You could also put little RCs at the source of digital signals to attenuate the high frequency components that are more likely to radiate. Requires that you know your maximum frequency to select the RC. (has this been said already?) ie. LSB changes fastest... Craig -----Original Message----- From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Dan Michaels Sent: Monday, April 24, 2000 10:47 PM To: PICLIST@MITVMA.MIT.EDU Subject: Re: [OT] [EE] 24-bit A/D. Are We in the Twilite Zone Here? Joe, Grif, Wagner, Dave, thank you all for the responses. I think I have been around the track a couple of times with pretty much everything you all mentioned. Bypass caps everywhere, as much physical separation between analog and digital sections as possible given a 3"x5" pcb, "separate gnd busses" for analog and digital, analog gnd single point connection returned directly to power entry point, as much gnd planing as possible on a 2 layer pcb, wide power traces, separate analog regulators, lowest feasible R values on opamp ckts, 2 opamp inverting stages in series, not using non-inverting stages, ferrite bead in power in lead and Rs-232 line. Also, pcb is in an EMI-shielded ABS case, and with case removed, the noise *is* always 2x to 3x to 5x higher. [I wonder if this isn't "the" crucial clue?]. I assume this means pickup from local digital sections of pcb is what is being shielded. I don't think it's coming from outside. ================= Some specifics: - 2 mV noise is referenced to input, with input grounded directly and measured by reading A/D output, not with scope. - for this measurement, input range = +/-1.25v, so 2 mV represents only about 9 bits - so am losing ~3 A/D LSBs. - Power train: - wall wart -> 7812 --> 7805 (digital). " --> 78L06 (analog +). " --> 7662 (-12v) --> 79L06 (analog -). - all regs bypassed, filtered high side, low side, etc. - Gnd planes present under noisy 7662 (766x derivative). Noise referred to above is broadband noise, not correlated to 7662 switching. - Analog circuitry: 2 inverting op amp stages (superior CMR to non-inverting), currently using TLE2072, 4 gains set via '4052. - Lowest range: +/-1.25v, Av1=0.4, Av2=4. - R values: 1st stage - 1M in, T-section feedback to lower effective R, with 82K-21K-68K. 2nd stage 12K in, 50K fb. - both stages switch gain via '4052 mentioned above. - output to A/D thru small R. - opamp/CMOS switch bypassed directly at pins. - A/D: LTC1400, SOIC, 12-bit, 8-pin, internal Vref, SPI control at 6.4 Mhz. Vref bypassed directly at chip. Vdd, Vss bypassed close to chip. - Digital control: PIC76 @ 20 Mhz, bypass caps in several places, gnd planes under chip, gnd island around xtal. - no hi-speed signals passing near analog circuitry. - 2 DC control signals from PIC to '4052, bypassed. - RS-232 connection to pcb: lots of noise was originally measured on the TX line from the PC, and now has a ferrite bead inline. Note - overall, this is a "system-level" problem. Mixed analog-digital, small space, noisy 7662, R values as small as I can make them, RS-232 to PC, wall wart power, etc. I have isolated and dealt with any number of noise sources already. Working on next pcb layout: - moving analog regs further away from PIC. - removing 7812 from inline with 7805. - considering moving single-point analog gnd connection from 78L06/79L06<->power_entry_point to under A/D. Note - I have several times fiddled with the opamp ckt design and R values, and they are about as low as I can go now, and still keep Rin = 1M. I may need to go to a completely different front-end design, ie FET follower amp - unfortunately this is much more complicated, requires lots more space, it drifts, etc/etc. I was hoping to keep the front-end design simple and straightforward, since BW is only 1 Mhz, but I am beginning to wonder. ============= ============= Re what Dave said: >Decouple your supplies into the op-amp. A series R and local C to ground >should work well. Bypass caps are "everywhere", but I haven't tried series R in the opamp power leads. However, they have their own regulators, bypassed and separate from the digital regs, plus a separate analog gnd, too. So I don't think a series R here will help much. ================= >Check the op-amp PSRR AT the frequency of your main interference. > >Use a spectrum analyzer to check everything, not just a scope. Measured noise is pretty wideband, not correlated to PIC xtal or 7662 - [xtal is outside passband, 7662 has been dealt with]. =============== >In the circuit itself, lower the impedances if you can, keep layout tight, >don't ground or decouple to any foreign points. I already have lowered the R values in the analog section as far as I think I can go. The 2 mV noise was measured with input BNC shorted. "... gnd or decouple to a foreign point". What does "foreign" point" mean? ==================== ==================== Re what Joe mentioned: >And yes, you may be missing something. You can trade off bandwidth for >noise! The narrower the bandwidth, the lower the noise figure. Every time >you reduce bandwidth to 50% noise is decreased to 35%. Its one of those >square law formulas I learned in Signals. I am pretty familiar with signal theory, but this is a wideband ckt, and the analog has to be made to tow the line. BW here is 1 Mhz, and I wish I could make it 20 Mhz!! [I could probably get Walter's 0.12 uV, if I LP filtered down to 3 hz. Yeah, right]. -------> My wondering was whether there isn't some magical formula saying something like "for a 1 Mhz BW, no matter how damn hard you try, you aren't ever gonna get the noise below ___ mV, unless you do ........". [as in, "make the Rs all <= 1K", whatever]. =============== >There are other exotic methods for squeezing the last little bit out of a >signal path, synchronous detectors, digital filtering, extremely long (over) >sampling, etc. My case is 1-shot waveform captures, and I want to make the ckt work as well as possible *before* the DSP stuff comes in [which it does later on]. =============== =============== RE what Wagner mentioned, I am still mulling over your comments. Lots of good info there. The Vref on my LTC1400 A/D is inside the chip, bypass caps outside. Noise measured via A/D binary output, with front-end amp tied hard to gnd. >Here again, expensive and good polypropylene caps makes a hell of a >difference from cheap ones. My only caps are ceramic bypass and electrolytic P/S filtering. Where did you use polypropylene? For coupling? ============== Whew, what a marathon. Given all this, if there is something I have clearly overlooked, ....... ???????? Best regards, - Dan Michaels