Floating above Gnd?

G

George Herold

Guest
I've got the following two lead circuit measuring the thermal voltage of a diode.
(to control temp.) R1 had to be added to keep the circuit above Gnd, which is a mess
at the ~1mV level.

Instrment amp.

10uA-> |\
+--OO----+-------|+\
| | | >---
Gnd | +-|-/
| | |/
| |
| +--(R1 1k)--Gnd
| |
R_l R_l R_lead is about 1 ohm
| |
+-|>|-+

diode is some pnp, diode connected transistor...
Sot3? pac with the nice thermal tab on the collector.


So I got this working, but I had to add a bunch of LPF's to the circuit.
(roll off every invertin' amp you have)

And now I'm laying out what I hacked together last month...
and starting to think more about it.
(does anyone else work like this?)
And I'm thinking R1 is not enough.. I can easily add more R between
the R1 node and the inverting input.
Any other thoughts more than welcome. :^)

George H.
 
Perhaps a voltage divider between whatever V- and V+ that amp is using,
to get a "mid-way" voltage, bypass it, and send it through a voltage
follower. Use that as a "virtual ground" for the diode circuit.
 
On Tue, 23 Apr 2019 16:54:16 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

I've got the following two lead circuit measuring the thermal voltage of a diode.
(to control temp.) R1 had to be added to keep the circuit above Gnd, which is a mess
at the ~1mV level.

Instrment amp.

10uA-> |\
+--OO----+-------|+\
| | | >---
Gnd | +-|-/
| | |/
| |
| +--(R1 1k)--Gnd
| |
R_l R_l R_lead is about 1 ohm
| |
+-|>|-+

diode is some pnp, diode connected transistor...
Sot3? pac with the nice thermal tab on the collector.


So I got this working, but I had to add a bunch of LPF's to the circuit.
(roll off every invertin' amp you have)

And now I'm laying out what I hacked together last month...
and starting to think more about it.
(does anyone else work like this?)
And I'm thinking R1 is not enough.. I can easily add more R between
the R1 node and the inverting input.
Any other thoughts more than welcome. :^)

George H.
Click to expand...

Why not make R1 zero ohms?

You might put a cap across the diode, so it doesn't rectify RF and
spikes and such. 10 uA isn't much current.

Are you measuring cryo temps?


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
 
George Herold wrote...
>

The best way to meas temp with a "diode" is
with dV/dt for between two currents. I.e., I
changes by 10x, dV=58mV at room temp. With a
fairly-fast dI measurement, DC offsets are OK.


--
Thanks,
- Win
 
On Tuesday, April 23, 2019 at 7:54:22 PM UTC-4, George Herold wrote:
I've got the following two lead circuit measuring the thermal voltage of a diode.
(to control temp.) R1 had to be added to keep the circuit above Gnd, which is a mess
at the ~1mV level.
Click to expand...
Sorry that's not the whole story,
there was also a 'big' DC offset when the heater circuit
turns on and puts ~100mA through 'local' ground.

GH

Instrment amp.

10uA-> |\
+--OO----+-------|+\
| | | >---
Gnd | +-|-/
| | |/
| |
| +--(R1 1k)--Gnd
| |
R_l R_l R_lead is about 1 ohm
| |
+-|>|-+

diode is some pnp, diode connected transistor...
Sot3? pac with the nice thermal tab on the collector.


So I got this working, but I had to add a bunch of LPF's to the circuit.
(roll off every invertin' amp you have)

And now I'm laying out what I hacked together last month...
and starting to think more about it.
(does anyone else work like this?)
And I'm thinking R1 is not enough.. I can easily add more R between
the R1 node and the inverting input.
Any other thoughts more than welcome. :^)

George H.
Click to expand...
 
On Tuesday, April 23, 2019 at 7:54:22 PM UTC-4, George Herold wrote:
I've got the following two lead circuit measuring the thermal voltage of a diode.
(to control temp.) R1 had to be added to keep the circuit above Gnd, which is a mess
at the ~1mV level.

Instrment amp.

10uA-> |\
+--OO----+-------|+\
| | | >---
Gnd | +-|-/
| | |/
| |
| +--(R1 1k)--Gnd
| |
R_l R_l R_lead is about 1 ohm
| |
+-|>|-+

diode is some pnp, diode connected transistor...
Sot3? pac with the nice thermal tab on the collector.


So I got this working, but I had to add a bunch of LPF's to the circuit.
(roll off every invertin' amp you have)

And now I'm laying out what I hacked together last month...
and starting to think more about it.
(does anyone else work like this?)
And I'm thinking R1 is not enough.. I can easily add more R between
the R1 node and the inverting input.
Any other thoughts more than welcome. :^)

George H.
Click to expand...

Current source is a 10 V reference, through 1 Meg, out and
into fet opamp to float the reference.
GH
 
On Tuesday, April 23, 2019 at 8:29:03 PM UTC-4, Winfield Hill wrote:
George Herold wrote...


