scope preamp (2023 Update)

lørdag den 10. september 2022 kl. 23.34.00 UTC+2 skrev snipped-for-privacy@highlandsniptechnology.com:

I my math adds up the LM2941 is 150uV typical at 5V, the ancient LM7805 is 40uV

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** An L/C or even RC filter after the reg will do the job. Trim the output to make up for any voltage drop.

.... Phil

I'd like the noise to be low at low frequencies, down to 1 Hz or less, so an LC isn't practical. And a load of a few hundred mA makes an RC impractical too.

I do want low dropout to minimize dissipation. We'll probably switch down to +6.25 maybe and then LDO to make a very quiet +5.

This is important so maybe I'll make my own regulator, which would likely be lower noise than an IC reg.

The classic dodge is a switcher driving a linear regulator, followed by Phil H's favorite capacitance multiplier, with RC filters all along.

Joe Gwinn

On Sat, 10 Sep 2022 19:19:24 -0400, Joe Gwinn snipped-for-privacy@comcast.net wrote:

That adds a lot of dissipation.

This looks pretty good. It's actually not bad without R5+C3, kinda surprising. Using a quiet 3 volt reference helps a lot.

Version 4 SHEET 1 1524 680 WIRE 144 16 -112 16 WIRE 336 16 240 16 WIRE 400 16 336 16 WIRE 544 16 400 16 WIRE 656 16 544 16 WIRE 736 16 656 16 WIRE 832 16 736 16 WIRE 896 16 832 16 WIRE 1008 16 976 16 WIRE 1056 16 1008 16 WIRE 656 48 656 16 WIRE -112 64 -112 16 WIRE 400 64 400 16 WIRE 544 64 544 16 WIRE 832 64 832 16 WIRE 224 96 224 64 WIRE 400 160 400 128 WIRE 544 160 544 128 WIRE 656 160 656 128 WIRE -112 176 -112 144 WIRE 832 176 832 144 WIRE 224 224 224 176 WIRE 336 224 224 224 WIRE 464 224 416 224 WIRE 592 224 528 224 WIRE -80 256 -112 256 WIRE -48 256 -80 256 WIRE 48 256 16 256 WIRE 80 256 48 256 WIRE -112 288 -112 256 WIRE 16 288 16 256 WIRE 368 320 336 320 WIRE 400 320 368 320 WIRE 400 336 400 320 WIRE 592 352 592 224 WIRE 592 352 432 352 WIRE 624 352 592 352 WIRE 736 352 736 16 WIRE 736 352 704 352 WIRE 224 368 224 224 WIRE 368 368 224 368 WIRE 464 384 432 384 WIRE 592 384 592 352 WIRE -112 400 -112 368 WIRE 16 400 16 368 WIRE 400 432 400 400 WIRE 592 496 592 464 FLAG -112 176 0 FLAG -112 400 0 FLAG 400 160 0 FLAG 544 160 0 FLAG 16 400 0 FLAG -80 256 +20 FLAG 48 256 +3 FLAG 336 16 +5 FLAG 656 160 0 FLAG 464 384 +3 FLAG 592 496 0 FLAG 400 432 0 FLAG 368 320 +20 FLAG 832 176 0 FLAG 1008 16 N SYMBOL Opamps\\AD823 400 304 M0 WINDOW 0 70 91 Left 2 WINDOW 3 48 120 Left 2 SYMATTR InstName U1 SYMBOL nmos 144 64 R270 WINDOW 0 22 -62 VRight 2 WINDOW 3 -6 -84 VRight 2 SYMATTR InstName M1 SYMATTR Value SP8K2 SYMBOL res 208 80 R0 WINDOW 0 48 40 Left 2 WINDOW 3 49 69 Left 2 SYMATTR InstName R1 SYMATTR Value 50 SYMBOL voltage -112 48 R0 WINDOW 0 54 49 Left 2 WINDOW 3 38 87 Left 2 WINDOW 123 45 120 Left 2 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value SINE(7 100m 1K) SYMATTR Value2 AC 1 SYMBOL voltage -112 272 R0 WINDOW 0 34 82 Left 2 WINDOW 3 35 108 Left 2 SYMATTR InstName V2 SYMATTR Value 20 SYMBOL cap 384 64 R0 WINDOW 0 46 27 Left 2 WINDOW 3 30 57 Left 2 SYMATTR InstName C1 SYMATTR Value 100µ SYMATTR SpiceLine Rser=5m SYMBOL cap 528 64 R0 WINDOW 0 40 9 Left 2 WINDOW 3 40 51 Left 2 SYMATTR InstName C2 SYMATTR Value 10µ SYMBOL voltage 16 272 R0 WINDOW 0 34 82 Left 2 WINDOW 3 35 108 Left 2 SYMATTR InstName V3 SYMATTR Value 3 SYMBOL res 640 32 R0 WINDOW 0 40 78 Left 2 WINDOW 3 32 107 Left 2 SYMATTR InstName R2 SYMATTR Value 100 SYMBOL res 720 336 R90 WINDOW 0 71 56 VBottom 2 WINDOW 3 78 56 VTop 2 SYMATTR InstName R3 SYMATTR Value 2K SYMBOL res 608 480 R180 WINDOW 0 59 53 Left 2 WINDOW 3 58 22 Left 2 SYMATTR InstName R4 SYMATTR Value 3K SYMBOL res 432 208 R90 WINDOW 0 68 81 VBottom 2 WINDOW 3 43 26 VTop 2 SYMATTR InstName R5 SYMATTR Value 1k SYMBOL cap 528 208 R90 WINDOW 0 71 52 VBottom 2 WINDOW 3 47 9 VTop 2 SYMATTR InstName C3 SYMATTR Value 10n SYMBOL current 832 64 R0 WINDOW 0 22 89 Left 2 WINDOW 3 -76 158 Left 2 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName I1 SYMATTR Value PULSE(0 250m 25m 0 0 10m) SYMBOL voltage 880 16 R270 WINDOW 0 -68 55 VTop 2 WINDOW 3 -76 54 VBottom 2 SYMATTR InstName V4 SYMATTR Value 5 TEXT 280 456 Left 2 ;OPA197 TEXT 816 408 Left 2 !.tran 50m startup TEXT 808 296 Left 2 ;T660 5 VOLT LDO TEXT 824 352 Left 2 ;JL Sep 10 2022 TEXT 560 144 Left 2 ;+5mr

