Attila Kinali

Hi, I've been again reading a bit and stumbled over a few application notes by Jim Williams. What stuck out on those, that he uses at times JFET differnetial pairs as a kind of pre-amplifier before opamps (e.g. [1], [2]) The only reason I can think of using JFETs is low gate input leakage. But in [1], the LSK389 has a specified max input leakage of 200pA while the LTC6241 has 75pA. Could someone be so kind and enlighten me, why a JFET diff pair makes sense in such applications? Attila Kinali [1] "40nVpp Noise, 0.05uV/°C Drift, Chopped FET Amplifier", by Jim Williams, 2006 http://www.linear.com/product/LTC6241 (page 39) [2] "1Hz to 10MHz V->F Converter", http://cds.linear.com/docs/en/application-note/an13f.pdf (page 8) -- I must not become metastable. Metastability is the mind-killer. Metastability is the little-death that brings total obliteration. I will face my metastability. I will permit it to pass over me and through me. And when it has gone past I will turn the inner eye to see its path. Where the metastability has gone there will be nothing. Only I will remain.

		Attila Kinali

Hello, the LSK389 is usually used for very low noise applications. (the title already states this: 40nVpp). What 1/f noise has the LTC6241? with best regards Andreas Am 29.08.2015 um 20:06 schrieb Attila Kinali: > Hi, > > I've been again reading a bit and stumbled over a few application > notes by Jim Williams. What stuck out on those, that he uses at times > JFET differnetial pairs as a kind of pre-amplifier before opamps (e.g. [1], [2]) > > The only reason I can think of using JFETs is low gate input leakage. > But in [1], the LSK389 has a specified max input leakage of 200pA > while the LTC6241 has 75pA. > > Could someone be so kind and enlighten me, why a JFET diff pair > makes sense in such applications? > > > Attila Kinali > > [1] "40nVpp Noise, 0.05uV/°C Drift, Chopped FET Amplifier", > by Jim Williams, 2006 > http://www.linear.com/product/LTC6241 (page 39) > > [2] "1Hz to 10MHz V->F Converter", > http://cds.linear.com/docs/en/application-note/an13f.pdf (page 8)

		Attila Kinali

On Sat, 29 Aug 2015 20:17:55 +0200 Andreas Jahn <Andreas_-_Jahn@t-online.de> wrote: > the LSK389 is usually used for very low noise applications. > (the title already states this: 40nVpp). > > What 1/f noise has the LTC6241? Ah! Now it makes sense! The LTC6241 has an input noise voltage of typ 550nV p-p (0.1Hz to 10Hz). Thanks! Attila Kinali -- I must not become metastable. Metastability is the mind-killer. Metastability is the little-death that brings total obliteration. I will face my metastability. I will permit it to pass over me and through me. And when it has gone past I will turn the inner eye to see its path. Where the metastability has gone there will be nothing. Only I will remain.

                     Attila Kinali

Interesting that the Keithley 148 has an input noise of 0.2nV RMS and had its heyday in the 1970s. The147 is not as good but good enough to have a 30nV lowest range. I have reliably measured 5nV with my 147. Most of a day is required to warm up the instrument and let the DUT, connections and cables stabilize. Setting the DUT on top of the instrument shows some coupling of the 147 transformer and the DUT/cables. Had to move the DUT away from the instrument. I used a Keithley 2450 to force 1A into a 5 nano ohms precision 4-wire short manufactured by Ohm Labs. Brian VintageNut on eevblog On Sat, Aug 29, 2015 at 3:54 PM, Attila Kinali <attila@kinali.ch> wrote: > On Sat, 29 Aug 2015 20:17:55 +0200 > Andreas Jahn <Andreas_-_Jahn@t-online.de> wrote: > > > the LSK389 is usually used for very low noise applications. > > (the title already states this: 40nVpp). > > > > What 1/f noise has the LTC6241? > > Ah! Now it makes sense! The LTC6241 has an input noise voltage > of typ 550nV p-p (0.1Hz to 10Hz). > > Thanks! > > Attila Kinali > > -- > I must not become metastable. > Metastability is the mind-killer. > Metastability is the little-death that brings total obliteration. > I will face my metastability. > I will permit it to pass over me and through me. > And when it has gone past I will turn the inner eye to see its path. > Where the metastability has gone there will be nothing. Only I will remain. > > _______________________________________________ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there. >

		Attila Kinali

Attila, That is a most excellent signature. I understood exactly to what you were referring after the first line. DaveD On 8/29/2015 12:06 PM, Attila Kinali wrote: > Hi, > > I've been again reading a bit and stumbled over a few application > notes by Jim Williams. What stuck out on those, that he uses at times > JFET differnetial pairs as a kind of pre-amplifier before opamps (e.g. [1], [2]) > > The only reason I can think of using JFETs is low gate input leakage. > But in [1], the LSK389 has a specified max input leakage of 200pA > while the LTC6241 has 75pA. > > Could someone be so kind and enlighten me, why a JFET diff pair > makes sense in such applications? > > > Attila Kinali > > [1] "40nVpp Noise, 0.05uV/°C Drift, Chopped FET Amplifier", > by Jim Williams, 2006 > http://www.linear.com/product/LTC6241 (page 39) > > [2] "1Hz to 10MHz V->F Converter", > http://cds.linear.com/docs/en/application-note/an13f.pdf (page 8)