Opamp recommendation - low noise, high BW  - Page 1
Forum section: electronics / projects

author: dobsonr741
author_rank: Frequent Contributor
timestamp: June 24, 2023, 06:34:58 pm
content: I'm modeling and measuring a 2 stage 1000x AC amplifier built with 2x LME49713. First stage gain is 32. Noise over 20MHz bandwidth comes out ~10mV rms on the output in real life as well in LTspice.Can you recommend a better opamp for this range? Expected to work from +/-12V supply. A 10x improvement would be nice.

author: macaba
author_rank: Regular Contributor
timestamp: June 24, 2023, 06:58:28 pm
content: With that input RC, try something with less input current noise (<0.5pA/rHz). Maybe LT1037? No free lunch though - half the bandwidth.

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 24, 2023, 08:24:35 pm
content: As a note, I could find the TI datasheet elsewhere, but I'm not able to find the LME49713 on TI's website. I've read that it was EOL. Not completely sure.The OPA847 would be worth a shot, but it's +/-6V max only.Otherwise, to lower the overall noise, increasing the gain of the first stage as much as your bandwidth requirement allows, while decreasing the gain of the second stage, should help.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: June 24, 2023, 09:29:34 pm
content: OPA847 came out from the simulation with 0.0066V noise, and surprisingly the AD8099, I found by Analog's parametric search, had only 0.00025V.It was a surprise as OPA had a bit lower voltage noise density (0.85 vs 0.95)

author: magic
author_rank: Super Contributor
timestamp: June 24, 2023, 09:30:30 pm
content: Assuming it's white noise, 10mV over 20MHz bandwidth means 2.2μV/rtHz density at the output and 2.2nV/rtHz input-referred. This is almost bang on the noise of your combined source resistance (80Ω - 1.1nV/rtHz) plus the 1.9nV/rtHz specified voltage noise of the chip.You need a chip with lower voltage noise.LT1037 is a step backwards. OPA847 appears to possibly be a contender, but I suspect it has a ton of 1/f noise which you may or may not find an issue. AD797 has low 1/f noise but also less GBW. LT1028 for marginally less noise and much less GBW. All of them are probably still at a point where current noise is not a problem with such low source impedances.Maximization of 1st stage gain is not so very important, just 10x gain makes the contribution of your LME49713 second stage negligible.Since 1.1nV/rtHz is coming from resistors alone, no improvement beyond 50% is possible without significantly cooling the circuit.

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 24, 2023, 10:08:04 pm
content: Quote from: magic on June 24, 2023, 09:30:30 pmAgreed.Now the OP must figure out if further lowering noise is really needed in his application. What kind of signal is it meant to condition?Since the noise is essentially going to be 'random' noise, if averaging a number of repeated takes is an option, that may be enough to decrease the resulting noise to a negligible level for your application.

author: moffy
author_rank: Super Contributor
timestamp: June 24, 2023, 11:41:46 pm
content: If you can reduce R3 from 10k to 1k and boost the input cap to 10u then you will reduce the 1/f noise due to current, should provide a small improvement. The OPA1612 is not a bad choice.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: June 25, 2023, 12:20:16 am
content: I gravitate to replace U1 with OPA847. The source impedance (R7) will be 150 ohm in the worst case scenario.The nominal input is 1mV AC, the simulated output noise is 9mV. It will give me 40dB dynamic range, which is in turn acceptable. Thx for the comments.

author: magic
author_rank: Super Contributor
timestamp: June 25, 2023, 06:20:11 am
content: Of course, you can reduce noise by limiting the bandwidth of the amplifier. At the high end and also at the low end.