Looking for instrumentation amp w/ input range <-1V, gain<1 - Page 1
Forum section: electronics / projects

author: Siwastaja
author_rank: Super Contributor
timestamp: March 30, 2016, 09:08:30 am
content: Hi,I'm measuring voltage between sense wires (battery cell voltage) in a bidirectional converter. Due to resistive drop in the power leads, the negative terminal goes below ground.I want to sense the voltage differentially with input impedance of at least 2-3 k, with BW(-3dB) of at least 10kHz. Desired gain is 0.66 (sensing 5V on a 3.3V ADC).Currently, I'm using three jellybean opamps (LM358) with dual supplies to construct an instrumentation amp. This works just fine, no problems whatsoever. I just want to get rid of the negative supply, and, secondarily, it would be nice to have one integrated amp, so smaller footprint and BOM; even if it's a bit more expensive.Many amps have input range that goes to approx. -0.2V, but unfortunately, this is not enough.I need input range of at least -1.0V to 5V. With 5V supply, it would be great, but I can supply it with 8V.Now, many current sense amplifiers and some generic instrumentation amps provide this CMRR range, but I want gain <1, not hundreds.Output range should go to at least 0.3V above negative supply.Otherwise, no need for high specs. LM358 has been good enough with software calibration.Any suggestions welcome. Two-sided supply just for a few jellybean opamps doing such a simple task is something I'd like to get rid of. Of course, I can bias things with resistors and achieve a barely acceptable result with poor input impedance, and maybe I'll do just that, but I'm sure there is an integrated solution out there I just haven't found yet!Thanks for the people who suggested me LT1999 last time I was looking for IC suggestions.

author: nctnico
author_rank: Super Contributor
timestamp: March 30, 2016, 10:26:31 am
content: Look for current sensing amplifiers like the INA283 (and similar). The gain will be higher than you want though.

author: danadak
author_rank: Super Contributor
timestamp: March 30, 2016, 11:01:45 am
content: There are simple techniques, if you don't need high accuracy, ofeither generating negative rails for powering parts or a two resistortechnique for handling negative inputs in a single supply application.https://www.dropbox.com/s/k1yn4m7heaixvmg/ADinput%20negative%20inputs.zip?dl=0Regards, Dana.

author: Marco
author_rank: Super Contributor
timestamp: March 30, 2016, 01:25:39 pm
content: I googled fordifference amplifier below rail, I don't think you really need an in-amp since you don't use amplification. One of the results was INA152, very large beyond rail capability, it ain't cheap though. There might be more, I stopped looking.Just making your own difference amplifier with a precision resistor array might be an option too.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 30, 2016, 02:08:50 pm
content: Yeah I have been considering just skipping the stage which gives me the high impedance of the instrumentation amp structure. So, I could use any jellybean opamp really. Simulation as an attachment.I have simulated this before, so I don't know why I did reject this idea earlier, and went with the exactly same schematic but with two unity buffer stages (before R3&R4) powered from a two-sided supply, forming an instrumentation amp.Maybe it was because I was using smallish-value resistors (330R) directly connected from sense inputs to the supply itself, so that the unit "automatically" works without connecting the sense wires (and when the sense wiresareconnected, they bypass the resistors easily at <1 ohm). In this case, having limited input impedance causes error, when not connecting the sense wires but going through the 330R. On the other hand, anyone who decides not to use the sense wires in a battery testing equipment, is asking for really poor accuracy to begin with, so a little bit of extra error doesn't matter too much. Actual sense wires are low enough in impedance to work with the plain difference amplifier.Maybe I'll just go with this. Comments still appreciated. Thanks for inspiration!

author: danadak
author_rank: Super Contributor
timestamp: March 30, 2016, 08:15:20 pm
content: That sim is not right as your V source, 3.5V, is fixed, so output of OpAmpstays fixed, it always sees Vdiff = 3.5.This might help -http://ww1.microchip.com/downloads/en/AppNotes/01332B.pdfRegards, Dana.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 31, 2016, 08:03:08 am
content: It's a li-ion cell, so a voltage source. Naturally, I also simulated with different voltages (between 0.5V and 4.7V) and internal resistance added to the voltage source. Seems to work well over the expected operating range and a bit beyond. It starts failing with cell voltages near 0V under discharge, when the in- node gets below about -0.3V; or if the sense lead resistance is unacceptably high. But in conditions otherwise acceptable, the opamp input ranges of even LM358 are satisfied, even with a 5V supply. In some special cases, in- (plotted in dark cyan) might get a bit negative, but not much below -0.3V which would start causing real issues (at least with LM358).Thanks.

author: dannyf
author_rank: Super Contributor
timestamp: March 31, 2016, 11:18:34 am
content: Search for beyond the rail opamps and there are a few of them.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 31, 2016, 01:11:15 pm
content: Quote from: dannyf on March 31, 2016, 11:18:34 amYes, that was the question (kind of, before I realized that any standard opamp is good enough). I haven't found the right search terms! It seems that "beyond-the-rails" is a legal trademark of Maxim Integrated, and they use it to denote a range that goes to -0.2V, which is not that special anyway (and I was thinking along the lines of going down to -1V). For future reference, if you have any particular part in mind, maybe you could share.Almost all "rail-to-rail input" opamps actually go 100 to 200mV, sometimes near 300mV, beyond the rails, without special marketing terms applied.