The best way to meas temp with a "diode" is
with dV/dt for between two currents. I.e., I
changes by 10x, dV=58mV at room temp. With a
fairly-fast dI measurement, DC offsets are OK.
Click to expand...
Right. I've done that, diode connected transistors
are more 'ideal' than just diodes, or either junction.

for dV/dT the diode voltage is better.
(not as good as thermistors... )
George H.
--
Thanks,
- Win
Click to expand...
 
On Tuesday, April 23, 2019 at 8:33:37 PM UTC-4, John Larkin wrote:
On Tue, 23 Apr 2019 16:54:16 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

I've got the following two lead circuit measuring the thermal voltage of a diode.
(to control temp.) R1 had to be added to keep the circuit above Gnd, which is a mess
at the ~1mV level.

Instrment amp.

10uA-> |\
+--OO----+-------|+\
| | | >---
Gnd | +-|-/
| | |/
| |
| +--(R1 1k)--Gnd
| |
R_l R_l R_lead is about 1 ohm
| |
+-|>|-+

diode is some pnp, diode connected transistor...
Sot3? pac with the nice thermal tab on the collector.


So I got this working, but I had to add a bunch of LPF's to the circuit.
(roll off every invertin' amp you have)

And now I'm laying out what I hacked together last month...
and starting to think more about it.
(does anyone else work like this?)
And I'm thinking R1 is not enough.. I can easily add more R between
the R1 node and the inverting input.
Any other thoughts more than welcome. :^)

George H.

Why not make R1 zero ohms?
Click to expand...
That's how it started, and when the heater turned on the
'error' voltage got bigger... yuck.
The heater and diode sensor share a DB9 connector...
two layer pcb, traces get skinny around the connector.

Anyway I've got a current source and can float it several volts
above ground. (There's x10 gain in the first Int amp. I didn't
show that.)

You might put a cap across the diode, so it doesn't rectify RF and
spikes and such. 10 uA isn't much current.
Click to expand...
Right, I think I have a BFC (1uF film?) across the Int amp inputs.
I've never tested caps at low temps. Are them some that work/ survive
(say) 77K?* smd ceramics?
Are you measuring cryo temps?
Click to expand...
Right. Well the sensor craps out at ~130K (or something)
so we want to keep it above that.

George H.
--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
Click to expand...
 
On 24/04/2019 1:17 am, George Herold wrote:
On Tuesday, April 23, 2019 at 7:54:22 PM UTC-4, George Herold wrote:
I've got the following two lead circuit measuring the thermal voltage of a diode.
(to control temp.) R1 had to be added to keep the circuit above Gnd, which is a mess
at the ~1mV level.
Sorry that's not the whole story,
there was also a 'big' DC offset when the heater circuit
turns on and puts ~100mA through 'local' ground.

GH


Instrment amp.

10uA-> |\
+--OO----+-------|+\
| | | >---
Gnd | +-|-/
| | |/
| |
| +--(R1 1k)--Gnd
| |
R_l R_l R_lead is about 1 ohm
| |
+-|>|-+

diode is some pnp, diode connected transistor...
Sot3? pac with the nice thermal tab on the collector.


So I got this working, but I had to add a bunch of LPF's to the circuit.
(roll off every invertin' amp you have)

And now I'm laying out what I hacked together last month...
and starting to think more about it.
(does anyone else work like this?)
And I'm thinking R1 is not enough.. I can easily add more R between
the R1 node and the inverting input.
Any other thoughts more than welcome. :^)

George H.
Click to expand...

Sharing a DE9 connector is not a problem as long as the sensor ground
and heater ground are on different pins. Perhaps you can keep the
grounds separate until they meet at the power supply. R1 could be zero -
unless you actually want the zero point to be 10mV?

piglet
 
On Wednesday, April 24, 2019 at 8:12:55 AM UTC-4, piglet wrote:
On 24/04/2019 1:17 am, George Herold wrote:
On Tuesday, April 23, 2019 at 7:54:22 PM UTC-4, George Herold wrote:
I've got the following two lead circuit measuring the thermal voltage of a diode.
(to control temp.) R1 had to be added to keep the circuit above Gnd, which is a mess
at the ~1mV level.
Sorry that's not the whole story,
there was also a 'big' DC offset when the heater circuit
turns on and puts ~100mA through 'local' ground.

GH


Instrment amp.

10uA-> |\
+--OO----+-------|+\
| | | >---
Gnd | +-|-/
| | |/
| |
| +--(R1 1k)--Gnd
| |
R_l R_l R_lead is about 1 ohm
| |
+-|>|-+

diode is some pnp, diode connected transistor...
Sot3? pac with the nice thermal tab on the collector.


