Jim Williams' App Note 124 - 775 Nanovolt Noise Measurement - Page 1
Forum section: electronics / projects

author: tree
author_rank: Regular Contributor
timestamp: March 17, 2014, 04:35:14 am
content: http://cds.linear.com/docs/en/application-note/an124f.pdfI'm reading this Jim Williams' app note regarding measuring ultra-low noise. I'm having difficulty understanding how A1 provides DC stability for Q1 and Q2 and why Q1 and Q2 are needed?Thanks.

author: Vgkid
author_rank: Super Contributor
timestamp: March 17, 2014, 05:30:53 am
content: Q1/Q2 are working as a dual jfet differential input buffer circuit(provides very high input impedance). A1 is a bit interesting...Deleted my thoughts on it as it is late.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 17, 2014, 06:53:34 am
content: Note that a high impedance isn't really necessary -- the 1.2k pulldown covers this.One reason might be minimal current noise; JFETs have low gate current and thus intrinsically low current noise. Bipolar amps have input impedances on the order of kohms (for a precision amp, perhaps a meg or so, I think?), the noise of which would compete with the thermal noise of the resistor or DUT. Even so, JFETs are an odd choice; I'll explain later.I'm thinking the main reason is to avoid AC coupling in this part of the circuit -- which is tricky business, as talked about in the capacitor selection part. So, A1 is a servo amp which keeps the JFET gates at equal voltages. The two 100k resistors and 1uF caps provide ample slow-down, while the 0.15uF compensation cap keeps things stable (and it's going to keep bandwidth down as well; I'd have to look at the datasheet to say how much). Oddly, 1uF + 100k is a 1.6Hz pole, so I'm guessing (hoping?) the compensation dominates, keeping the bandwidth to 0.1Hz (at least) as claimed.Anyway, two random discrete JFETs can have wildly different characteristics, and even matched (but still discrete) JFETs can be off by tens of mV. You only get good matching (single digit mV or less) with laser-trimmed monolithic devices (typically only found inside whole op-amps). And even then, the precision amps have some compensation mechanism to keep drift down. Anyway, we're talking microvolts inside a high gain,  DC coupled circuit, so a servo is a good idea.In general terms, a servo is a loop placed around, and coupled to, another block, to gradually control its DC state. For amplifiers, this usually means controlling DC bias, while the signal is AC coupled. Even if the signal is not AC coupled, the servo has the same effect, because it provides DC feedback; a DC offset in the input signal has the same effect as offset in the amplifier, and gets nulled out either way.As for noise, JFETs are an odd choice because they exhibit 1/f noise. Even the rather beefy J105 has a 1/f knee of a few hundred Hz (as a rule, bigger JFETs, running at higher currents, are less noisy). 2SK369 I don't think is any better. The LSK389 specifies 2.5 nV/rtHz at 10Hz (but only 2mA, so it might be better at the ~10mA they're using them at here), which isn't as good as it is at 1kHz for example. The LT1097 is a whole 10-20 nV/rtHz down there, but that's added after the JFET gain, which looks like it might be around 16, so noise at A2 input looks like 40 nV/rtHz, so the amp is a reasonable choice, and doesn't account for much input noise (maybe 10 or 20%).So, ~3 nV/rtHz at 10Hz BW is around 10nV, but the measured performance is 160nV. What gives? It will tend to be higher due to the lower frequency contributions (it's not a flat 3 nV/rtHz), but I'm not sure where 16 times comes from. It could be the 3V battery used to test it is that noisy, I don't know.The unequal drain resistances (450 + 200 on the left side, 900 on the right) are strange; I don't know if this is for voltage reasons (better adjustment range given the 1k 'tweak' resistor and such?), important (Miller effect? ...But at 10Hz?), inconsequential (it looks bad, but doesn't harm anything?), or what. It could very well be a combination of voltage reasons and laziness (not to accuse Jim of anything, but we are talking a circuit built up in a cookie tin here).Tim

author: tree
author_rank: Regular Contributor
timestamp: March 17, 2014, 02:39:37 pm
content: I understand the physics behind the devices themselves, but how does the entire circuit function from a loop perspective, specifically Q1/Q2 and A1.Why do we want the JFETs to be DC stable?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 17, 2014, 04:06:15 pm
content: Well, with a gain of 10^4, it doesn't take much DC offset to rail the circuit.Keeping the gate voltages equal seems like an odd constraint, but I guess as long as it relates to the output (+/- 50uV at the input is +/- 500mV at the output, plenty of headroom), that's fine.I think if I were doing it, I'd add in the offset through a resistor divider to -in. Although that might not be the best choice: it's the most sensitive node, and therefore any noise from A1 is going right in. So even though the loop gain is lower (by about 16x) at the drain, the noise gain is lower by the same amount.Tim

author: tree
author_rank: Regular Contributor
timestamp: March 17, 2014, 05:56:49 pm
content: Quote from: T3sl4co1l on March 17, 2014, 04:06:15 pm50uV of Q1?So let me get this straight. The servo amp makes sure that when there is no signal, the gate of Q2 remains at ~0 volts?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 17, 2014, 06:15:34 pm
content: 50uV being around the Vos of A1. But yes, that appears to be the point.I would think a cheaper amp (like 1/2 LM358, or heck, just a BJT or two) taking feedback from the output, rather than the feedback divider, would achieve better stability (Vos at the output, not Vos * gain), but perhaps there's a reason (excess loop gain necessitating extremely long time constants?), or he simply didn't bother to do it that way.Tim

author: blackdog
author_rank: Frequent Contributor
timestamp: March 17, 2014, 08:56:03 pm
content: Hi,This is a beautiful design...Low Voltage NoiseLow Current NoiseHigh Gain 10.000And still enough openloop gain left over to be precise.LM358, jou must be kidding...Kind regarts,Blackdog

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 17, 2014, 08:58:18 pm
content: You clearly misunderstood my point...

author: David Hess
author_rank: Super Contributor
timestamp: March 18, 2014, 12:18:03 pm
content: I designed and built a low noise differential amplifier using the same correction idea from Jim Williams which could have used a dual JFET but since the input impedance was much lower, I used LT1028 operational amplifiers and fed the correction signal in via the offset pins like he shows in several of the applications notes he wrote. Back when I did this, I was dimly aware of the 2SK369 but availability was poor and the LSK389 was not available yet.A1 corrects for the offset and offset drift of Q1/Q2. Since it operates as an integrator with effectively infinite DC gain, the low frequency breakpoint where it contributes depends both on the integration constant *and* the attenuation going into Q1/Q2 via their load network. That makes adjusting for lowest noise easy because you can just change the value of the attenuation network for lowest output noise assuming you can measure it reliably.As far as why a low noise JFET was used, a bipolar input amplifier optimized for a 1k source resistance like an LT1007 could have about 50 percent higher input noise at 10 Hz and 100% higher input noise at higher frequencies. As an alternative I would try to grade LT1007s for low noise.I wonder why he did not use a chopper stabilized amplifier for A1 but my guess is it had too much charge injection. Using amplifiers which support external overcompensaton make this kind of design a lot easier.