects/front-end-voltage-limiter-for-instrumentation-amplifier/While the circuit is working, there's an underlying issue of one of the batteries being more heavily loaded than the other when the DUT is in place. The constant current source is sending 100mA through the DUT.The other battery feeding the op amp is clearly there to create a negative rail to allow the op amp to limit the clamped output to 230mV. If I simply run the op amp on a single supply, the lowest it can clamp the output is ~800-900mV. Too high for my use.Looking for suggestions on how to create at least -0.5v negative rail for the op amp amongst the no-so-simple ground references already in place. Perhaps a suggestion to clean that up also?

author: PSR B1257
author_rank: Frequent Contributor
timestamp: November 29, 2023, 10:01:10 am
content: Would be a classical application for an ICL7660.

author: Berni
author_rank: Super Contributor
timestamp: November 29, 2023, 10:06:00 am
content: Yep there are chips for this exact use case.Id recommend the LM7705 (cheaper and more easily available than the part suggested above)

author: foetusmachine
author_rank: Contributor
timestamp: November 29, 2023, 10:52:25 am
content: Thanks both.Just looking for something I can model quickly in LTspice, I've given an LTC1044A a go (pin equivalent ICL7660).I've used the example in the datasheet and read the data and there seems to be little to fine-tune, so i'm guessing it's fairly foolproof.I'm getting a pretty choppy current on the output of the LTC1044A on startup, is this pretty normal?

author: Berni
author_rank: Super Contributor
timestamp: November 29, 2023, 11:04:06 am
content: Well it is a charge pump, that's how they work by using capacitors to bucket brigade the charge up or down to the needed voltage.Don't really see why you would need to simulate it anyway. Just model the output as a current limited voltage source according to the numbers from the datasheet.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 29, 2023, 11:35:15 am
content: Depending on the currents and voltage really needed I would try to get away with a single 3.7 V (likely more 4.2 to 3 V) supply and create the virtual ground with a diode to the negative side. So one would have some +3 V and -0.7 relative to the virtual ground.The LT3092 current source is simple, but it may not be the best to dissipate a lot of heat. So the external 1.25 V ref. can make sense. If an ADC and digital display is used one shoudl use the same voltage also as the ADC reference and may get away with a less accurate one. One would normally not need a regulated supply.

author: ArdWar
author_rank: Frequent Contributor
timestamp: November 29, 2023, 11:48:41 am
content: I'm sorry but after looking at your schematic and reading your description in both of your threads I still not sure what exactly you want to build (and maybe infer what kind of supply quality you may need or if virtual ground is fine).It looks like a complicated ohmmeter instead of millivolt meter.

author: foetusmachine
author_rank: Contributor
timestamp: November 29, 2023, 07:12:51 pm
content: QuoteBecause learning.

author: foetusmachine
author_rank: Contributor
timestamp: November 29, 2023, 07:20:57 pm
content: Quote from: Kleinstein on November 29, 2023, 11:35:15 amThanks. I think ultimately, a single battery is where I'd ideally get to. It's likely the main part of the circuit can run off a single cell, which ultimately would be the milli-ohm to millivolt converter part.However, depending on the ADC/millivolt meter part, I may or may not need/prefer two cells. My options are currently:1. Use an external DMM for the display.2. Use a 0-2v LED voltmeter with a 5v input isolated supply. (higher current demand and boost converter requirement using one cell).3. Use a 0-2v LCD voltmeter with a 9v input, but requiring an additional isolated supply (much lower current, but additional boost, isolated supply components).My project had originally intended to have an integrated display, so it is an all-in-one product - that is what I would still prefer.

author: foetusmachine
author_rank: Contributor
timestamp: November 29, 2023, 07:22:26 pm
content: Quote from: ArdWar on November 29, 2023, 11:48:41 amA bit of both. A milliohm to millivolt style ohmmeter. Will possible further development to a microohm meter.

author: shapirus
author_rank: Super Contributor
timestamp: November 29, 2023, 08:24:01 pm
content: For an alternative, there are also isolated DC-DC switching converters such ashttps://www.aliexpress.com/item/32995009243.html,which can be used to create an isolated (which isn't a requirement in this case) negative rail.They'll probably be much noisier than the ICs suggested in previous posts though.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 29, 2023, 08:31:22 pm
content: I've used single batteries and these-https://www.ti.com/lit/ds/symlink/tle2426.pdf?ts=1701248095088&ref_url=https%253A%252F%252Fwww.google.com%252Fin my projects, but you need more current. Any way to reduce your current needs, or put the high current loop from pos to neg, not stressing ground?

author: Andy Chee
author_rank: Super Contributor
timestamp: November 29, 2023, 10:16:24 pm
content: How about adding an extra op-amp and creating a new ground reference like this picture:The +V and -V are determined by the voltage divider ratio and the rail-to-rail capability of the op-amp used.

author: coppercone2
author_rank: Super Contributor
timestamp: December 01, 2023, 10:40:21 pm
content: I wonder if you can make some kind of switch that switches the batteries connections to ensure a more equal drain rate without the fake grounds. controlled by a metering circuit that knows battery health.