So I got this working, but I had to add a bunch of LPF's to the circuit.
(roll off every invertin' amp you have)

And now I'm laying out what I hacked together last month...
and starting to think more about it.
(does anyone else work like this?)
And I'm thinking R1 is not enough.. I can easily add more R between
the R1 node and the inverting input.
Any other thoughts more than welcome. :^)

George H.


Sharing a DE9 connector is not a problem as long as the sensor ground
and heater ground are on different pins. Perhaps you can keep the
grounds separate until they meet at the power supply. R1 could be zero -
unless you actually want the zero point to be 10mV?

piglet
Click to expand...

Yeah, I guess that is right. I had both pins 'grounded' at the DB9
connector. I don't really care about 10 mV offset. It could be more.

George h.
 
On Tuesday, April 23, 2019 at 4:54:22 PM UTC-7, George Herold wrote:
> I've got the following two lead circuit measuring the thermal voltage of a diode.

It'd be preferable (having an instrument amp) to make a three-resistors-and-a-diode
Wheatstone bridge, and really USE the instrument amp common mode rejection.

Ground-wire currents won't change the bridge output, unless it changes the
excitation current. It does, however, mean you need more wires as far as the bridge
resistors' position (though not necessarily to the diode).
 
On Wednesday, April 24, 2019 at 4:32:44 PM UTC-4, whit3rd wrote:
On Tuesday, April 23, 2019 at 4:54:22 PM UTC-7, George Herold wrote:
I've got the following two lead circuit measuring the thermal voltage of a diode.

It'd be preferable (having an instrument amp) to make a three-resistors-and-a-diode
Wheatstone bridge, and really USE the instrument amp common mode rejection.

Ground-wire currents won't change the bridge output, unless it changes the
excitation current. It does, however, mean you need more wires as far as the bridge
resistors' position (though not necessarily to the diode).
Click to expand...

huh, I never thought (saw) a diode in a bridge.
I was hacking one circuit to do something else.
The original had a four point measurement for the diode.

But I found three leads was good enough.. a remote voltage sense of
the diode 'Gnd'. And then I posted this question about DC offsets
of sensors... and piglet said put in two, one external.
And Lasse said, put in two back to back. ~No offset and double the
signal. So that's what I did. (Thanks guys, :^)
If I've made a mistake about who suggested what..
I'm sorry, my error.

But I had to give up my third wire of the diode,
for the extra sensor lead.
(the exact T is not important, stability first.)

So here I am. I'm thinking first; return the (big) heater
current right to PS ground. (Which is a lesson I learned
years ago, go figure.)
And second I'll make a spot for an offset R,
(and think harder about local gnd around the input Int amp....)

Thanks again guys!
(I think I've been lazy in 'ground everything hard'
and that isn't always best at DC- ~1kHz ranges.
It's more work to think about ground currents. :^)
George H.
 
Winfield Hill wrote:
George Herold wrote...


The best way to meas temp with a "diode" is
with dV/dt for between two currents. I.e., I
changes by 10x, dV=58mV at room temp. With a
fairly-fast dI measurement, DC offsets are OK.


In other words, the Tektronix way.
Click to expand...


---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus
 
George Herold wrote:
On Tuesday, April 23, 2019 at 7:54:22 PM UTC-4, George Herold wrote:
I've got the following two lead circuit measuring the thermal voltage of a diode.
(to control temp.) R1 had to be added to keep the circuit above Gnd, which is a mess
at the ~1mV level.
Sorry that's not the whole story,
there was also a 'big' DC offset when the heater circuit
turns on and puts ~100mA through 'local' ground.

GH


Instrment amp.

10uA-> |\
+--OO----+-------|+\
| | | >---
Gnd | +-|-/
| | |/
| |
| +--(R1 1k)--Gnd
| |
R_l R_l R_lead is about 1 ohm
| |
+-|>|-+

diode is some pnp, diode connected transistor...
Sot3? pac with the nice thermal tab on the collector.


So I got this working, but I had to add a bunch of LPF's to the circuit.
(roll off every invertin' amp you have)

And now I'm laying out what I hacked together last month...
and starting to think more about it.
(does anyone else work like this?)
And I'm thinking R1 is not enough.. I can easily add more R between
the R1 node and the inverting input.
Any other thoughts more than welcome. :^)

George H.
Click to expand...
Reference the "ground" of the electronics(INCLUDING supply) to that
'local' ground.
That should be the only "float".


---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus
 
On 4/25/19 2:21 AM, Robert Baer wrote:
Winfield Hill wrote:
George Herold wrote...


  The best way to meas temp with a "diode" is
  with dV/dt for between two currents.  I.e., I
  changes by 10x, dV=58mV at room temp.  With a
  fairly-fast dI measurement, DC offsets are OK.


  In other words, the Tektronix way.
Click to expand...

Also used in combination bandgap reference/temperature sensors, e.g. the
REF01, and other PTAT sensors.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
 
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