CA3240 Advantage over TL082?

[Frank-Stefan Müller]

Hi there,

a client of mine insists in using ONLY Ca3240s instead of TL082s.
Anybody knows a reason exept Intersil shareholdership?

Frank

 

[Winfield Hill]

Frank-Stefan_Mueller wrote...

A client of mine insists in using ONLY Ca3240s instead of TL082s.
Anybody knows a reason except Intersil shareholdership?

Most specs favor the JFET '082, but some favor the '3240.
If he needs operation to the negative rail, either IN or
OUT, the CA3240 gets the nod. Being a mosfet, it also has
less leakage at high ambient temps. But many would consider
the lack of a second source to be a strong mark against it.

view in monospace font:

item CA3240A TL082A
input: mosfet jfet

Vcm input range:
to -rail: yes no
to +rail: no yes

output to rail?
Vee: yes no
Vos max: 5 6 mV
Ib max: 0.64 7 nA at +70C
e_n typ: 40 18 nV
slew typ: 9 13 V/us
f_T typ: 4.5 3 MHz
Iq (both) 8.0 2.8 mA
cost: $1.18 $0.43 for 100 pieces
2nd source: no yes

 

[Frank-Stefan Müller]

Frank-Stefan_Mueller wrote...

Most specs favor the JFET '082, but some favor the '3240.
If he needs operation to the negative rail, either IN or
OUT, the CA3240 gets the nod. Being a mosfet, it also has
less leakage at high ambient temps. But many would consider
the lack of a second source to be a strong mark against it.

view in monospace font:

item CA3240A TL082A
input: mosfet jfet

Vcm input range:
to -rail: yes no
to +rail: no yes

output to rail?
Vee: yes no
Vos max: 5 6 mV
Ib max: 0.64 7 nA at +70C
e_n typ: 40 18 nV
slew typ: 9 13 V/us
f_T typ: 4.5 3 MHz
Iq (both) 8.0 2.8 mA
cost: $1.18 $0.43 for 100 pieces
2nd source: no yes

Thanks for your effort

Frank-Stefan

 

[Phil Allison]

"Winfield Hill"

Most specs favor the JFET '082, but some favor the '3240.


item CA3240A TL082A

e_n typ: 40 nV 18 nV

** Examination of the EIN curves for both devices shows big differences in
the way noise varies with frequency.

The MOSFET input device has a constantly falling curve which is quite unlike
most JFET and BJT input amplifiers which have curves that remain level above
a certain break frequency - usually at a few hundred Hz.

See fig 18 on page 13.

http://www.intersil.com/data/fn/fn1050.pdf

See fig 21 on page 14.

http://focus.ti.com/lit/ds/symlink/tl081a.pdf

Means the MOSFET device is significantly *less* noisy above about 5 kHz than
the JFET one.

Could be useful to know....



...... Phil

 

[Winfield Hill]

George Herold wrote...

Interesting, Thanks Phil. Say I noticed the other day that the opa134
has current noise that starts to rise dramatically with frequency
above 2kHz or so. http://focus.ti.com/lit/ds/symlink/opa134.pdf
See the graph at the bottom of page 4.

Is this common for all jfet opamps? I've never seen other spec
sheets that plot current noise vs. frequency for FET opamps.

The current noise is so low, 3fA/rt-Hz, that it's scarcely
significant. But it's still interesting to analyze.

Often this type of very low spectral-density noise, rising
proportional to frequency, is due to a capacitively-coupled
signal from a spectrally-flat voltage-noise source.

We can calculate, i_n = e_n 2pi f Cx. For example, assume
the JFETs have 60k drain resistors, which gives about 10nV
of Johnson noise. If this is coupled to the input via some
capacitance, etc., we can calculate, Cx = I_n / 2pi f e_n.
For noise at some frequency on the plot, we get Cx = 0.06pF.
Is that due to Cdg, or is it some other small capacitance?

If the opamp had a cascode input stage for the JFETs, their
drains should not be exposed to high voltage noise. It's
possible the opa134 doesn't have a cascode input. TI says,
"The p-channel JFETs in the FET input stage exhibit a varying
input capacitance with applied common-mode input voltage."

 

[bw]

George Herold wrote...



The current noise is so low, 3fA/rt-Hz, that it's scarcely
significant. But it's still interesting to analyze.

Often this type of very low spectral-density noise, rising
proportional to frequency, is due to a capacitively-coupled
signal from a spectrally-flat voltage-noise source.

We can calculate, i_n = e_n 2pi f Cx. For example, assume
the JFETs have 60k drain resistors, which gives about 10nV
of Johnson noise. If this is coupled to the input via some
capacitance, etc., we can calculate, Cx = I_n / 2pi f e_n.
For noise at some frequency on the plot, we get Cx = 0.06pF.
Is that due to Cdg, or is it some other small capacitance?

If the opamp had a cascode input stage for the JFETs, their
drains should not be exposed to high voltage noise. It's
possible the opa134 doesn't have a cascode input. TI says,
"The p-channel JFETs in the FET input stage exhibit a varying
input capacitance with applied common-mode input voltage."

Thanks Win, The capacitive coupling makes sense. The 3 fA/rtHz is no
worry. But at 100kHz the nosie has risen to 100fA/rtHz.. If my source
impedance is 100k ohm.. that looks like 10nV/rtHz of voltage noise.
That's starting to look significant.

any better way to measure current noise than something like this?

+--100k ohm--+
| |\ |
+-- \ |
| >-------+---out
+-+ /
| |/
|
GND

Thanks,

George H.
----------------------------------------------------
Some low noise circuits by VLF listeners here
http://www.infiltec.com/SID-GRB@home/
http://www.vlf.it/
http://www.vlf.it/ewer3/spectrum1.html
http://www.qsl.net/vk5br/VLFLFRx.htm

I've tested some simple pre-amp/voltage amps over the years, found out 2 key
points.
1. Use a discrete JFET with careful bias set by low noise resistors.
2. Then find a place FAR FAR away from any grid power lines.

 

[Winfield Hill]

bw wrote...

any better way to measure current noise than something like this?

+--100k ohm--+
| |\ |
+-- \ |
| >-------+---out
+-+ /
| |/
|
GND

100k is a bit on the low side.

 

[Phil Allison]

"George Hairoil"

Thanks Win, The capacitive coupling makes sense. The 3 fA/rtHz is no
worry. But at 100kHz the nosie has risen to 100fA/rtHz.. If my source
impedance is 100k ohm.. that looks like 10nV/rtHz of voltage noise.
That's starting to look significant.


** Not compared to the self noise of a 100 kohm resistor it ain't.

My god you are a tedious wanker.



...... Phil