Looking for a 10mA constant current source.

[Dave]

Jim Thompson wrote:

I need to ground (0V) the anode of my noise diode. Anything else can be
grounded too if need be. The cathode must be connected to the 28V supply
via the current control element.

Would you call that floating or not? I think it does need to be floating.


[snip]

Is current FORWARD or REVERSE thru the noise diode? Is 28V supply
+28V or -28V?

...Jim Thompson
The diode is reverse biased and will break down around 10V. The supply

is +28V

 

[Dave]

Fred Bloggs wrote:

You can make a good current reference out of the LM185. The typical
performance on temperature stability is 30ppm calculated over the entire
operating range, but these parts are optimized for nearly zero tempco
in the vicinity of 25oC, and should be very much less over 15o-30o like
3ppm average or less.
View in a fixed-width font such as Courier.

28V>--|>|-------+--------------+
| |
LM185 | [120]
_|_/ |
// \ ------------+
--- |
| e
| |/
| --|
| | |\
| e c
| |/ |
+-------| |
| |\ |
| c |
[270K] | |
| ----+
| |
| [R] R~120 ohm
| |
| |
| |
| -
| ^ ND
| |
+------+-------+
|
gnd

Thanks. I'll look at that in some detail later, but the spec seems good
if that can be achieved.

 

[Dave]

RST Engineering (jw) wrote:

A resistor is certianly not a bad idea, but I don't think it is accurate
enough.


Why would you think this? Remember the cardinal engineering law - KISS.

Yes, if it is not going to compromise performance.

HP certinaly do not use a simple resistor in their noise sources.

And your point is?

Well HP have been making noise figure meters and noise sources for ages.
I think they know a thing or two about them. The HP 436B

http://www.home.agilent.com/USeng/nav/-536888365.536880068/pd.html

is the noise source most suited to the meter I have. The operation
manual for that

http://www.home.agilent.com/cgi-bin/pub/agilent/reuse/cp_ObservationLogRedirector.jsp?NAV_ID=-536888365.536880068.03&LANGUAGE_CODE=eng&CONTENT_KEY=1000002290-1%3aepsg%3aman&COUNTRY_CODE=US&LINKTYPE=LitStation&CONTENT_TYPE=AGILENT_EDITORIAL

certainly shows a constant current source (page 36 of the pdf). I
suspect they would use a resistor if they felt it adequate.

That is specified (see page 9 in the pdf) for operation over the range 0
to 55 deg C, which is wider than what I need, but note that the spec is
only guaranteed at the calibration temperature - *not* over the full range.

These things are quite critical. The room temperature has to be entered
into the instrument, as it will affect the amount of noise the
attenuator generates on the output (noise power = k T B).

There are reasons for this.

1) Diode noise output changes with current - both in terms of amount and
the frequency spectrum.

Yes.

2) Diode voltage changes with temperature.

Over your given temperature range it won't change squat.

Assuming squat means an insigificant amount, then I would agree with you
. Hence it will change a significant amount.

3) The output voltage of the noise meter is speced as 28 +/- 0.1 V.


And this means???

It can be anywhere from 27.9 to 28.1.

4) A 10% change in current will typically cause a 0.5dB change in Excess
noise ratio (ENR). But the ENR will need to be measured to much greater
accuracy than that.


How you would get 1% error is beyond me, but if you want to futz around with
exotic solutions, have at it.

Jim

Calibration data on a diode will show resolution of 0.01 dB.

 

[Dave]

ehsjr wrote:

Dave wrote:

Hi,
I'm looking for a constant current source that must

a) Be floating.

b) Provide 10mA - exact value is not too critical.

c) Have as low a temperature coefficient as possible. Operating range
15-30 deg C.

It is to be used to provide current to an avalanche noise diode. The
input voltage will be 28V, the diode will drop about 8 V, so I need to
drop 20V or so across a resistor and constant current source. The
input voltage will be pretty stable (28 +/- 0.2V), so it does not need
to work over a wide range.

I was thinking about an LM334 with the temperature compensation diode
added. The only problem is 10mA is on its limit, and might cause
significant internal heating, so it will be difficult to keep the
temperature compensation diode at the same temperature as the
regulator IC.



So put two LM334/resistor/diode 5 mA circuits in parallel.

Ed



Why the hell NI did not build the diode into the package I do not know.

I was considering that, athough it will need 3, since they say self
heating is a problem above 4mA.

 

[Dave]

Fred Bloggs wrote:

You can make a good current reference out of the LM185. The typical
performance on temperature stability is 30ppm calculated over the entire
operating range, but these parts are optimized for nearly zero tempco
in the vicinity of 25oC, and should be very much less over 15o-30o like
3ppm average or less.
View in a fixed-width font such as Courier.