Am 11.09.22 um 00:55 schrieb snipped-for-privacy@highlandsniptechnology.com:

I did not measure the LM2941, but the Lm2940 is about the worst choice possible. It would require real work to be better than a LT3042. Its main drawback is the small load current because of the tiny package.

< > 0 dB is 1nV/rtHz.

In the LT3042 data sheet is a a circuit with an external NPN to take the heat. It measures about the same as the 3042 alone. The noise peak climbs to 100 KHz but does not grow in size.

Adding more output C increases peaking. Cset may be increased at the cost of larger startup time. Cset=100u can make sense and lowers the 1/f corner to a few Hz. It is not really 1/f but

1/f**2 or 3. More than 100u seems not to help. < >

Some empty boardlets are left over. 1.5A@5V from a 6.6V raw source is ok. The transistor costs a diode drop vs. bare 3042.

The worse-than-1/f did haunt the preamp I used in 2016 when the spectra were taken. It was a much too small input cap. That must be much larger than needed for f-3dB to be able to short the thermal bias-r noise of the amplifier through the low impedance DUT. The DUT _must_ be low impedance or the 220pV/rtHz of the preamp is not useful. (20*ADA4898 in parallel summed up) A 4700uF wet slug tantalum is about right, but that opens another can of worms.

Even board layout counts. Above 500 KHz, voltage noise of the amplifier did increase. on flickr, there is a picture showing the routing with a felt pen. I suspected the tantalum, but it wasn't the culprit as shown by a mesh of wires.

