Input stage mess

Looks like a cakewalk to me ;-)

...Jim Thompson

I've done things like that and have a couple of suggestions: note the points A and B.

(1) If you add a switch and to switch in a lower resistance between A and B, you can get the circuit settled more quickly.

(2) If you add a series RC from A to B you can add a zero to the bias point servo. You can use this to get rid of peaking at the gain cross over point.

"Ken Smith" a écrit dans le message de news:dmkfp8$52l$ snipped-for-privacy@blue.rahul.net...

Ken, the pb here, as noted by Phil, isn't introducing a zero somewhere, which I stupidly forgot in the ascii art, and finding where to insert it. It's mainly the 1G resistor that limits the slewing rate to about 15mV/s which imposes impractical settling times if left as is. So I also have to switch some "low" value resistor in parallel to the 1G to allow fast settling times, then once settled, switch in the rigth time constants to obtain the full specs (and obviously preserve stability). This was purposely omitted in the ascii art for clarity.

One thing to take care of is the switch leakages (shot noise in the protection diodes), hopefully not more than 1 or 2 JFETs gate leakage (so under 4pA) in order to preserve the low low frequency noise.

The first place I saw this was in an early FET-input oscilloscope probe. Unfortunately I can't recall the model. The idea is pretty simple, really:

| |--+ | in ------------->|--+ | | | |------ out | c | _|_ \\ | ___ \\|______ | /| | e

"Mark" a écrit dans le message de news: snipped-for-privacy@g43g2000cwa.googlegroups.com...

Yep, they make this. What I want is 200pV/rtHz (yes that's pV), which is a

Do the maths, its gfs is a pitiful 17mS typ. Plus the J310 has a huge low frequency noise, spec'ed at 10nV/rtHz @ 100Hz, missing the target by a 50 factor. Not great and probably much much worse at 1Hz and 0.1Hz.

No.

They are SMPS and are not a pb, but I won't give you all the details.

The specs. by far, I'm afraid :-)

Now you're welcomed to throw in a few components and submit your solution here. Here is the target:

- BW 0.1Hz - 1MHz

- noise floor 200pV/rtHz

- noise density at 0.1Hz not greater than 2nV/rtHz

- must cut the DC input voltage (up to +/- 15V)

- fast settling time (seconds)

- must also withstand 500mV input pulses and be able to characterise a recovery to 200nV in 50uS

- must be floating

- should be user friendly

Oh, and you deliver it mid february.

"Jim Thompson" a écrit dans le message de news: snipped-for-privacy@4ax.com...

Nah. You don't know how to design transistors by square meters :-)

"Mark" a écrit dans le message de news: snipped-for-privacy@g14g2000cwa.googlegroups.com...

I guess so.

Thanks.

"Fred Bartoli" a écrit dans le message de news:438cb657$0$21244$ snipped-for-privacy@news.free.fr...

so

Have just tested a Philips (now Vishay-BC component) MKP378 2.2uF 250V. They are amazingly good. Under 20V bias, they show less than 1pA (probably 0.5pA) leakage.

That's more than 20000 Gohm isolation and an impressive 44.10^6 seconds (>500days) time constant, for a 10^5 min specified.

I guess they'll be good enough :-)

In article , Fred Bartoli wrote: [...]

You could make the op-amp swing to some outragous voltage like 100V to speed things up a bit.

You may be able to make the protection diodes serve double purpose. If their far ends are hooked to active circuits. you could drive them into conduction to center the circuit.

No, he needs an amplifier that has less noise voltage than the supply does. The resistive component of the impedance at the output of the supply is very low and hence the thermal noise floor is very low.

Try assuming the impedance of the system is 1 Ohm not the 50 Ohms you normally think about.

"Ken Smith" a écrit dans le message de news:dmlpdc$f9m$ snipped-for-privacy@blue.rahul.net...

wrote:

That'll reduce the settling time from 3 hours to about 20min. Much better but the requirement is about 1 second (a few seconds at worst).

So will need 100kV with this scheme. Well probably not because when the arc will trigger the 10u input cap will charge pretty fast :-)

Good idea. It'll require that I push the clamp level to 3 diodes drop instead of 2 but that's probably OK. Thanks.

OK so the OP is trying to measure a very small voltage signal FROM A VERY LOW SOURCE Z.

In which case he needs to look at amplifers with very low input noise voltage but the input noise current is not critical. I think that leads back to topologies that have a low input Z like common base or common gate so that they are able to extract the most "power" from the desired signal. A high Z input to an FET gate is not the correct approach.

Mark

going back to a higher level approach to the problem,,,,

if it is so hard to make this measurment, how is it that the application circuit (that is being powered by this power supply) is able to detect this noise so readily?

Mark

"Mark" a écrit dans le message de news: snipped-for-privacy@g44g2000cwa.googlegroups.com...

You're still missing the point. The source is a power supply so I have to block the DC component with... tadam! a cap which has a rising impedance at low frequency, so the input current noise is of primary importance too. And a fet input _is_ the correct approach.

If I had not that pb, the correct approach wouldn't be a discrete input stage but toss 16 AD797 together and it's done. Unfortunately it isn't.

Except the correlation method, which only works for rms measurements. OTOH, it can reach noise levels otherwise unattainable to room-temperature circuitry.

Cheers,

Phil Hobbs

In article , Mark wrote: [.. 200pV/sqrt(hz) ...]

The input noise of a bipolar or FET appears as though it is a small voltage source in series with the gate or base lead. Changing the topology to gommon gate/base doesn't change the noise voltage. It just lowers the power gain.

The OP is basically stuck. There is no way around the need to make the input devices have a low noise voltage.

In article , Fred Bartoli wrote: [....]

You could use a couple of hundred TL074 input stages. :)

BTW: You could use a common gate stage if you floated the whole thing at the average input voltage. Sort of like this:

Input --+------ --------- To more stuff ! ! ! ! ----- ! ^ ! ! -\\/\\/---+ 1G ! --- 1F --- ! GND

You will notice that I didn't say this was a good idea.

I assume in this case you mean:

(1) Connect two amplifiers to the signal.

(2) Multiply the outputs of said amplifiers.

(3) Average the product and then SQRT() to get RMS.

This works ok but it does require longish averaging times if you want to do a lot better than the amplifiers do on their own.

If you run each amplifier into an ADC and do a bit of math with them, You can make a bandwidth filtered measurement to get the noise within some band.

BTW: I've used this with an averaging time equal to one weekend.

"Mark" a écrit dans le message de news: snipped-for-privacy@g49g2000cwa.googlegroups.com...

Eye seeing an whole region averages a lot of pixels + "contrast zooming" and some other "small" things.

Having seen it, I can tell you it's absolutly obvious, even to the not trained eyes, which are certainly not the final users.