28V>--|>|-------+--------------+
| |
LM185 | [120]
_|_/ |
// \ ------------+
--- |
| e
| |/
| --|
| | |\
| e c
| |/ |
+-------| |
| |\ |
| c |
[270K] | |
| ----+
| |
| [R] R~120 ohm
| |
| |
| |
| -
| ^ ND
| |
+------+-------+
|
gnd

I think you have the device wrong there - the LM185 is a 2 terminal
device and you seem to be showing 3. I assume the transistors are both PNP.

 

[Fred Bloggs]

I'm looking for a constant current source that must

a) Be floating.

b) Provide 10mA - exact value is not too critical.

c) Have as low a temperature coefficient as possible. Operating range
15-30 deg C.

It is to be used to provide current to an avalanche noise diode. The
input voltage will be 28V, the diode will drop about 8 V, so I need to
drop 20V or so across a resistor and constant current source. The input
voltage will be pretty stable (28 +/- 0.2V), so it does not need to work
over a wide range.

I was thinking about an LM334 with the temperature compensation diode
added. The only problem is 10mA is on its limit, and might cause
significant internal heating, so it will be difficult to keep the
temperature compensation diode at the same temperature as the regulator
IC. Why the hell NI did not build the diode into the package I do not
know.

The 28V input is actually switched, the frequency of which I do not
know. But I don't think it will be too high, and so the rise/fall time
of the constant current source should not be an issue.

Any better suggestions?

The datasheet shows you how to achieve higher current:
View in a fixed-width font such as
Courier.


Vin
|
|
e
|/
+-------|
| |\
(+)| c
+---+ |
| L | |
| M | |
| 3 |R------+
| 3 | |
| 4 | |
+---+ [Rset]
(-)| |
| |
+---------+
|
|
v

Vref
----
Rset

The drawback is there is no temperature compensation of the Vref
+214uV/oC in this circuit. But since the '334 servo's the transistor in
order to maintain Vref across the R-(-) terminals, the diode
temperature compensation technique still applies:
View in a fixed-width font such as Courier.


Vin
|
|
e
|/
+------------| PN4250A
| |\
| c
| |
+-[1.5K]---||--+
| 0.1u |
| |
(+)| |
+---+ |
| L | |
| M | |
| 3 |R-+---------+
| 3 | | |
| 4 | | |
+---+ [14] [143]
(-)| | | 14 and 143R
| | |
| | | 1% metal film
| | |
| | |
+----+--|>|----+
1N457A |
| Vref Vref + VD
| ---- + --------- ~ 10mA
| R1 R2
|
|
|
[1.2K]
|
|
|
-
^ ND
|
|
gnd

 

I have an 8.67ma current source right here in my hot little hand. You can
have it if you want it. E-mail me.

Norm Strong

"Dave" <nospam@nowhere.com> wrote in message news:42dfdb96@212.67.96.135...

Hi,
I'm looking for a constant current source that must

a) Be floating.

b) Provide 10mA - exact value is not too critical.

c) Have as low a temperature coefficient as possible. Operating range
15-30 deg C.

It is to be used to provide current to an avalanche noise diode. The input
voltage will be 28V, the diode will drop about 8 V, so I need to drop 20V
or so across a resistor and constant current source. The input voltage
will be pretty stable (28 +/- 0.2V), so it does not need to work over a
wide range.

I was thinking about an LM334 with the temperature compensation diode
added. The only problem is 10mA is on its limit, and might cause
significant internal heating, so it will be difficult to keep the
temperature compensation diode at the same temperature as the regulator
IC. Why the hell NI did not build the diode into the package I do not
know.

The 28V input is actually switched, the frequency of which I do not know.
But I don't think it will be too high, and so the rise/fall time of the
constant current source should not be an issue.

Any better suggestions?


HP / Agilent's 346B noise source

http://www.home.agilent.com/USeng/nav/-536888365.536880068/pd.html

is what I am trying to copy, but a look at the manual (page 36 of 81)

http://www.home.agilent.com/cgi-bin/pub/agilent/reuse/cp_ObservationLogRedirector.jsp?NAV_ID=-536888365.536880068.03&LANGUAGE_CODE=eng&CONTENT_KEY=1000002290-1%3aepsg%3aman&COUNTRY_CODE=US&LINKTYPE=LitStation&CONTENT_TYPE=AGILENT_EDITORIAL

shows a degree of complexity I can't really understand. They use the 28V
to make an oscillator, regulate that and use that to drive the constant
current source. I can't understand the logic for just not using the 28V.