analog output 0 ... 15mv with noise - Page 1
Forum section: electronics / projects

author: ixps
author_rank: Newbie
timestamp: April 23, 2021, 01:08:26 am
content: Hello,I'm building an circuit to generate an analog output with range 0 .. 15mV however i have getting a noise on my output when i measure (with instrument to read load cells, with range 0..50mV and 15bit). I can calibrate the output, and works fine, but sometimes the noise is too much.On oscilloscope, I cant measure "ripples or noises" because its very small.Follow the schematic:My 3.3V is used as reference in my DAC (and i use an voltage reference to generate it). I would like to supply the OP07 with +-5V, however my charge pump only generate -1.4V on negative supply, but, i think this doenst it the problem.I attempted to put an low pass filter on AO+ and AO- with 159Hz and 0,15Hz, but the problem persist.Ps: my resistors does not precision and stable (are standards, with tol of 5%)Someone has a suggest?thnx

author: David Hess
author_rank: Super Contributor
timestamp: April 23, 2021, 10:26:14 am
content: I have designed differential amplifiers which directly drove a simulated load cell output before and never had a problem with noise, but the output was differential and I used a precision operational amplifier in the class of an LT1001. I think it was the LT1112 but there are better choices unless you need such a low input bias current.In another design I started with a higher voltage, and then used a resistive attenuator to reach the levels that a load cell would produce.However you definitely should be using 1% metal film resistors or better.

author: ixps
author_rank: Newbie
timestamp: April 23, 2021, 11:13:04 am
content: Hello David,Thank you for your suggest. I never worked with output using differential op amps. Is it like this?https://ars.els-cdn.com/content/image/3-s2.0-B9781856175050000119-gr5.jpgFor example, I need a reference voltage (precision) in one of inputs, and my DAC output in another and the output will be diference between they?Thanks again.

author: David Hess
author_rank: Super Contributor
timestamp: April 23, 2021, 11:55:59 am
content: Quote from: ixps on April 23, 2021, 11:13:04 amNo, like the left side of the three operational amplifier instrumentation amplifier:https://en.wikipedia.org/wiki/Instrumentation_amplifierQuoteI do not understand what you are trying to accomplish. Load cell outputs are differential even if they only use a half bridge or one active element. However the outputs do *not* need to be symmetrical as long as their output resistance is identical. So for instance you could use a precision reference, probably the same as the DAC reference, divide it in half for one of the outputs, and then use a voltage divider between that output and the DAC output to make the other output. I remember seeing a load cell simulator which did exactly this although the reference used was the excitation voltage.

author: jmelson
author_rank: Super Contributor
timestamp: April 23, 2021, 11:22:01 pm
content: Large capacitors to ground on the output of opamps can make them unstable. You could put a small capacitor across R5 and let the op-amp do the work of smoothing the output. Possibly as little as 100 pF could solve it.If you MUST put a large cap across the output, place a 100 Ohm resistor at the output pin of the op-amp. if you connect the feedback resistor to the far side of the 100 Ohm resistor, then the feedback loop will compensate for any DC error introduced by the series resistor.Jon

author: mikerj
author_rank: Super Contributor
timestamp: April 24, 2021, 02:31:19 pm
content: Quote from: ixps on April 23, 2021, 01:08:26 amCharge pump style supplies can be extremely noisy. Is you circuit on a PCB with good grounding, or on a breadboard?

author: ixps
author_rank: Newbie
timestamp: April 24, 2021, 04:04:23 pm
content: Quote from: ixps on April 23, 2021, 11:13:04 amI would to connect my circuit in analog input of a indicator (in your input of 0 .. 50mV).Thanks, I will study about that you say!Quote from: jmelson on April 23, 2021, 11:22:01 pmGot it! But, i must to keep only capacitor in output of Op amp, without a load resistor? And others capacitors I should take out? I will try now! Thanks.Quote from: mikerj on April 24, 2021, 02:31:19 pmIm using a board FR4 that I made and manufactured in China, but, i'm learning about good practices in PCBs. So, i consider that this ground is horrible, if I made today, I'd make totally diferent.I'm developing a new board, and i changed the charge pump that I made with microcontroller to IC (that is charge pump also), but, believe that better than me.Thank for all.

author: David Hess
author_rank: Super Contributor
timestamp: April 24, 2021, 06:55:18 pm
content: Quote from: jmelson on April 23, 2021, 11:22:01 pmThat is not always true; it depends on the details. If the capacitance is large enough, then it will result in dominant pole compensation when combined with the operational amplifier's output resistance. It works especially well with an aluminum electrolytic or solid tantalum capacitor because the ESR of the capacitor adds some phase lead to the response. 10 to 100 microfarads commonly works but tests should be done to verify stability.QuoteThat works for any operational amplifier but if a low impedance is required at middle frequencies, then a capacitor as I described above directly across the output works better.