But I forgot about the shorted tantalum when I tried to measure the noise of some Li ion batteries next week. That ended the life of the preamp in a cruel way. Post mortem Pic is on Flickr, more to the right.

I have a new amplifier with 16 * CPH3910 FETs, only 320nV/rtHz instead of 220. But much less noise current, which helps for cross correlation with 2 amplifiers. The Agilent 89441A can do that. 2 b published in october.

cheers Gerhard

Am 11.09.22 um 05:01 schrieb Gerhard Hoffmann:

I did not get the url right. <

>

Gerhard

I think I'll make my own LDO with an opamp and an n-fet. By the time I have a dpak fet and some giant caps, the opamp and a few passives add not much more board area.

If I use a good 3 volt reference (which I'll have) I expect much less low frequency noise than using the 1.25v bandgap and feedback point of a regulator.

None of that stuff will be on the same die as the big hot pass transistor.

I'll prowl the dungeon and see if I have a AM503 amplifier, and see if it still works, to measure the LDO noise. Or maybe just trust Spice.

One trick I sometimes use is to let the regulator worry about the DC and the load regulation, and then apply an AC-coupled op amp loop to null out the noise.

That way, the AC-coupling goes on the high-Z end (the op amp input), so you can have any corner frequency you like. (If it's very very slow, you'll want to restrict the output swing of the amp to prevent the rail going out of spec.)

Cheers

Phil Hobbs

That's the reason to start with a switcher - most of the voltage drop is there.

But dissipation be damned - we want low noise.

Interesting, but I won't get to this SPICE for a while.

Joe Gwinn

Be sure to make your noise measurements in-situ.

The environment can influence the regulator's performance.

Low noise linears react to simple board rotation.

RL

It may not be too critical - almost any low noise, battery operated AC-coupled amplifier will give a good indication of relative noise levels, in an iterative process.

You'll want to see a good margin on your actual requirements, then recheck in the physical application.

I used to use a simple BPO two-transistor version of the 'liniac', with increased decoupling capacitors, running off a PP9 battery. It used to take some 10s of seconds to DC-stabilize after turn-on. Switched gains of x10, x100.

RL

On Sun, 11 Sep 2022 11:11:05 -0400, Joe Gwinn snipped-for-privacy@comcast.net wrote:

This is better. The mosfet was basically ohmic, but an NPN still acts like a follower.

No comp on the opamp is weird but seems to work. I'd better breadboard this.

Version 4 SHEET 1 1524 680 WIRE 16 16 -64 16 WIRE 176 16 16 16 WIRE 368 16 272 16 WIRE 480 16 368 16 WIRE 592 16 480 16 WIRE 672 16 592 16 WIRE 736 16 672 16 WIRE 800 16 736 16 WIRE -64 64 -64 16 WIRE 368 64 368 16 WIRE 480 64 480 16 WIRE 736 64 736 16 WIRE 592 80 592 16 WIRE 224 112 224 80 WIRE 368 160 368 128 WIRE 480 160 480 128 WIRE -64 176 -64 144 WIRE 736 176 736 144 WIRE -32 256 -64 256 WIRE 0 256 -32 256 WIRE 96 256 64 256 WIRE 128 256 96 256 WIRE 368 272 336 272 WIRE 400 272 368 272 WIRE -64 288 -64 256 WIRE 64 288 64 256 WIRE 400 304 400 272 WIRE 592 320 592 160 WIRE 592 320 432 320 WIRE 224 336 224 176 WIRE 368 336 224 336 WIRE 464 352 432 352 WIRE 592 368 592 320 WIRE -64 400 -64 368 WIRE 64 400 64 368 WIRE 400 400 400 368 WIRE 592 480 592 448 FLAG -64 176 0 FLAG -64 400 0 FLAG 368 160 0 FLAG 480 160 0 FLAG 64 400 0 FLAG -32 256 +20 FLAG 96 256 +3 FLAG 672 16 +5 FLAG 464 352 +3 FLAG 592 480 0 FLAG 400 400 0 FLAG 368 272 +20 FLAG 736 176 0 FLAG 16 16 IN SYMBOL voltage -64 48 R0 WINDOW 0 54 49 Left 2 WINDOW 3 38 87 Left 2 WINDOW 123 45 120 Left 2 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value SINE(7 100m 1K) SYMATTR Value2 AC 1 SYMBOL voltage -64 272 R0 WINDOW 0 34 82 Left 2 WINDOW 3 35 108 Left 2 SYMATTR InstName V2 SYMATTR Value 20 SYMBOL cap 352 64 R0 WINDOW 0 -46 22 Left 2 WINDOW 3 -57 54 Left 2 SYMATTR InstName C1 SYMATTR Value 500µ SYMATTR SpiceLine Rser=10m SYMBOL cap 464 64 R0 WINDOW 0 -52 20 Left 2 WINDOW 3 -56 52 Left 2 SYMATTR InstName C2 SYMATTR Value 20µ SYMBOL voltage 64 272 R0 WINDOW 0 34 82 Left 2 WINDOW 3 35 108 Left 2 SYMATTR InstName V3 SYMATTR Value 3 SYMBOL res 608 176 R180 WINDOW 0 60 93 Left 2 WINDOW 3 60 57 Left 2 SYMATTR InstName R3 SYMATTR Value 2K SYMBOL res 608 464 R180 WINDOW 0 59 53 Left 2 WINDOW 3 58 22 Left 2 SYMATTR InstName R4 SYMATTR Value 3K SYMBOL current 736 64 R0 WINDOW 0 -45 90 Left 2 WINDOW 3 -233 149 Left 2 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName I1 SYMATTR Value PULSE(10m 250m 10m 10n 10n 5m) SYMBOL Opamps\\UniversalOpamp2 400 336 M0 WINDOW 0 -88 -88 Left 2 SYMATTR InstName U1 SYMATTR Value2 Avol=1Meg GBW=10Meg Slew=20Meg SYMBOL npn 176 80 R270 WINDOW 0 31 -54 VRight 2 WINDOW 3 1 -105 VRight 2 SYMATTR InstName Q1 SYMATTR Value BSS4130A SYMBOL FerriteBead 224 144 R0 WINDOW 0 -53 31 Left 2 SYMATTR InstName L1 SYMATTR Value 258n SYMATTR SpiceLine Ipk=0.5 Rser=0.3 Rpar=75 Cpar=250f mfg="Würth Elektronik" pn="74279267 WE-CBF 0603" TEXT 688 408 Left 2 !.tran 0 20m 0 20n TEXT 680 312 Left 2 ;T660 5 VOLT LDO TEXT 696 360 Left 2 ;JL Sep 11 2022 TEXT 288 144 Left 2 ;+10m TEXT 424 280 Left 2 ;OPA197 TEXT 672 448 Left 2 !; .ac dec 50 1 1e6

Am 11.09.22 um 05:30 schrieb snipped-for-privacy@highlandsniptechnology.com:

LT3042 uses a precision current source to develop the output voltage over a resistor. That is easily filtered.

They even have a proposal of using a post-filtered LT6555 reference with the *1 output stage of the LT3042.

Like their external cheap D44H10 NPN

You can trust spice, but you cannot trust the models. Few have 1/f noise, or even correct data. Many transistor models have been made with the Orcad parts program. You recognize them since all have the same RBB.

Gerhard

Am 11.09.22 um 05:30 schrieb snipped-for-privacy@highlandsniptechnology.com:

The current probe amplifier?

No, sorry, Am502.

Turns out we had one of these tucked away in the lab.

Measures 5 uV RMS noise into my scope at 1k gain and 1 MHz bandwidth. Not bad.

There is a little spikey stuff from the USB power wart, about 10 uV.